Runge-Kutta method: Difference between revisions
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{{Trans| |
{{Trans|ALGOL 68}} |
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As originally defined, the signature of a procedure parameter could not be specified in Algol W (as here), modern compilers may require parameter specifications for the "f" parameter of rk4. |
As originally defined, the signature of a procedure parameter could not be specified in Algol W (as here), modern compilers may require parameter specifications for the "f" parameter of rk4. |
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<lang algolw>begin |
<lang algolw>begin |
Revision as of 13:24, 5 March 2021
You are encouraged to solve this task according to the task description, using any language you may know.
Given the example Differential equation:
With initial condition:
- and
This equation has an exact solution:
- Task
Demonstrate the commonly used explicit fourth-order Runge–Kutta method to solve the above differential equation.
- Solve the given differential equation over the range with a step value of (101 total points, the first being given)
- Print the calculated values of at whole numbered 's () along with error as compared to the exact solution.
- Method summary
Starting with a given and calculate:
then:
11l
<lang 11l>F rk4(f, x0, y0, x1, n)
V vx = [0.0] * (n + 1) V vy = [0.0] * (n + 1) V h = (x1 - x0) / Float(n) V x = x0 V y = y0 vx[0] = x vy[0] = y L(i) 1..n V k1 = h * f(x, y) V k2 = h * f(x + 0.5 * h, y + 0.5 * k1) V k3 = h * f(x + 0.5 * h, y + 0.5 * k2) V k4 = h * f(x + h, y + k3) vx[i] = x = x0 + i * h vy[i] = y = y + (k1 + k2 + k2 + k3 + k3 + k4) / 6 R (vx, vy)
F f(Float x, Float y) -> Float
R x * sqrt(y)
V (vx, vy) = rk4(f, 0.0, 1.0, 10.0, 100) L(x, y) zip(vx, vy)[(0..).step(10)]
print(‘#2.1 #4.5 #2.8’.format(x, y, y - (4 + x * x) ^ 2 / 16))</lang>
- Output:
0.0 1.00000 0.00000000 1.0 1.56250 -1.45721892e-7 2.0 4.00000 -9.194792e-7 3.0 10.56250 -0.00000291 4.0 24.99999 -0.00000623 5.0 52.56249 -0.00001082 6.0 99.99998 -0.00001659 7.0 175.56248 -0.00002352 8.0 288.99997 -0.00003157 9.0 451.56246 -0.00004072 10.0 675.99995 -0.00005098
Ada
<lang Ada>with Ada.Text_IO; use Ada.Text_IO; with Ada.Numerics.Generic_Elementary_Functions; procedure RungeKutta is
type Floaty is digits 15; type Floaty_Array is array (Natural range <>) of Floaty; package FIO is new Ada.Text_IO.Float_IO(Floaty); use FIO; type Derivative is access function(t, y : Floaty) return Floaty; package Math is new Ada.Numerics.Generic_Elementary_Functions (Floaty); function calc_err (t, calc : Floaty) return Floaty; procedure Runge (yp_func : Derivative; t, y : in out Floaty_Array; dt : Floaty) is dy1, dy2, dy3, dy4 : Floaty; begin for n in t'First .. t'Last-1 loop dy1 := dt * yp_func(t(n), y(n)); dy2 := dt * yp_func(t(n) + dt / 2.0, y(n) + dy1 / 2.0); dy3 := dt * yp_func(t(n) + dt / 2.0, y(n) + dy2 / 2.0); dy4 := dt * yp_func(t(n) + dt, y(n) + dy3); t(n+1) := t(n) + dt; y(n+1) := y(n) + (dy1 + 2.0 * (dy2 + dy3) + dy4) / 6.0; end loop; end Runge; procedure Print (t, y : Floaty_Array; modnum : Positive) is begin for i in t'Range loop if i mod modnum = 0 then Put("y("); Put (t(i), Exp=>0, Fore=>0, Aft=>1); Put(") = "); Put (y(i), Exp=>0, Fore=>0, Aft=>8); Put(" Error:"); Put (calc_err(t(i),y(i)), Aft=>5); New_Line; end if; end loop; end Print;
function yprime (t, y : Floaty) return Floaty is begin return t * Math.Sqrt (y); end yprime; function calc_err (t, calc : Floaty) return Floaty is actual : constant Floaty := (t**2 + 4.0)**2 / 16.0; begin return abs(actual-calc); end calc_err; dt : constant Floaty := 0.10; N : constant Positive := 100; t_arr, y_arr : Floaty_Array(0 .. N);
begin
t_arr(0) := 0.0; y_arr(0) := 1.0; Runge (yprime'Access, t_arr, y_arr, dt); Print (t_arr, y_arr, 10);
end RungeKutta;</lang>
- Output:
y(0.0) = 1.00000000 Error: 0.00000E+00 y(1.0) = 1.56249985 Error: 1.45722E-07 y(2.0) = 3.99999908 Error: 9.19479E-07 y(3.0) = 10.56249709 Error: 2.90956E-06 y(4.0) = 24.99999377 Error: 6.23491E-06 y(5.0) = 52.56248918 Error: 1.08197E-05 y(6.0) = 99.99998341 Error: 1.65946E-05 y(7.0) = 175.56247648 Error: 2.35177E-05 y(8.0) = 288.99996843 Error: 3.15652E-05 y(9.0) = 451.56245928 Error: 4.07232E-05 y(10.0) = 675.99994902 Error: 5.09833E-05
ALGOL 68
<lang ALGOL68> BEGIN
PROC rk4 = (PROC (REAL, REAL) REAL f, REAL y, x, dx) REAL : BEGIN CO Fourth-order Runge-Kutta method CO REAL dy1 = dx * f(x, y); REAL dy2 = dx * f(x + dx / 2.0, y + dy1 / 2.0); REAL dy3 = dx * f(x + dx / 2.0, y + dy2 / 2.0); REAL dy4 = dx * f(x + dx, y + dy3); y + (dy1 + 2.0 * dy2 + 2.0 * dy3 + dy4) / 6.0 END; REAL x0 = 0, x1 = 10, y0 = 1.0; CO Boundary conditions. CO REAL dx = 0.1; CO Step size. CO INT num points = ENTIER ((x1 - x0) / dx + 0.5); CO Add 0.5 for rounding errors. CO [0:num points]REAL y; y[0] := y0; CO Grid and starting point.CO PROC dy by dx = (REAL x, y) REAL : x * sqrt(y); CO Differential equation. CO FOR i TO num points DO y[i] := rk4 (dy by dx, y[i-1], x0 + dx * (i - 1), dx) OD; print ((" x true y calc y relative error", newline)); FOR i FROM 0 BY 10 TO num points DO REAL x = x0 + dx * i; REAL true y = (x * x + 4.0) ^ 2 / 16.0; printf (($3(-zzd.7dxxx), -d.4de-ddl$, x, true y, y[i], y[i] / true y - 1.0)) OD
END </lang>
- Output:
x true y calc y relative error 0.0000000 1.0000000 1.0000000 0.0000e 00 1.0000000 1.5625000 1.5624999 -9.3262e-08 2.0000000 4.0000000 3.9999991 -2.2987e-07 3.0000000 10.5625000 10.5624971 -2.7546e-07 4.0000000 25.0000000 24.9999938 -2.4940e-07 5.0000000 52.5625000 52.5624892 -2.0584e-07 6.0000000 100.0000000 99.9999834 -1.6595e-07 7.0000000 175.5625000 175.5624765 -1.3396e-07 8.0000000 289.0000000 288.9999684 -1.0922e-07 9.0000000 451.5625000 451.5624593 -9.0183e-08 10.0000000 676.0000000 675.9999490 -7.5419e-08
ALGOL W
As originally defined, the signature of a procedure parameter could not be specified in Algol W (as here), modern compilers may require parameter specifications for the "f" parameter of rk4. <lang algolw>begin
real procedure rk4 ( real procedure f ; real value y, x, dx ) ; begin % Fourth-order Runge-Kutta method % real dy1, dy2, dy3, dy4; dy1 := dx * f(x, y); dy2 := dx * f(x + dx / 2.0, y + dy1 / 2.0); dy3 := dx * f(x + dx / 2.0, y + dy2 / 2.0); dy4 := dx * f(x + dx, y + dy3); y + (dy1 + 2.0 * dy2 + 2.0 * dy3 + dy4) / 6.0 end rk4; real x0, x1, y0, dx; integer numPoints; x0 := 0; x1 := 10; y0 := 1.0; % Boundary conditions. % dx := 0.1; % Step size. % numPoints := entier ((x1 - x0) / dx + 0.5); % Add 0.5 for rounding errors. % begin real procedure dyByDx ( real value x, y ) ; x * sqrt(y); % Differential equation. % real array y ( 0 :: numPoints); y(0) := y0; % Grid and starting point. % for i := 1 until numPoints do y(i) := rk4 (dyByDx, y(i-1), x0 + dx * (i - 1), dx); write( " x true y calc y relative error" ); for i := 0 step 10 until numPoints do begin real x, trueY; x := x0 + dx * i; trueY := (x * x + 4.0) ** 2 / 16.0; write( r_format := "A", r_w := 12, r_d := 7, s_w := 3, x, trueY, y( i ) , r_format := "S", r_w := 12, y( i ) / trueY - 1 ) end for_i end
end.</lang>
- Output:
x true y calc y relative error 0.0000000 1.0000000 1.0000000 0.0000e+000 1.0000000 1.5625000 1.5624998 -9.3262e-008 2.0000000 4.0000000 3.9999990 -2.2986e-007 3.0000000 10.5625000 10.5624971 -2.7546e-007 4.0000000 25.0000000 24.9999937 -2.4939e-007 5.0000000 52.5625000 52.5624891 -2.0584e-007 6.0000000 100.0000000 99.9999834 -1.6594e-007 7.0000000 175.5625000 175.5624764 -1.3395e-007 8.0000000 289.0000000 288.9999684 -1.0922e-007 9.0000000 451.5625000 451.5624592 -9.0182e-008 10.0000000 676.0000000 675.9999490 -7.5419e-008
APL
<lang APL>
∇RK4[⎕]∇ ∇
[0] Z←R(Y¯ RK4)Y;T;YN;TN;∆T;∆Y1;∆Y2;∆Y3;∆Y4 [1] (T R ∆T)←R [2] LOOP:→(R≤TN←¯1↑T)/EXIT [3] ∆Y1←∆T×TN Y¯ YN←¯1↑Y [4] ∆Y2←∆T×(TN+∆T÷2)Y¯ YN+∆Y1÷2 [5] ∆Y3←∆T×(TN+∆T÷2)Y¯ YN+∆Y2÷2 [6] ∆Y4←∆T×(TN+∆T)Y¯ YN+∆Y3 [7] Y←Y,YN+(∆Y1+(2×∆Y2)+(2×∆Y3)+∆Y4)÷6 [8] T←T,TN+∆T [9] →LOOP [10] EXIT:Z←T,[⎕IO+.5]Y
∇
∇PRINT[⎕]∇ ∇
[0] PRINT;TABLE [1] TABLE←0 10 .1({⍺×⍵*.5}RK4)1 [2] ⎕←'T' 'RK4 Y' 'ERROR'⍪TABLE,TABLE[;2]-{((4+⍵*2)*2)÷16}TABLE[;1]
∇
</lang>
- Output:
PRINT T RK4 Y ERROR 0 1 0.000000000E0 0.1 1.005006249 ¯1.303701147E¯9 0.2 1.020099995 ¯5.215366805E¯9 0.3 1.045506238 ¯1.174457109E¯8 0.4 1.081599979 ¯2.093284546E¯8 0.5 1.128906217 ¯3.288601591E¯8 0.6 1.188099952 ¯4.780736740E¯8 0.7 1.260006184 ¯6.602350622E¯8 0.8 1.345599912 ¯8.799725681E¯8 0.9 1.446006136 ¯1.143253423E¯7 . . .
AWK
<lang AWK>
- syntax: GAWK -f RUNGE-KUTTA_METHOD.AWK
- converted from BBC BASIC
BEGIN {
print(" t y error") y = 1 for (i=0; i<=100; i++) { t = i / 10 if (t == int(t)) { actual = ((t^2+4)^2) / 16 printf("%2d %12.7f %g\n",t,y,actual-y) } k1 = t * sqrt(y) k2 = (t + 0.05) * sqrt(y + 0.05 * k1) k3 = (t + 0.05) * sqrt(y + 0.05 * k2) k4 = (t + 0.10) * sqrt(y + 0.10 * k3) y += 0.1 * (k1 + 2 * (k2 + k3) + k4) / 6 } exit(0)
} </lang>
- Output:
t y error 0 1.0000000 0 1 1.5624999 1.45722e-007 2 3.9999991 9.19479e-007 3 10.5624971 2.90956e-006 4 24.9999938 6.23491e-006 5 52.5624892 1.08197e-005 6 99.9999834 1.65946e-005 7 175.5624765 2.35177e-005 8 288.9999684 3.15652e-005 9 451.5624593 4.07232e-005 10 675.9999490 5.09833e-005
BASIC
BBC BASIC
<lang bbcbasic> y = 1.0
FOR i% = 0 TO 100 t = i% / 10 IF t = INT(t) THEN actual = ((t^2 + 4)^2) / 16 PRINT "y("; t ") = "; y TAB(20) "Error = "; actual - y ENDIF k1 = t * SQR(y) k2 = (t + 0.05) * SQR(y + 0.05 * k1) k3 = (t + 0.05) * SQR(y + 0.05 * k2) k4 = (t + 0.10) * SQR(y + 0.10 * k3) y += 0.1 * (k1 + 2 * (k2 + k3) + k4) / 6 NEXT i%</lang>
- Output:
y(0) = 1 Error = 0 y(1) = 1.56249985 Error = 1.45721892E-7 y(2) = 3.99999908 Error = 9.19479201E-7 y(3) = 10.5624971 Error = 2.90956245E-6 y(4) = 24.9999938 Error = 6.23490936E-6 y(5) = 52.5624892 Error = 1.08196974E-5 y(6) = 99.9999834 Error = 1.65945964E-5 y(7) = 175.562476 Error = 2.35177287E-5 y(8) = 288.999968 Error = 3.15652015E-5 y(9) = 451.562459 Error = 4.07231605E-5 y(10) = 675.999949 Error = 5.09832905E-5
IS-BASIC
<lang IS-BASIC>100 PROGRAM "Runge.bas" 110 LET Y=1 120 FOR T=0 TO 10 STEP .1 130 IF T=INT(T) THEN PRINT "y(";STR$(T);") =";Y;TAB(21);"Error =";((T^2+4)^2)/16-Y 140 LET K1=T*SQR(Y) 150 LET K2=(T+.05)*SQR(Y+.05*K1) 160 LET K3=(T+.05)*SQR(Y+.05*K2) 170 LET K4=(T+.1)*SQR(Y+.1*K3) 180 LET Y=Y+.1*(K1+2*(K2+K3)+K4)/6 190 NEXT</lang>
C
<lang c>#include <stdio.h>
- include <stdlib.h>
- include <math.h>
double rk4(double(*f)(double, double), double dx, double x, double y) { double k1 = dx * f(x, y), k2 = dx * f(x + dx / 2, y + k1 / 2), k3 = dx * f(x + dx / 2, y + k2 / 2), k4 = dx * f(x + dx, y + k3); return y + (k1 + 2 * k2 + 2 * k3 + k4) / 6; }
double rate(double x, double y) { return x * sqrt(y); }
int main(void) { double *y, x, y2; double x0 = 0, x1 = 10, dx = .1; int i, n = 1 + (x1 - x0)/dx; y = (double *)malloc(sizeof(double) * n);
for (y[0] = 1, i = 1; i < n; i++) y[i] = rk4(rate, dx, x0 + dx * (i - 1), y[i-1]);
printf("x\ty\trel. err.\n------------\n"); for (i = 0; i < n; i += 10) { x = x0 + dx * i; y2 = pow(x * x / 4 + 1, 2); printf("%g\t%g\t%g\n", x, y[i], y[i]/y2 - 1); }
return 0; }</lang>
- Output:
(errors are relative)
x y rel. err. ------------ 0 1 0 1 1.5625 -9.3262e-08 2 4 -2.2987e-07 3 10.5625 -2.75462e-07 4 25 -2.49396e-07 5 52.5625 -2.05844e-07 6 100 -1.65946e-07 7 175.562 -1.33956e-07 8 289 -1.09222e-07 9 451.562 -9.01828e-08 10 676 -7.54191e-08
C#
<lang csharp> using System;
namespace RungeKutta {
class Program { static void Main(string[] args) { //Incrementers to pass into the known solution double t = 0.0; double T = 10.0; double dt = 0.1;
// Assign the number of elements needed for the arrays int n = (int)(((T - t) / dt)) + 1;
// Initialize the arrays for the time index 's' and estimates 'y' at each index 'i' double[] y = new double[n]; double[] s = new double[n];
// RK4 Variables double dy1; double dy2; double dy3; double dy4;
// RK4 Initializations int i = 0; s[i] = 0.0; y[i] = 1.0;
Console.WriteLine(" ===================================== "); Console.WriteLine(" Beging 4th Order Runge Kutta Method "); Console.WriteLine(" ===================================== ");
Console.WriteLine(); Console.WriteLine(" Given the example Differential equation: \n"); Console.WriteLine(" y' = t*sqrt(y) \n"); Console.WriteLine(" With the initial conditions: \n"); Console.WriteLine(" t0 = 0" + ", y(0) = 1.0 \n"); Console.WriteLine(" Whose exact solution is known to be: \n"); Console.WriteLine(" y(t) = 1/16*(t^2 + 4)^2 \n"); Console.WriteLine(" Solve the given equations over the range t = 0...10 with a step value dt = 0.1 \n"); Console.WriteLine(" Print the calculated values of y at whole numbered t's (0.0,1.0,...10.0) along with the error \n"); Console.WriteLine();
Console.WriteLine(" y(t) " +"RK4" + " ".PadRight(18) + "Absolute Error"); Console.WriteLine(" -------------------------------------------------"); Console.WriteLine(" y(0) " + y[i] + " ".PadRight(20) + (y[i] - solution(s[i])));
// Iterate and implement the Rk4 Algorithm while (i < y.Length - 1) {
dy1 = dt * equation(s[i], y[i]); dy2 = dt * equation(s[i] + dt / 2, y[i] + dy1 / 2); dy3 = dt * equation(s[i] + dt / 2, y[i] + dy2 / 2); dy4 = dt * equation(s[i] + dt, y[i] + dy3);
s[i + 1] = s[i] + dt; y[i + 1] = y[i] + (dy1 + 2 * dy2 + 2 * dy3 + dy4) / 6;
double error = Math.Abs(y[i + 1] - solution(s[i + 1])); double t_rounded = Math.Round(t + dt, 2);
if (t_rounded % 1 == 0) { Console.WriteLine(" y(" + t_rounded + ")" + " " + y[i + 1] + " ".PadRight(5) + (error)); }
i++; t += dt;
};//End Rk4
Console.ReadLine(); }
// Differential Equation public static double equation(double t, double y) { double y_prime; return y_prime = t*Math.Sqrt(y); }
// Exact Solution public static double solution(double t) { double actual; actual = Math.Pow((Math.Pow(t, 2) + 4), 2)/16; return actual; } }
}</lang>
C++
Using Lambdas <lang cpp>/*
* compiled with: * g++ (Debian 8.3.0-6) 8.3.0 * * g++ -std=c++14 -o rk4 % * */
- include <iostream>
- include <math.h>
auto rk4(double f(double, double)) {
return [f](double t, double y, double dt) -> double { double dy1 { dt * f( t , y ) }, dy2 { dt * f( t+dt/2, y+dy1/2 ) }, dy3 { dt * f( t+dt/2, y+dy2/2 ) }, dy4 { dt * f( t+dt , y+dy3 ) }; return ( dy1 + 2*dy2 + 2*dy3 + dy4 ) / 6; };
}
int main(void) {
constexpr double TIME_MAXIMUM { 10.0 }, T_START { 0.0 }, Y_START { 1.0 }, DT { 0.1 }, WHOLE_TOLERANCE { 1e-12 };
auto dy = rk4( [](double t, double y) -> double { return t*sqrt(y); } ) ; for ( double y { Y_START }, t { T_START }; t <= TIME_MAXIMUM; y += dy(t,y,DT), t += DT ) if (ceilf(t)-t < WHOLE_TOLERANCE) printf("y(%4.1f)\t=%12.6f \t error: %12.6e\n", t, y, std::fabs(y - pow(t*t+4,2)/16)); return 0;
}</lang>
Common Lisp
<lang lisp>(defun runge-kutta (f x y x-end n)
(let ((h (float (/ (- x-end x) n) 1d0)) k1 k2 k3 k4) (setf x (float x 1d0) y (float y 1d0)) (cons (cons x y) (loop for i below n do (setf k1 (* h (funcall f x y)) k2 (* h (funcall f (+ x (* 0.5d0 h)) (+ y (* 0.5d0 k1)))) k3 (* h (funcall f (+ x (* 0.5d0 h)) (+ y (* 0.5d0 k2)))) k4 (* h (funcall f (+ x h) (+ y k3))) x (+ x h) y (+ y (/ (+ k1 k2 k2 k3 k3 k4) 6))) collect (cons x y)))))
(let ((sol (runge-kutta (lambda (x y) (* x (sqrt y))) 0 1 10 100)))
(loop for n from 0 for (x . y) in sol when (zerop (mod n 10)) collect (list x y (- y (/ (expt (+ 4 (* x x)) 2) 16)))))
((0.0d0 1.0d0 0.0d0)
(0.9999999999999999d0 1.562499854278108d0 -1.4572189210859676d-7) (2.0000000000000004d0 3.9999990805207988d0 -9.194792029987298d-7) (3.0000000000000013d0 10.562497090437557d0 -2.9095624576314094d-6) (4.000000000000002d0 24.999993765090643d0 -6.234909392333066d-6) (4.999999999999998d0 52.56248918030259d0 -1.081969734428867d-5) (5.999999999999995d0 99.9999834054036d0 -1.659459609015812d-5) (6.999999999999991d0 175.56247648227117d0 -2.3517728038768837d-5) (7.999999999999988d0 288.9999684347983d0 -3.156520000402452d-5) (8.999999999999984d0 451.56245927683887d0 -4.072315812209126d-5) (9.99999999999998d0 675.9999490167083d0 -5.0983286655537086d-5))</lang>
Crystal
<lang ruby>y, t = 1, 0 while t <= 10
k1 = t * Math.sqrt(y) k2 = (t + 0.05) * Math.sqrt(y + 0.05 * k1) k3 = (t + 0.05) * Math.sqrt(y + 0.05 * k2) k4 = (t + 0.1) * Math.sqrt(y + 0.1 * k3) printf("y(%4.1f)\t= %12.6f \t error: %12.6e\n", t, y, (((t**2 + 4)**2 / 16) - y )) if (t.round - t).abs < 1.0e-5 y += 0.1 * (k1 + 2 * (k2 + k3) + k4) / 6 t += 0.1
end</lang>
- Output:
y( 0.0) = 1.000000 error: 0.000000e+00 y( 1.0) = 1.562500 error: 1.457219e-07 y( 2.0) = 3.999999 error: 9.194792e-07 y( 3.0) = 10.562497 error: 2.909562e-06 y( 4.0) = 24.999994 error: 6.234909e-06 y( 5.0) = 52.562489 error: 1.081970e-05 y( 6.0) = 99.999983 error: 1.659460e-05 y( 7.0) = 175.562476 error: 2.351773e-05 y( 8.0) = 288.999968 error: 3.156520e-05 y( 9.0) = 451.562459 error: 4.072316e-05 y(10.0) = 675.999949 error: 5.098329e-05
D
<lang d>import std.stdio, std.math, std.typecons;
alias FP = real; alias FPs = Typedef!(FP[101]);
void runge(in FP function(in FP, in FP)
pure nothrow @safe @nogc yp_func, ref FPs t, ref FPs y, in FP dt) pure nothrow @safe @nogc { foreach (immutable n; 0 .. t.length - 1) { immutable FP dy1 = dt * yp_func(t[n], y[n]), dy2 = dt * yp_func(t[n] + dt / 2.0, y[n] + dy1 / 2.0), dy3 = dt * yp_func(t[n] + dt / 2.0, y[n] + dy2 / 2.0), dy4 = dt * yp_func(t[n] + dt, y[n] + dy3); t[n + 1] = t[n] + dt; y[n + 1] = y[n] + (dy1 + 2.0 * (dy2 + dy3) + dy4) / 6.0; }
}
FP calc_err(in FP t, in FP calc) pure nothrow @safe @nogc {
immutable FP actual = (t ^^ 2 + 4.0) ^^ 2 / 16.0; return abs(actual - calc);
}
void main() {
enum FP dt = 0.10; FPs t_arr, y_arr;
t_arr[0] = 0.0; y_arr[0] = 1.0; runge((t, y) => t * y.sqrt, t_arr, y_arr, dt);
foreach (immutable i; 0 .. t_arr.length) if (i % 10 == 0) writefln("y(%.1f) = %.8f Error: %.6g", t_arr[i], y_arr[i], calc_err(t_arr[i], y_arr[i]));
}</lang>
- Output:
y(0.0) = 1.00000000 Error: 0 y(1.0) = 1.56249985 Error: 1.45722e-07 y(2.0) = 3.99999908 Error: 9.19479e-07 y(3.0) = 10.56249709 Error: 2.90956e-06 y(4.0) = 24.99999377 Error: 6.23491e-06 y(5.0) = 52.56248918 Error: 1.08197e-05 y(6.0) = 99.99998341 Error: 1.65946e-05 y(7.0) = 175.56247648 Error: 2.35177e-05 y(8.0) = 288.99996843 Error: 3.15652e-05 y(9.0) = 451.56245928 Error: 4.07232e-05 y(10.0) = 675.99994902 Error: 5.09833e-05
Dart
<lang dart>import 'dart:math' as Math;
num RungeKutta4(Function f, num t, num y, num dt){
num k1 = dt * f(t,y); num k2 = dt * f(t+0.5*dt, y + 0.5*k1); num k3 = dt * f(t+0.5*dt, y + 0.5*k2); num k4 = dt * f(t + dt, y + k3); return y + (1/6) * (k1 + 2*k2 + 2*k3 + k4);
}
void main(){
num t = 0; num dt = 0.1; num tf = 10; num totalPoints = ((tf-t)/dt).floor()+1; num y = 1; Function f = (num t, num y) => t * Math.sqrt(y); Function actual = (num t) => (1/16) * (t*t+4)*(t*t+4); for (num i = 0; i <= totalPoints; i++){ num relativeError = (actual(t) - y)/actual(t); if (i%10 == 0){ print('y(${t.round().toStringAsPrecision(3)}) = ${y.toStringAsPrecision(11)} Error = ${relativeError.toStringAsPrecision(11)}'); } y = RungeKutta4(f, t, y, dt); t += dt; }
}</lang>
- Output:
y(0.00) = 1.0000000000 Error = 0.0000000000 y(1.00) = 1.5624998543 Error = 9.3262010950e-8 y(2.00) = 3.9999990805 Error = 2.2986980086e-7 y(3.00) = 10.562497090 Error = 2.7546153479e-7 y(4.00) = 24.999993765 Error = 2.4939637555e-7 y(5.00) = 52.562489180 Error = 2.0584442034e-7 y(6.00) = 99.999983405 Error = 1.6594596090e-7 y(7.00) = 175.56247648 Error = 1.3395644308e-7 y(8.00) = 288.99996843 Error = 1.0922214534e-7 y(9.00) = 451.56245928 Error = 9.0182772312e-8 y(10.0) = 675.99994902 Error = 7.5419063100e-8
EDSAC order code
The EDSAC subroutine library had two Runge-Kutta subroutines: G1 for 35-bit values and G2 for 17-bit values. A demo of G1 is given here. Setting up the parameters is rather complicated, but after that it's just a matter of calling G1 once for every step in the Runge-Kutta process.
Since EDSAC real numbers are restricted to -1 <= x < 1, the values in the Rosetta Code task have to be scaled down. For comparison with other languages it's convenient to divide the y values by 1000. With 100 steps, a convenient time interval is 1/128.
G1 can solve equations in several variables, say y_1, ..., y_n. The user must provide an auxiliary subroutine which calculates dy_1/dt, ..., dy_n/dt from y_1, ..., y_n. If the derivatives also depend on t (as in the Rosetta Code task) it's necessary to add a dummy y variable which is identical with t. <lang edsac>
[Demo of EDSAC library subroutine G1: Runge-Kutta solution of differential equations. Full description is in Wilkes, Wheeler & Gill, 1951 edn, pages 32-34, 86-87, 132-134.
Before using G1, we need to fix n, m, a, b, c, d, as defined in WWG pages 86-87: n = number of equations (2 for the Rosetta Code example). 2^m = multiplier for the hy', as large as possible without causing numeric overflow; with the scaling chosen here, m = 5. Variables y are stored in n consecutive long locations, the last of which is aD. Scaled derivatives (2^m)hy' in n consecutive long locations, the last of which is bD. G1 uses working variables in n consecutive long locations, the last of which is cD. d = address of user-supplied auxiliary subroutine, which calculates the (2^m)hy'.
For convenience, keep G1 and its storage together. Start at (say) 400 and place: variables y at 400D, 402D; scaled derivatives at 404D, 406D; workspace for G1 at 408D, 410D; G1 itself at 412. If the base address is placed in location 51 at load time, all the above addresses can be accessed via the G parameter:] T 51 K P 400 F [Now set up the 6 preset parameters specified in WWG:] T 45 K P 2#G [H parameter: P a D] P 4 F [N parameter: P 2n F] P 4 F [M parameter: P (b-a) F, or V (2048-a+b) F if a > b] P 4 F [& parameter: P (c-b) F, or V (2048-b+c) F if b > c] P 8 F [L parameter: P 2^(m-2) F] P 300 F [X parameter: P d F] [For other addresses in the program we can optionally use some more parameters:] T 52 K P 120 F [A parameter: main routine] P 56 F [B parameter: print subroutine P1 from EDSAC library] P 350 F [C parameter: constants for Rosetta code example] P 78 F [V parameter: square root subroutine]
[Library subroutine to read constants; runs at load time and is then overwritten. R5, for decimal fractions, seems to be unavailable (lost?), so the values are here read in as 35-bit integers (i.e. times 2^34) by R2. Values are: 0.001, initial value of y (2^23)/(10^7) and 25/(2^10) for use in calculations 0.5/(10^9) for rounding to 9 d.p. (print routine P1 doesn't do this)] GKT20FVDL8FA40DUDTFI40FA40FS39FG@S2FG23FA5@T5@E4@E13Z T#C 17179869F14411518808F419430400F9# TZ
[Library subroutine M3; prints header at load time and is then overwritten.] PFGKIFAFRDLFUFOFE@A6FG@E8FEZPF *SCALED!FOR!EDSAC@&!!TIME!!!!!!!!!Y!VIA!RK!!!!!Y!DIRECT@& ....PK [end text with some blank tape] [Runge-Kutta: auxiliary subroutine to calculate (2^m)*h*(dy1/dt) and (2^m)*h*(dy2/dt) from y1, y2, where y1 is the function y in Rosetta Code (but scaled) and y2 = t. For the Rosetta code example we're using m = 5, h = 2^(-7)] E25K TX GK A3F T20@ [set up return as usual] H2#G V2#G TD [acc := t^2, temp store in 0D] H#G VD LD YF TD [y1 times t^2, shift left, round, temp store in 0D] H2#C VD YF T4D [times (2^23)/(10^7), round, to 4D for square root] [14] A14@ GV A4D T4#G [call square root, result in 4D, copy to (2^m)hy'] A21@ T6#G [1/4, i.e. (2^m)h with m and h as above, to (2^m)ht'] [20] ZF [overwritten by jump back to caller] [21] RF [constant 1/4]
[Main routine, with two subroutines in the same address block as the main routine.] E25K TA GK [0] #F [figures shift on teleprinter] [1] MF [decimal point (in figures mode)] [2] !F @F &F [space, carriage return, line feed,] [5] K4096F [null char] [6] P100F [constant: nr of Runge-Kutta steps (in address field)] [7] PF [negative count of Runge-Kutta steps] [8] P10F [constant: number of steps between printed values] [9] PF [negative count of steps between printed values] [Enter with acc = 0] [10] O@ [set teleprinter to figures] S6@ T7@ [init negative count of R-K steps] S8@ T9@ [init negative count of print steps] [Before using library subroutine G1, clear its working registers (WWG page 33)] T8#G T10#G [Set up initial values of y1 and y2 (where y2 = t)] A#C T#G [load 0.001 from constants section, store in y1] T2#G [y2 = t = 0] [20] A20@ G40@ [call subroutine to print initial values] [Loop round Runge-Kutta steps] [22] TF A23@ G12G [clear accumulator, call G1 for Runge-Kutta step] A9@ A2F U9@ [update negative print count] G33@ [skip printing if not reached 0] S8@ T9@ [reset negative print count] A31@ G40@ [call subroutine to print values] [33] TF [clear accumulator] A7@ A2F U7@ [increment negative count of Runge-Kutta steps] G22@ [loop till count = 0] O5@ ZF [flush teleprinter buffer; stop]
[Subroutine to print y1 as calculated (1) by Runge-Kutta (2) direct from formula] [40] A3F T71@ [set up return as usual] A2#G TD [latest t (= y2) from Runge-Kutta, to 0D for printing] [44] A44@ G72@ [call subroutine to print t] O2@ O2@ [followed by 2 spaces] A#G TD [latest y1 from Runge-Kutta, to 0D for printing] [50] A50@ G72@ [call subroutine to print y1] O2@ O2@ [followed by 2 spaces] A 4#C [load constant 25/(2^10)] H2#G V2#G TD [add t^2, temp store result in 0D] HD VD LD YF TD [square, shift 1 left, round, result to 0D] H2#C VD YF TD [times (2^23)/(10^7), round, to 0D for printing] [67] A67@ G72@ [call subroutine to print y] O3@ O4@ [print CR, LF] [71] ZF [overwritten by jump back to caller]
[Second-level subroutine to print number in 0D to 9 decimal places] [72] A3F T82@ [set up return as usual] AD A6#C TD [load number, add decimal rounding, to 0D for printing] O81@ O1@ [print '0.' since P1 doesn't do so] A79@ GB [call library subroutine P1 for printing] [81] P9F [parameter for P1, 9 decimals] [82] ZF [overwritten by jump back to caller]
[Library subroutine G1 for Runge-Kutta process. 66 locations, even address.] E25K T12G GKT4#ZH682DT6#ZPNT12#Z!1405DT14#ZTHT16#ZT2HTZA3FT61@A31@G63@&FT6ZPN T8ZMMO&H4@A20@E23@T14ZAHT16ZA2HT18ZH12#@S12#@T12#@E28@H4#@T4DUFS38@ A25@T38@S6#@A16#@U46#@A8@U37@A9@U55@A24@T39@ZFR1057#@ZFYFU6DV6DRLYF UDZFZFADLDADLLS6DN4DYFZFA46#@S14#@G29@A65@S11@ZFA35@U65@GXZF
[Replacement for library routine S2 (square root). 38 locations, even address. Advantages: More accurate for small values of the argument. Calculates sqrt(0) without going into an infinite loop. Disadvantages: Longer and slower than S2 (calculates one bit at a time).] E25K TV GKA3FT31@A4DG32@A33@T36#@T4DA33@RDU34#@RDS4DS33@A36#@G22@T36#@A4DS34#@ T4DA36#@A33@G25@TFA36#@S33@A36#@T36#@A34#@RDYFG9@ZFZFK4096FPFPFPFPF
[Library subroutine P1 - print a single positive number. 21 locations. Prints number in 0D to n places of decimals, where n is specified by 'P n F' pseudo-order after subroutine call.] E25K TB GKA18@U17@S20@T5@H19@PFT5@VDUFOFFFSFL4FTDA5@A2FG6@EFU3FJFM1F
[Define entry point in main routine] E25K TA GK E10Z PF [enter at relative address 10 with accumulator = 0]
</lang>
- Output:
SCALED FOR EDSAC TIME Y VIA RK Y DIRECT 0.000000000 0.001000000 0.001000000 0.078125000 0.001562499 0.001562500 0.156250000 0.003999998 0.004000000 0.234375000 0.010562495 0.010562500 0.312500000 0.024999992 0.025000000 0.390625000 0.052562487 0.052562500 0.468750000 0.099999981 0.100000000 0.546875000 0.175562474 0.175562500 0.625000000 0.288999965 0.289000000 0.703125000 0.451562456 0.451562500 0.781250000 0.675999945 0.676000000
ERRE
<lang ERRE> PROGRAM RUNGE_KUTTA
CONST DELTA_T=0.1
FUNCTION Y1(T,Y)
Y1=T*SQR(Y)
END FUNCTION
BEGIN
Y=1.0 FOR I%=0 TO 100 DO T=I%*DELTA_T
IF T=INT(T) THEN ! print every tenth ACTUAL=((T^2+4)^2)/16 ! exact solution PRINT("Y(";T;")=";Y;TAB(20);"Error=";ACTUAL-Y) END IF
K1=Y1(T,Y) K2=Y1(T+DELTA_T/2,Y+DELTA_T/2*K1) K3=Y1(T+DELTA_T/2,Y+DELTA_T/2*K2) K4=Y1(T+DELTA_T,Y+DELTA_T*K3) Y+=DELTA_T*(K1+2*(K2+K3)+K4)/6 END FOR
END PROGRAM</lang>
- Output:
Y( 0 )= 1 Error= 0 Y( 1 )= 1.5625 Error= 2.384186E-07 Y( 2 )= 3.999999 Error= 7.152558E-07 Y( 3 )= 10.5625 Error= 1.907349E-06 Y( 4 )= 25 Error= 3.814697E-06 Y( 5 )= 52.56249 Error= 7.629395E-06 Y( 6 )= 100 Error= 0 Y( 7 )= 175.5625 Error= 0 Y( 8 )= 289 Error= 0 Y( 9 )= 451.5625 Error= 0 Y( 10 )= 676.0001 Error=-6.103516E-05
F#
<lang fsharp> open System
let y'(t,y) = t * sqrt(y)
let RungeKutta4 t0 y0 t_max dt =
let dy1(t,y) = dt * y'(t,y) let dy2(t,y) = dt * y'(t+dt/2.0, y+dy1(t,y)/2.0) let dy3(t,y) = dt * y'(t+dt/2.0, y+dy2(t,y)/2.0) let dy4(t,y) = dt * y'(t+dt, y+dy3(t,y))
(t0,y0) |> Seq.unfold (fun (t,y) -> if ( t <= t_max) then Some((t,y), (Math.Round(t+dt, 6), y + ( dy1(t,y) + 2.0*dy2(t,y) + 2.0*dy3(t,y) + dy4(t,y))/6.0)) else None )
let y_exact t = (pown (pown t 2 + 4.0) 2)/16.0
RungeKutta4 0.0 1.0 10.0 0.1
|> Seq.filter (fun (t,y) -> t % 1.0 = 0.0 ) |> Seq.iter (fun (t,y) -> Console.WriteLine("y({0})={1}\t(relative error:{2})", t, y, (y / y_exact(t))-1.0) )</lang>
- Output:
y(0)=1 (relative error:0) y(1)=1.56249985427811 (relative error:-9.32620110027926E-08) y(2)=3.9999990805208 (relative error:-2.29869800194571E-07) y(3)=10.5624970904376 (relative error:-2.75461533583155E-07) y(4)=24.9999937650906 (relative error:-2.49396374552013E-07) y(5)=52.5624891803026 (relative error:-2.05844421730106E-07) y(6)=99.9999834054036 (relative error:-1.65945964192282E-07) y(7)=175.562476482271 (relative error:-1.33956447156969E-07) y(8)=288.999968434799 (relative error:-1.09222150213029E-07) y(9)=451.56245927684 (relative error:-9.01827772459285E-08) y(10)=675.99994901671 (relative error:-7.54190684348899E-08)
Fortran
<lang fortran>program rungekutta
implicit none integer, parameter :: dp = kind(1d0) real(dp) :: t, dt, tstart, tstop real(dp) :: y, k1, k2, k3, k4 tstart = 0.0d0 tstop = 10.0d0 dt = 0.1d0 y = 1.0d0 t = tstart write (6, '(a,f4.1,a,f12.8,a,es13.6)') 'y(', t, ') = ', y, ' error = ', & abs(y-(t**2+4)**2/16) do while (t < tstop) k1 = dt*f(t, y) k2 = dt*f(t+dt/2, y+k1/2) k3 = dt*f(t+dt/2, y+k2/2) k4 = dt*f(t+dt, y+k3) y = y+(k1+2*(k2+k3)+k4)/6 t = t+dt if (abs(nint(t)-t) <= 1d-12) then write (6, '(a,f4.1,a,f12.8,a,es13.6)') 'y(', t, ') = ', y, ' error = ', & abs(y-(t**2+4)**2/16) end if end do
contains
function f(t,y) real(dp), intent(in) :: t, y real(dp) :: f
f = t*sqrt(y) end function f
end program rungekutta</lang>
- Output:
y( 0.0) = 1.00000000 Error = 0.000000E+00 y( 1.0) = 1.56249985 Error = 1.457219E-07 y( 2.0) = 3.99999908 Error = 9.194792E-07 y( 3.0) = 10.56249709 Error = 2.909562E-06 y( 4.0) = 24.99999377 Error = 6.234909E-06 y( 5.0) = 52.56248918 Error = 1.081970E-05 y( 6.0) = 99.99998341 Error = 1.659460E-05 y( 7.0) = 175.56247648 Error = 2.351773E-05 y( 8.0) = 288.99996843 Error = 3.156520E-05 y( 9.0) = 451.56245928 Error = 4.072316E-05 y(10.0) = 675.99994902 Error = 5.098329E-05
FreeBASIC
<lang freebasic>' version 03-10-2015 ' compile with: fbc -s console ' translation of BBC BASIC
Dim As Double y = 1, t, actual, k1, k2, k3, k4
For i As Integer = 0 To 100
t = i / 10
If t = Int(t) Then actual = ((t ^ 2 + 4) ^ 2) / 16 Print "y("; Str(t); ") ="; y ; Tab(27); "Error = "; actual - y End If
k1 = t * Sqr(y) k2 = (t + 0.05) * Sqr(y + 0.05 * k1) k3 = (t + 0.05) * Sqr(y + 0.05 * k2) k4 = (t + 0.10) * Sqr(y + 0.10 * k3) y += 0.1 * (k1 + 2 * (k2 + k3) + k4) / 6
Next i
' empty keyboard buffer
While Inkey <> "" : Wend
Print : Print "hit any key to end program"
Sleep
End</lang>
- Output:
y(0) = 1 Error = 0 y(1) = 1.562499854278108 Error = 1.457218921085968e-007 y(2) = 3.999999080520799 Error = 9.194792012223729e-007 y(3) = 10.56249709043755 Error = 2.909562448749625e-006 y(4) = 24.99999376509064 Error = 6.234909363911356e-006 y(5) = 52.56248918030259 Error = 1.081969741534294e-005 y(6) = 99.99998340540358 Error = 1.659459641700778e-005 y(7) = 175.5624764822713 Error = 2.351772874931157e-005 y(8) = 288.9999684347985 Error = 3.156520148195341e-005 y(9) = 451.5624592768396 Error = 4.072316039582802e-005 y(10) = 675.9999490167097 Error = 5.098329029351589e-005
FutureBasic
<lang futurebasic> include "ConsoleWindow"
def tab 9
local fn dydx( x as double, y as double ) as double end fn = x * sqr(y)
local fn exactY( x as long ) as double end fn = ( x ^2 + 4 ) ^2 / 16
dim as long i dim as double h, k1, k2, k3, k4, x, y, result
h = 0.1 y = 1 for i = 0 to 100 x = i * h if x == int(x) result = fn exactY( x ) print "y("; mid$( str$(x), 2, len(str$(x) )); ") = "; y, "Error = "; result - y end if
k1 = h * fn dydx( x, y ) k2 = h * fn dydx( x + h / 2, y + k1 / 2 ) k3 = h * fn dydx( x + h / 2, y + k2 / 2 ) k4 = h * fn dydx( x + h, y + k3 )
y = y + 1 / 6 * ( k1 + 2 * k2 + 2 * k3 + k4 ) next </lang> Output:
y(0) = 1 Error = 0 y(1) = 1.5624998543 Error = 1.45721892e-7 y(2) = 3.9999990805 Error = 9.19479201e-7 y(3) = 10.5624970904 Error = 2.90956245e-6 y(4) = 24.9999937651 Error = 6.23490936e-6 y(5) = 52.56248918 Error = 1.08196974e-5 y(6) = 99.999983405 Error = 1.65945964e-5 y(7) = 175.562476482 Error = 2.35177287e-5 y(8) = 288.99996843 Error = 3.15652014e-5 y(9) = 451.56245928 Error = 4.07231603e-5 y(10) = 675.99994902 Error = 5.09832903e-5
Go
<lang go>package main
import (
"fmt" "math"
)
type ypFunc func(t, y float64) float64 type ypStepFunc func(t, y, dt float64) float64
// newRKStep takes a function representing a differential equation // and returns a function that performs a single step of the forth-order // Runge-Kutta method. func newRK4Step(yp ypFunc) ypStepFunc {
return func(t, y, dt float64) float64 { dy1 := dt * yp(t, y) dy2 := dt * yp(t+dt/2, y+dy1/2) dy3 := dt * yp(t+dt/2, y+dy2/2) dy4 := dt * yp(t+dt, y+dy3) return y + (dy1+2*(dy2+dy3)+dy4)/6 }
}
// example differential equation func yprime(t, y float64) float64 {
return t * math.Sqrt(y)
}
// exact solution of example func actual(t float64) float64 {
t = t*t + 4 return t * t / 16
}
func main() {
t0, tFinal := 0, 10 // task specifies times as integers, dtPrint := 1 // and to print at whole numbers. y0 := 1. // initial y. dtStep := .1 // step value.
t, y := float64(t0), y0 ypStep := newRK4Step(yprime) for t1 := t0 + dtPrint; t1 <= tFinal; t1 += dtPrint { printErr(t, y) // print intermediate result for steps := int(float64(dtPrint)/dtStep + .5); steps > 1; steps-- { y = ypStep(t, y, dtStep) t += dtStep } y = ypStep(t, y, float64(t1)-t) // adjust step to integer time t = float64(t1) } printErr(t, y) // print final result
}
func printErr(t, y float64) {
fmt.Printf("y(%.1f) = %f Error: %e\n", t, y, math.Abs(actual(t)-y))
}</lang>
- Output:
y(0.0) = 1.000000 Error: 0.000000e+00 y(1.0) = 1.562500 Error: 1.457219e-07 y(2.0) = 3.999999 Error: 9.194792e-07 y(3.0) = 10.562497 Error: 2.909562e-06 y(4.0) = 24.999994 Error: 6.234909e-06 y(5.0) = 52.562489 Error: 1.081970e-05 y(6.0) = 99.999983 Error: 1.659460e-05 y(7.0) = 175.562476 Error: 2.351773e-05 y(8.0) = 288.999968 Error: 3.156520e-05 y(9.0) = 451.562459 Error: 4.072316e-05 y(10.0) = 675.999949 Error: 5.098329e-05
Groovy
<lang Groovy> class Runge_Kutta{ static void main(String[] args){ def y=1.0,t=0.0,counter=0; def dy1,dy2,dy3,dy4; def real; while(t<=10) {if(counter%10==0) {real=(t*t+4)*(t*t+4)/16; println("y("+t+")="+ y+ " Error:"+ (real-y)); }
dy1=dy(dery(y,t)); dy2=dy(dery(y+dy1/2,t+0.05)); dy3=dy(dery(y+dy2/2,t+0.05)); dy4=dy(dery(y+dy3,t+0.1));
y=y+(dy1+2*dy2+2*dy3+dy4)/6; t=t+0.1; counter++; } } static def dery(def y,def t){return t*(Math.sqrt(y));} static def dy(def x){return x*0.1;} } </lang>
- Output:
y(0.0)=1.0 Error:0.0000 y(1.0)=1.562499854278108 Error:1.4572189210859676E-7 y(2.0)=3.999999080520799 Error:9.194792007782837E-7 y(3.0)=10.562497090437551 Error:2.9095624487496252E-6 y(4.0)=24.999993765090636 Error:6.234909363911356E-6 y(5.0)=52.562489180302585 Error:1.0819697415342944E-5 y(6.0)=99.99998340540358 Error:1.659459641700778E-5 y(7.0)=175.56247648227125 Error:2.3517728749311573E-5 y(8.0)=288.9999684347986 Error:3.156520142510999E-5 y(9.0)=451.56245927683966 Error:4.07231603389846E-5 y(10.0)=675.9999490167097 Error:5.098329029351589E-5
Haskell
Using GHC 7.4.1.
<lang haskell>dv
:: Floating a => a -> a -> a
dv = (. sqrt) . (*)
fy t = 1 / 16 * (4 + t ^ 2) ^ 2
rk4
:: (Enum a, Fractional a) => (a -> a -> a) -> a -> a -> a -> [(a, a)]
rk4 fd y0 a h = zip ts $ scanl (flip fc) y0 ts
where ts = [a,h ..] fc t y = sum . (y :) . zipWith (*) [1 / 6, 1 / 3, 1 / 3, 1 / 6] $ scanl (\k f -> h * fd (t + f * h) (y + f * k)) (h * fd t y) [1 / 2, 1 / 2, 1]
task =
mapM_ (print . (\(x, y) -> (truncate x, y, fy x - y))) (filter (\(x, _) -> 0 == mod (truncate $ 10 * x) 10) $ take 101 $ rk4 dv 1.0 0 0.1)</lang>
Example executed in GHCi: <lang haskell>*Main> task (0,1.0,0.0) (1,1.5624998542781088,1.4572189122041834e-7) (2,3.9999990805208006,9.194792029987298e-7) (3,10.562497090437557,2.909562461184123e-6) (4,24.999993765090654,6.234909399438493e-6) (5,52.56248918030265,1.0819697635611192e-5) (6,99.99998340540378,1.6594596999652822e-5) (7,175.56247648227165,2.3517730085131916e-5) (8,288.99996843479926,3.1565204153594095e-5) (9,451.562459276841,4.0723166534917254e-5) (10,675.9999490167125,5.098330132113915e-5)</lang>
(See Euler method#Haskell for implementation of simple general ODE-solver)
Or, disaggregated a little, and expressed in terms of a single scanl:
<lang haskell>rk4 :: Double -> Double -> Double -> Double
rk4 y x dx =
let f x y = x * sqrt y k1 = dx * f x y k2 = dx * f (x + dx / 2.0) (y + k1 / 2.0) k3 = dx * f (x + dx / 2.0) (y + k2 / 2.0) k4 = dx * f (x + dx) (y + k3) in y + (k1 + 2.0 * k2 + 2.0 * k3 + k4) / 6.0
actual :: Double -> Double actual x = (1 / 16) * (x * x + 4) * (x * x + 4)
step :: Double step = 0.1
ixs :: [Int] ixs = [0 .. 100]
xys :: [(Double, Double)] xys =
scanl (\(x, y) _ -> (((x * 10) + (step * 10)) / 10, rk4 y x step)) (0.0, 1.0) ixs
samples :: [(Double, Double, Double)] samples =
zip ixs xys >>= (\(i, (x, y)) -> [ (x, y, actual x - y) | 0 == mod i 10 ])
main :: IO () main =
(putStrLn . unlines) $ (\(x, y, v) -> unwords [ "y" ++ justifyRight 3 ' ' ('(' : show (round x)) ++ ") = " , justifyLeft 19 ' ' (show y) , '±' : show v ]) <$> samples where justifyLeft n c s = take n (s ++ replicate n c) justifyRight n c s = drop (length s) (replicate n c ++ s)</lang>
- Output:
y (0) = 1.0 ±0.0 y (1) = 1.562499854278108 ±1.4572189210859676e-7 y (2) = 3.999999080520799 ±9.194792007782837e-7 y (3) = 10.562497090437551 ±2.9095624487496252e-6 y (4) = 24.999993765090636 ±6.234909363911356e-6 y (5) = 52.562489180302585 ±1.0819697415342944e-5 y (6) = 99.99998340540358 ±1.659459641700778e-5 y (7) = 175.56247648227125 ±2.3517728749311573e-5 y (8) = 288.9999684347986 ±3.156520142510999e-5 y (9) = 451.56245927683966 ±4.07231603389846e-5 y(10) = 675.9999490167097 ±5.098329029351589e-5
J
Solution: <lang j>NB.*rk4 a Solve function using Runge-Kutta method NB. y is: y(ta) , ta , tb , tstep NB. u is: function to solve NB. eg: fyp rk4 1 0 10 0.1 rk4=: adverb define
'Y0 a b h'=. 4{. y T=. a + i.@>:&.(%&h) b - a Y=. Yt=. Y0 for_t. }: T do. ty=. t,Yt k1=. h * u ty k2=. h * u ty + -: h,k1 k3=. h * u ty + -: h,k2 k4=. h * u ty + h,k3 Y=. Y, Yt=. Yt + (%6) * 1 2 2 1 +/@:* k1, k2, k3, k4 end.
T ,. Y )</lang> Example: <lang j> fy=: (%16) * [: *: 4 + *: NB. f(t,y)
fyp=: (* %:)/ NB. f'(t,y) report_whole=: (10 * i. >:10)&{ NB. report at whole-numbered t values report_err=: (, {: - [: fy {.)"1 NB. report errors
report_err report_whole fyp rk4 1 0 10 0.1 0 1 0 1 1.5625 _1.45722e_7 2 4 _9.19479e_7 3 10.5625 _2.90956e_6 4 25 _6.23491e_6 5 52.5625 _1.08197e_5 6 100 _1.65946e_5 7 175.562 _2.35177e_5 8 289 _3.15652e_5 9 451.562 _4.07232e_5
10 676 _5.09833e_5</lang>
Alternative solution:
The following solution replaces the for loop as well as the calculation of the increments (ks) with an accumulating suffix. <lang j>rk4=: adverb define
'Y0 a b h'=. 4{. y T=. a + i.@>:&.(%&h) b-a (,. [: h&(u nextY)@,/\. Y0 ,~ }.)&.|. T
)
NB. nextY a Calculate Yn+1 of a function using Runge-Kutta method NB. y is: 2-item numeric list of time t and y(t) NB. u is: function to use NB. x is: step size NB. eg: 0.001 fyp nextY 0 1 nextY=: adverb define
tableau=. 1 0.5 0.5, x * u y ks=. (x * [: u y + (* x&,))/\. tableau ({:y) + 6 %~ +/ 1 2 2 1 * ks
)</lang>
Use:
report_err report_whole fyp rk4 1 0 10 0.1
Java
<lang java>import static java.lang.Math.*; import java.util.function.BiFunction;
public class RungeKutta {
static void runge(BiFunction<Double, Double, Double> yp_func, double[] t, double[] y, double dt) {
for (int n = 0; n < t.length - 1; n++) { double dy1 = dt * yp_func.apply(t[n], y[n]); double dy2 = dt * yp_func.apply(t[n] + dt / 2.0, y[n] + dy1 / 2.0); double dy3 = dt * yp_func.apply(t[n] + dt / 2.0, y[n] + dy2 / 2.0); double dy4 = dt * yp_func.apply(t[n] + dt, y[n] + dy3); t[n + 1] = t[n] + dt; y[n + 1] = y[n] + (dy1 + 2.0 * (dy2 + dy3) + dy4) / 6.0; } }
static double calc_err(double t, double calc) { double actual = pow(pow(t, 2.0) + 4.0, 2) / 16.0; return abs(actual - calc); }
public static void main(String[] args) { double dt = 0.10; double[] t_arr = new double[101]; double[] y_arr = new double[101]; y_arr[0] = 1.0;
runge((t, y) -> t * sqrt(y), t_arr, y_arr, dt);
for (int i = 0; i < t_arr.length; i++) if (i % 10 == 0) System.out.printf("y(%.1f) = %.8f Error: %.6f%n", t_arr[i], y_arr[i], calc_err(t_arr[i], y_arr[i])); }
}</lang>
y(0,0) = 1,00000000 Error: 0,000000 y(1,0) = 1,56249985 Error: 0,000000 y(2,0) = 3,99999908 Error: 0,000001 y(3,0) = 10,56249709 Error: 0,000003 y(4,0) = 24,99999377 Error: 0,000006 y(5,0) = 52,56248918 Error: 0,000011 y(6,0) = 99,99998341 Error: 0,000017 y(7,0) = 175,56247648 Error: 0,000024 y(8,0) = 288,99996843 Error: 0,000032 y(9,0) = 451,56245928 Error: 0,000041 y(10,0) = 675,99994902 Error: 0,000051
JavaScript
ES5
<lang JavaScript> function rk4(y, x, dx, f) {
var k1 = dx * f(x, y), k2 = dx * f(x + dx / 2.0, +y + k1 / 2.0), k3 = dx * f(x + dx / 2.0, +y + k2 / 2.0), k4 = dx * f(x + dx, +y + k3);
return y + (k1 + 2.0 * k2 + 2.0 * k3 + k4) / 6.0;
}
function f(x, y) {
return x * Math.sqrt(y);
}
function actual(x) {
return (1/16) * (x*x+4)*(x*x+4);
}
var y = 1.0,
x = 0.0, step = 0.1, steps = 0, maxSteps = 101, sampleEveryN = 10;
while (steps < maxSteps) {
if (steps%sampleEveryN === 0) { console.log("y(" + x + ") = \t" + y + "\t ± " + (actual(x) - y).toExponential()); }
y = rk4(y, x, step, f);
// using integer math for the step addition // to prevent floating point errors as 0.2 + 0.1 != 0.3 x = ((x * 10) + (step * 10)) / 10; steps += 1;
} </lang>
- Output:
y(0) = 1 ± 0e+0 y(1) = 1.562499854278108 ± 1.4572189210859676e-7 y(2) = 3.999999080520799 ± 9.194792007782837e-7 y(3) = 10.562497090437551 ± 2.9095624487496252e-6 y(4) = 24.999993765090636 ± 6.234909363911356e-6 y(5) = 52.562489180302585 ± 1.0819697415342944e-5 y(6) = 99.99998340540358 ± 1.659459641700778e-5 y(7) = 175.56247648227125 ± 2.3517728749311573e-5 y(8) = 288.9999684347986 ± 3.156520142510999e-5 y(9) = 451.56245927683966 ± 4.07231603389846e-5 y(10) = 675.9999490167097 ± 5.098329029351589e-5
ES6
<lang javascript>(() => {
'use strict';
// rk4 :: (Double -> Double -> Double) -> // Double -> Double -> Double -> Double const rk4 = f => (y, x, dx) => { const k1 = dx * f(x, y), k2 = dx * f(x + dx / 2.0, y + k1 / 2.0), k3 = dx * f(x + dx / 2.0, y + k2 / 2.0), k4 = dx * f(x + dx, y + k3); return y + (k1 + 2.0 * k2 + 2.0 * k3 + k4) / 6.0; };
// rk :: Double -> Double -> Double -> Double const rk = rk4((x, y) => x * Math.sqrt(y));
// actual :: Double -> Double const actual = x => (1 / 16) * ((x * x) + 4) * ((x * x) + 4);
// TEST -------------------------------------------------
// main :: IO () const main = () => { const step = 0.1, ixs = enumFromTo(0, 100), xys = scanl( xy => Tuple( ((xy[0] * 10) + (step * 10)) / 10, rk(xy[1], xy[0], step) ), Tuple(0.0, 1.0), ixs );
// samples :: [(Double, Double, Double)] const samples = concatMap( tpl => 0 === tpl[0] % 10 ? (() => { const [x, y] = Array.from(tpl[1]); return [TupleN(x, y, actual(x) - y)]; })() : [], zip(ixs, xys) );
console.log( unlines(map( tpl => { const [x, y, v] = Array.from(tpl), [sn, sm] = splitOn('.', y.toString()); return unwords([ 'y' + justifyRight(3, ' ', '(' + Math.round(x).toString()) + ') =', justifyRight(3, ' ', sn) + '.' + justifyLeft(15, ' ', sm || '0'), '± ' + v.toExponential() ]); }, samples )) ); };
// GENERIC FUNCTIONS ----------------------------
// Tuple (,) :: a -> b -> (a, b) const Tuple = (a, b) => ({ type: 'Tuple', '0': a, '1': b, length: 2 });
// TupleN :: a -> b ... -> (a, b ... ) function TupleN() { const args = Array.from(arguments), lng = args.length; return lng > 1 ? Object.assign( args.reduce((a, x, i) => Object.assign(a, { [i]: x }), { type: 'Tuple' + (2 < lng ? lng.toString() : ), length: lng }) ) : args[0]; };
// concatMap :: (a -> [b]) -> [a] -> [b] const concatMap = (f, xs) => xs.reduce((a, x) => a.concat(f(x)), []);
// enumFromTo :: Int -> Int -> [Int] const enumFromTo = (m, n) => Array.from({ length: 1 + n - m }, (_, i) => m + i)
// justifyLeft :: Int -> Char -> String -> String const justifyLeft = (n, cFiller, s) => n > s.length ? ( s.padEnd(n, cFiller) ) : s;
// justifyRight :: Int -> Char -> String -> String const justifyRight = (n, cFiller, s) => n > s.length ? ( s.padStart(n, cFiller) ) : s;
// Returns Infinity over objects without finite length // this enables zip and zipWith to choose the shorter // argument when one is non-finite, like cycle, repeat etc
// length :: [a] -> Int const length = xs => xs.length || Infinity;
// map :: (a -> b) -> [a] -> [b] const map = (f, xs) => xs.map(f);
// scanl :: (b -> a -> b) -> b -> [a] -> [b] const scanl = (f, startValue, xs) => xs.reduce((a, x) => { const v = f(a[0], x); return Tuple(v, a[1].concat(v)); }, Tuple(startValue, [startValue]))[1];
// splitOn :: String -> String -> [String] const splitOn = (pat, src) => src.split(pat);
// take :: Int -> [a] -> [a] // take :: Int -> String -> String const take = (n, xs) => xs.constructor.constructor.name !== 'GeneratorFunction' ? ( xs.slice(0, n) ) : [].concat.apply([], Array.from({ length: n }, () => { const x = xs.next(); return x.done ? [] : [x.value]; }));
// unlines :: [String] -> String const unlines = xs => xs.join('\n');
// unwords :: [String] -> String const unwords = xs => xs.join(' ');
// Use of `take` and `length` here allows for zipping with non-finite // lists - i.e. generators like cycle, repeat, iterate.
// zip :: [a] -> [b] -> [(a, b)] const zip = (xs, ys) => { const lng = Math.min(length(xs), length(ys)); return Infinity !== lng ? (() => { const bs = take(lng, ys); return take(lng, xs).map((x, i) => Tuple(x, bs[i])); })() : zipGen(xs, ys); };
// MAIN --- return main();
})();</lang>
- Output:
y (0) = 1.0 ± 0e+0 y (1) = 1.562499854278108 ± 1.4572189210859676e-7 y (2) = 3.999999080520799 ± 9.194792007782837e-7 y (3) = 10.562497090437551 ± 2.9095624487496252e-6 y (4) = 24.999993765090636 ± 6.234909363911356e-6 y (5) = 52.562489180302585 ± 1.0819697415342944e-5 y (6) = 99.99998340540358 ± 1.659459641700778e-5 y (7) = 175.56247648227125 ± 2.3517728749311573e-5 y (8) = 288.9999684347986 ± 3.156520142510999e-5 y (9) = 451.56245927683966 ± 4.07231603389846e-5 y(10) = 675.9999490167097 ± 5.098329029351589e-5
jq
In this section, two solutions are presented. They use "while" and/or "until" as defined in recent versions of jq (after version 1.4). To use either of the two programs with jq 1.4, simply include the lines in the following block: <lang jq>def until(cond; next):
def _until: if cond then . else (next|_until) end; _until;
def while(cond; update):
def _while: if cond then ., (update | _while) else empty end; _while;</lang>
The Example Differential Equation and its Exact Solution
<lang jq># yprime maps [t,y] to a number, i.e. t * sqrt(y) def yprime: .[0] * (.[1] | sqrt);
- The exact solution of yprime:
def actual:
. as $t | (( $t*$t) + 4 ) | . * . / 16;</lang>
dy/dt
The first solution presented here uses the terminology and style of the Raku version.
Generic filters: <lang jq># n is the number of decimal places of precision def round(n):
(if . < 0 then -1 else 1 end) as $s | $s*10*.*n | if (floor % 10) > 4 then (.+5) else . end | ./10 | floor/n | .*$s;
def abs: if . < 0 then -. else . end;
- Is the input an integer?
def integerq: ((. - ((.+.01) | floor)) | abs) < 0.01;</lang>
dy(f) <lang jq>def dt: 0.1;
- Input: [t, y]; yp is a filter that accepts [t,y] as input
def runge_kutta(yp):
.[0] as $t | .[1] as $y | (dt * yp) as $a | (dt * ([ ($t + (dt/2)), $y + ($a/2) ] | yp)) as $b | (dt * ([ ($t + (dt/2)), $y + ($b/2) ] | yp)) as $c | (dt * ([ ($t + dt) , $y + $c ] | yp)) as $d | ($a + (2*($b + $c)) + $d) / 6
- Input: [t,y]
def dy(f): runge_kutta(f);</lang> Example: <lang jq># state: [t,y] [0,1] | while( .[0] <= 10;
.[0] as $t | .[1] as $y | [$t + dt, $y + dy(yprime) ] )
| .[0] as $t | .[1] as $y | if $t | integerq then
"y(\($t|round(1))) = \($y|round(10000)) ± \( ($t|actual) - $y | abs)" else empty end</lang>
- Output:
<lang sh>$ time jq -r -n -f rk4.pl.jq y(0) = 1 ± 0 y(1) = 1.5625 ± 1.4572189210859676e-07 y(2) = 4 ± 9.194792029987298e-07 y(3) = 10.5625 ± 2.9095624576314094e-06 y(4) = 25 ± 6.234909392333066e-06 y(5) = 52.5625 ± 1.081969734428867e-05 y(6) = 100 ± 1.659459609015812e-05 y(7) = 175.5625 ± 2.3517728038768837e-05 y(8) = 289 ± 3.156520000402452e-05 y(9) = 451.5625 ± 4.072315812209126e-05 y(10) = 675.9999 ± 5.0983286655537086e-05
real 0m0.048s user 0m0.013s sys 0m0.006s</lang>
newRK4Step
The second solution follows the nomenclature and style of the Go solution on this page.
In the following notes:
- ypFunc denotes the type of a jq filter that maps [t, y] to a number;
- ypStepFunc denotes the type of a jq filter that maps [t, y, dt] to a number.
The heart of the program is the filter newRK4Step(yp), which is of type ypStepFunc and performs a single step of the fourth-order Runge-Kutta method, provided yp is of type ypFunc. <lang jq># Input: [t, y, dt] def newRK4Step(yp):
.[0] as $t | .[1] as $y | .[2] as $dt | ($dt * ([$t, $y]|yp)) as $dy1 | ($dt * ([$t+$dt/2, $y+$dy1/2]|yp)) as $dy2 | ($dt * ([$t+$dt/2, $y+$dy2/2]|yp)) as $dy3 | ($dt * ([$t+$dt, $y+$dy3] |yp)) as $dy4 | $y + ($dy1+2*($dy2+$dy3)+$dy4)/6
def printErr: # input: [t, y]
def abs: if . < 0 then -. else . end; .[0] as $t | .[1] as $y | "y(\($t)) = \($y) with error: \( (($t|actual) - $y) | abs )"
def main(t0; y0; tFinal; dtPrint):
def ypStep: newRK4Step(yprime) ;
0.1 as $dtStep # step value # [ t, y] is the state vector | [ t0, y0 ] | while( .[0] <= tFinal; .[0] as $t | .[1] as $y
| ($t + dtPrint) as $t1 | (((dtPrint/$dtStep) + 0.5) | floor) as $steps | [$steps, $t, $y] # state vector
| until( .[0] <= 1;
.[0] as $steps | .[1] as $t | .[2] as $y | [ ($steps - 1), ($t + $dtStep), ([$t, $y, $dtStep]|ypStep) ]
)
| .[1] as $t | .[2] as $y | [$t1, ([ $t, $y, ($t1-$t)] | ypStep)] # adjust step to integer time
) | printErr # print results
- main(t0; y0; tFinal; dtPrint)
main(0; 1; 10; 1)</lang>
- Output:
<lang sh>$ time jq -n -r -f runge-kutta.jq y(0) = 1 with error: 0 y(1) = 1.562499854278108 with error: 1.4572189210859676e-07 y(2) = 3.9999990805207974 with error: 9.194792025546406e-07 y(3) = 10.562497090437544 with error: 2.9095624558550526e-06 y(4) = 24.999993765090615 with error: 6.234909385227638e-06 y(5) = 52.562489180302656 with error: 1.081969734428867e-05 y(6) = 99.99998340540387 with error: 1.6594596132790684e-05 y(7) = 175.56247648227188 with error: 2.3517728124033965e-05 y(8) = 288.9999684347997 with error: 3.156520028824161e-05 y(9) = 451.56245927684154 with error: 4.0723158463151776e-05 y(10) = 675.9999490167129 with error: 5.0983287110284436e-05
real 0m0.023s user 0m0.014s sys 0m0.006s</lang>
Julia
Using lambda expressions
<lang julia>f(x, y) = x * sqrt(y) theoric(t) = (t ^ 2 + 4.0) ^ 2 / 16.0
rk4(f) = (t, y, δt) -> # 1st (result) lambda
((δy1) -> # 2nd lambda ((δy2) -> # 3rd lambda ((δy3) -> # 4th lambda ((δy4) -> ( δy1 + 2δy2 + 2δy3 + δy4 ) / 6 # 5th and deepest lambda: calc y_{n+1} )(δt * f(t + δt, y + δy3)) # calc δy₄ )(δt * f(t + δt / 2, y + δy2 / 2)) # calc δy₃ )(δt * f(t + δt / 2, y + δy1 / 2)) # calc δy₂ )(δt * f(t, y)) # calc δy₁
δy = rk4(f) t₀, δt, tmax = 0.0, 0.1, 10.0 y₀ = 1.0
t, y = t₀, y₀ while t ≤ tmax
if t ≈ round(t) @printf("y(%4.1f) = %10.6f\terror: %12.6e\n", t, y, abs(y - theoric(t))) end y += δy(t, y, δt) t += δt
end</lang>
- Output:
y( 0.0) = 1.000000 error: 0.000000e+00 y( 1.0) = 1.562500 error: 1.457219e-07 y( 2.0) = 3.999999 error: 9.194792e-07 y( 3.0) = 10.562497 error: 2.909562e-06 y( 4.0) = 24.999994 error: 6.234909e-06 y( 5.0) = 52.562489 error: 1.081970e-05 y( 6.0) = 99.999983 error: 1.659460e-05 y( 7.0) = 175.562476 error: 2.351773e-05 y( 8.0) = 288.999968 error: 3.156520e-05 y( 9.0) = 451.562459 error: 4.072316e-05 y(10.0) = 675.999949 error: 5.098329e-05
Alternative version
<lang julia>function rk4(f::Function, x₀::Float64, y₀::Float64, x₁::Float64, n)
vx = Vector{Float64}(undef, n + 1) vy = Vector{Float64}(undef, n + 1) vx[1] = x = x₀ vy[1] = y = y₀ h = (x₁ - x₀) / n for i in 1:n k₁ = h * f(x, y) k₂ = h * f(x + 0.5h, y + 0.5k₁) k₃ = h * f(x + 0.5h, y + 0.5k₂) k₄ = h * f(x + h, y + k₃) vx[i + 1] = x = x₀ + i * h vy[i + 1] = y = y + (k₁ + 2k₂ + 2k₃ + k₄) / 6 end return vx, vy
end
vx, vy = rk4(f, 0.0, 1.0, 10.0, 100) for (x, y) in Iterators.take(zip(vx, vy), 10)
@printf("%4.1f %10.5f %+12.4e\n", x, y, y - theoric(x))
end</lang>
Kotlin
<lang scala>// version 1.1.2
typealias Y = (Double) -> Double typealias Yd = (Double, Double) -> Double
fun rungeKutta4(t0: Double, tz: Double, dt: Double, y: Y, yd: Yd) {
var tn = t0 var yn = y(tn) val z = ((tz - t0) / dt).toInt() for (i in 0..z) { if (i % 10 == 0) { val exact = y(tn) val error = yn - exact println("%4.1f %10f %10f %9f".format(tn, yn, exact, error)) } if (i == z) break val dy1 = dt * yd(tn, yn) val dy2 = dt * yd(tn + 0.5 * dt, yn + 0.5 * dy1) val dy3 = dt * yd(tn + 0.5 * dt, yn + 0.5 * dy2) val dy4 = dt * yd(tn + dt, yn + dy3) yn += (dy1 + 2.0 * dy2 + 2.0 * dy3 + dy4) / 6.0 tn += dt }
}
fun main(args: Array<String>) {
println(" T RK4 Exact Error") println("---- ---------- ---------- ---------") val y = fun(t: Double): Double { val x = t * t + 4.0 return x * x / 16.0 } val yd = fun(t: Double, yt: Double) = t * Math.sqrt(yt) rungeKutta4(0.0, 10.0, 0.1, y, yd)
}</lang>
- Output:
T RK4 Exact Error ---- ---------- ---------- --------- 0.0 1.000000 1.000000 0.000000 1.0 1.562500 1.562500 -0.000000 2.0 3.999999 4.000000 -0.000001 3.0 10.562497 10.562500 -0.000003 4.0 24.999994 25.000000 -0.000006 5.0 52.562489 52.562500 -0.000011 6.0 99.999983 100.000000 -0.000017 7.0 175.562476 175.562500 -0.000024 8.0 288.999968 289.000000 -0.000032 9.0 451.562459 451.562500 -0.000041 10.0 675.999949 676.000000 -0.000051
Liberty BASIC
<lang lb> '[RC] Runge-Kutta method 'initial conditions x0 = 0 y0 = 1 'step h = 0.1 'number of points N=101
y=y0 FOR i = 0 TO N-1
x = x0+ i*h IF x = INT(x) THEN actual = exactY(x) PRINT "y("; x ;") = "; y; TAB(20); "Error = "; actual - y END IF
k1 = h*dydx(x,y) k2 = h*dydx(x+h/2,y+k1/2) k3 = h*dydx(x+h/2,y+k2/2) k4 = h*dydx(x+h,y+k3) y = y + 1/6 * (k1 + 2*k2 + 2*k3 + k4)
NEXT i
function dydx(x,y)
dydx=x*sqr(y)
end function
function exactY(x)
exactY=(x^2 + 4)^2 / 16
end function </lang>
- Output:
y(0) = 1 Error = 0 y(1) = 1.56249985 Error = 0.14572189e-6 y(2) = 3.99999908 Error = 0.9194792e-6 y(3) = 10.5624971 Error = 0.29095624e-5 y(4) = 24.9999938 Error = 0.62349094e-5 y(5) = 52.5624892 Error = 0.10819697e-4 y(6) = 99.9999834 Error = 0.16594596e-4 y(7) = 175.562476 Error = 0.23517729e-4 y(8) = 288.999968 Error = 0.31565201e-4 y(9) = 451.562459 Error = 0.4072316e-4 y(10) = 675.999949 Error = 0.5098329e-4
Mathematica
<lang Mathematica>(* Symbolic solution *) DSolve[{y'[t] == t*Sqrt[y[t]], y[0] == 1}, y, t] Table[{t, 1/16 (4 + t^2)^2}, {t, 0, 10}]
(* Numerical solution I (not RK4) *) Table[{t, y[t], Abs[y[t] - 1/16*(4 + t^2)^2]}, {t, 0, 10}] /.
First@NDSolve[{y'[t] == t*Sqrt[y[t]], y[0] == 1}, y, {t, 0, 10}]
(* Numerical solution II (RK4) *) f[{t_, y_}] := {1, t Sqrt[y]} h = 0.1; phi[y_] := Module[{k1, k2, k3, k4},
k1 = h*f[y]; k2 = h*f[y + 1/2 k1]; k3 = h*f[y + 1/2 k2]; k4 = h*f[y + k3]; y + k1/6 + k2/3 + k3/3 + k4/6]
solution = NestList[phi, {0, 1}, 101]; Table[{y1, y2, Abs[y2 - 1/16 (y1^2 + 4)^2]},
{y, solution1 ;; 101 ;; 10}]
</lang>
MATLAB
The normally-used built-in solver is the ode45 function, which uses a non-fixed-step solver with 4th/5th order Runge-Kutta methods. The MathWorks Support Team released a package of fixed-step RK method ODE solvers on MATLABCentral. The ode4 function contained within uses a 4th-order Runge-Kutta method. Here is code that tests both ode4 and my own function, shows that they are the same, and compares them to the exact solution. <lang MATLAB>function testRK4Programs
figure hold on t = 0:0.1:10; y = 0.0625.*(t.^2+4).^2; plot(t, y, '-k') [tode4, yode4] = testODE4(t); plot(tode4, yode4, '--b') [trk4, yrk4] = testRK4(t); plot(trk4, yrk4, ':r') legend('Exact', 'ODE4', 'RK4') hold off fprintf('Time\tExactVal\tODE4Val\tODE4Error\tRK4Val\tRK4Error\n') for k = 1:10:length(t) fprintf('%.f\t\t%7.3f\t\t%7.3f\t%7.3g\t%7.3f\t%7.3g\n', t(k), y(k), ... yode4(k), abs(y(k)-yode4(k)), yrk4(k), abs(y(k)-yrk4(k))) end
end
function [t, y] = testODE4(t)
y0 = 1; y = ode4(@(tVal,yVal)tVal*sqrt(yVal), t, y0);
end
function [t, y] = testRK4(t)
dydt = @(tVal,yVal)tVal*sqrt(yVal); y = zeros(size(t)); y(1) = 1; for k = 1:length(t)-1 dt = t(k+1)-t(k); dy1 = dt*dydt(t(k), y(k)); dy2 = dt*dydt(t(k)+0.5*dt, y(k)+0.5*dy1); dy3 = dt*dydt(t(k)+0.5*dt, y(k)+0.5*dy2); dy4 = dt*dydt(t(k)+dt, y(k)+dy3); y(k+1) = y(k)+(dy1+2*dy2+2*dy3+dy4)/6; end
end</lang>
- Output:
Time ExactVal ODE4Val ODE4Error RK4Val RK4Error 0 1.000 1.000 0 1.000 0 1 1.563 1.562 1.46e-007 1.562 1.46e-007 2 4.000 4.000 9.19e-007 4.000 9.19e-007 3 10.563 10.562 2.91e-006 10.562 2.91e-006 4 25.000 25.000 6.23e-006 25.000 6.23e-006 5 52.563 52.562 1.08e-005 52.562 1.08e-005 6 100.000 100.000 1.66e-005 100.000 1.66e-005 7 175.563 175.562 2.35e-005 175.562 2.35e-005 8 289.000 289.000 3.16e-005 289.000 3.16e-005 9 451.563 451.562 4.07e-005 451.562 4.07e-005 10 676.000 676.000 5.10e-005 676.000 5.10e-005
Maxima
<lang maxima>/* Here is how to solve a differential equation */ 'diff(y, x) = x * sqrt(y); ode2(%, y, x); ic1(%, x = 0, y = 1); factor(solve(%, y)); /* [y = (x^2 + 4)^2 / 16] */
/* The Runge-Kutta solver is builtin */
load(dynamics)$ sol: rk(t * sqrt(y), y, 1, [t, 0, 10, 1.0])$ plot2d([discrete, sol])$
/* An implementation of RK4 for one equation */
rk4(f, x0, y0, x1, n) := block([h, x, y, vx, vy, k1, k2, k3, k4],
h: bfloat((x1 - x0) / (n - 1)), x: x0, y: y0, vx: makelist(0, n + 1), vy: makelist(0, n + 1), vx[1]: x0, vy[1]: y0, for i from 1 thru n do ( k1: bfloat(h * f(x, y)), k2: bfloat(h * f(x + h / 2, y + k1 / 2)), k3: bfloat(h * f(x + h / 2, y + k2 / 2)), k4: bfloat(h * f(x + h, y + k3)), vy[i + 1]: y: y + (k1 + 2 * k2 + 2 * k3 + k4) / 6, vx[i + 1]: x: x + h ), [vx, vy]
)$
[x, y]: rk4(lambda([x, y], x * sqrt(y)), 0, 1, 10, 101)$
plot2d([discrete, x, y])$
s: map(lambda([x], (x^2 + 4)^2 / 16), x)$
for i from 1 step 10 thru 101 do print(x[i], " ", y[i], " ", y[i] - s[i]);</lang>
МК-61/52
ПП 38 П1 ПП 30 П2 ПП 35 П3 2 * ПП 30 ИП2 ИП3 + 2 * + ИП1 + 3 / ИП7 + П7 П8 С/П БП 00 ИП6 ИП5 + П6 <-> ИП7 + П8 ИП8 КвКор ИП6 * ИП5 * В/О
Input: 1/2 (h/2) - Р5, 1 (y0) - Р8 and Р7, 0 (t0) - Р6.
Nim
<lang nim>import math
proc fn(t, y: float): float =
result = t * math.sqrt(y)
proc solution(t: float): float =
result = (t^2 + 4)^2 / 16
proc rk(start, stop, step: float) =
let nsteps = int(round((stop - start) / step)) + 1 let delta = (stop - start) / float(nsteps - 1) var cur_y = 1.0 for i in 0..(nsteps - 1): let cur_t = start + delta * float(i)
if abs(cur_t - math.round(cur_t)) < 1e-5: echo "y(", cur_t, ") = ", cur_y, ", error = ", solution(cur_t) - cur_y let dy1 = step * fn(cur_t, cur_y) let dy2 = step * fn(cur_t + 0.5 * step, cur_y + 0.5 * dy1) let dy3 = step * fn(cur_t + 0.5 * step, cur_y + 0.5 * dy2) let dy4 = step * fn(cur_t + step, cur_y + dy3)
cur_y += (dy1 + 2.0 * (dy2 + dy3) + dy4) / 6.0
rk(start=0.0, stop=10.0, step=0.1)</lang>
- Output:
y(0.0) = 1.0, error = 0.0 y(1.0) = 1.562499854278108, error = 1.457218921085968e-007 y(2.0) = 3.9999990805208, error = 9.194792003341945e-007 y(3.0) = 10.56249709043755, error = 2.909562448749625e-006 y(4.0) = 24.99999376509064, error = 6.234909363911356e-006 y(5.0) = 52.56248918030259, error = 1.081969741534294e-005 y(6.0) = 99.99998340540358, error = 1.659459641700778e-005 y(7.0) = 175.5624764822713, error = 2.351772874931157e-005 y(8.0) = 288.9999684347986, error = 3.156520142510999e-005 y(9.0) = 451.5624592768397, error = 4.07231603389846e-005 y(10.0) = 675.9999490167097, error = 5.098329029351589e-005
Objeck
<lang objeck>class RungeKuttaMethod {
function : Main(args : String[]) ~ Nil { x0 := 0.0; x1 := 10.0; dx := .1; n := 1 + (x1 - x0)/dx; y := Float->New[n->As(Int)]; y[0] := 1; for(i := 1; i < n; i++;) { y[i] := Rk4(Rate(Float, Float) ~ Float, dx, x0 + dx * (i - 1), y[i-1]); }; for(i := 0; i < n; i += 10;) { x := x0 + dx * i; y2 := (x * x / 4 + 1)->Power(2.0); x_value := x->As(Int); y_value := y[i]; rel_value := y_value/y2 - 1.0; "y({$x_value})={$y_value}; error: {$rel_value}"->PrintLine(); }; }
function : native : Rk4(f : (Float, Float) ~ Float, dx : Float, x : Float, y : Float) ~ Float { k1 := dx * f(x, y); k2 := dx * f(x + dx / 2, y + k1 / 2); k3 := dx * f(x + dx / 2, y + k2 / 2); k4 := dx * f(x + dx, y + k3); return y + (k1 + 2 * k2 + 2 * k3 + k4) / 6; } function : native : Rate(x : Float, y : Float) ~ Float { return x * y->SquareRoot(); }
}</lang>
Output:
y(0)=1.0; error: 0.0 y(1)=1.563; error: -0.0000000933 y(2)=3.1000; error: -0.000000230 y(3)=10.563; error: -0.000000275 y(4)=24.1000; error: -0.000000249 y(5)=52.563; error: -0.000000206 y(6)=99.1000; error: -0.000000166 y(7)=175.563; error: -0.000000134 y(8)=288.1000; error: -0.000000109 y(9)=451.563; error: -0.0000000902 y(10)=675.1000; error: -0.0000000754
OCaml
<lang ocaml>let y' t y = t *. sqrt y let exact t = let u = 0.25*.t*.t +. 1.0 in u*.u
let rk4_step (y,t) h =
let k1 = h *. y' t y in let k2 = h *. y' (t +. 0.5*.h) (y +. 0.5*.k1) in let k3 = h *. y' (t +. 0.5*.h) (y +. 0.5*.k2) in let k4 = h *. y' (t +. h) (y +. k3) in (y +. (k1+.k4)/.6.0 +. (k2+.k3)/.3.0, t +. h)
let rec loop h n (y,t) =
if n mod 10 = 1 then Printf.printf "t = %f,\ty = %f,\terr = %g\n" t y (abs_float (y -. exact t)); if n < 102 then loop h (n+1) (rk4_step (y,t) h)
let _ = loop 0.1 1 (1.0, 0.0)</lang>
- Output:
t = 0.000000, y = 1.000000, err = 0 t = 1.000000, y = 1.562500, err = 1.45722e-07 t = 2.000000, y = 3.999999, err = 9.19479e-07 t = 3.000000, y = 10.562497, err = 2.90956e-06 t = 4.000000, y = 24.999994, err = 6.23491e-06 t = 5.000000, y = 52.562489, err = 1.08197e-05 t = 6.000000, y = 99.999983, err = 1.65946e-05 t = 7.000000, y = 175.562476, err = 2.35177e-05 t = 8.000000, y = 288.999968, err = 3.15652e-05 t = 9.000000, y = 451.562459, err = 4.07232e-05 t = 10.000000, y = 675.999949, err = 5.09833e-05
Octave
<lang octave>
- Applying the Runge-Kutta method (This code must be implement on a different file than the main one).
function temp = rk4(func,x,pvi,h)
K1 = h*func(x,pvi); K2 = h*func(x+0.5*h,pvi+0.5*K1); K3 = h*func(x+0.5*h,pvi+0.5*K2); K4 = h*func(x+h,pvi+K3); temp = pvi + (K1 + 2*K2 + 2*K3 + K4)/6;
endfunction
- Main Program.
f = @(t) (1/16)*((t.^2 + 4).^2); df = @(t,y) t*sqrt(y);
pvi = 1.0; h = 0.1; Yn = pvi;
for x = 0:h:10-h
pvi = rk4(df,x,pvi,h); Yn = [Yn pvi];
endfor
fprintf('Time \t Exact Value \t ODE4 Value \t Num. Error\n');
for i=0:10
fprintf('%d \t %.5f \t %.5f \t %.4g \n',i,f(i),Yn(1+i*10),f(i)-Yn(1+i*10));
endfor </lang>
- Output:
Time Exact Value ODE4 Value Num. Error 0 1.00000 1.00000 0 1 1.56250 1.56250 1.457e-007 2 4.00000 4.00000 9.195e-007 3 10.56250 10.56250 2.91e-006 4 25.00000 24.99999 6.235e-006 5 52.56250 52.56249 1.082e-005 6 100.00000 99.99998 1.659e-005 7 175.56250 175.56248 2.352e-005 8 289.00000 288.99997 3.157e-005 9 451.56250 451.56246 4.072e-005 10 676.00000 675.99995 5.098e-005
PARI/GP
<lang parigp>rk4(f,dx,x,y)={
my(k1=dx*f(x,y), k2=dx*f(x+dx/2,y+k1/2), k3=dx*f(x+dx/2,y+k2/2), k4=dx*f(x+dx,y+k3)); y + (k1 + 2*k2 + 2*k3 + k4) / 6
}; rate(x,y)=x*sqrt(y); go()={
my(x0=0,x1=10,dx=.1,n=1+(x1-x0)\dx,y=vector(n)); y[1]=1; for(i=2,n,y[i]=rk4(rate, dx, x0 + dx * (i - 1), y[i-1])); print("x\ty\trel. err.\n------------"); forstep(i=1,n,10, my(x=x0+dx*i,y2=(x^2/4+1)^2); print(x "\t" y[i] "\t" y[i]/y2 - 1) )
}; go()</lang>
- Output:
x y rel. err. ------------ 0.100000000 1 -0.00498131231 1.10000000 1.68999982 -0.00383519474 2.10000000 4.40999894 -0.00237694942 3.10000000 11.5599968 -0.00146924588 4.10000000 27.0399933 -0.000961094862 5.10000000 56.2499884 -0.000666538719 6.10000000 106.089982 -0.000485427212 7.10000000 184.959975 -0.000367681962 8.10000000 302.759966 -0.000287408941 9.10000000 470.889955 -0.000230470905
Pascal
This code has been compiled using Free Pascal 2.6.2.
<lang pascal>program RungeKuttaExample;
uses sysutils;
type
TDerivative = function (t, y : Real) : Real;
procedure RungeKutta(yDer : TDerivative;
var t, y : array of Real; dt : Real);
var
dy1, dy2, dy3, dy4 : Real; idx : Cardinal;
begin
for idx := Low(t) to High(t) - 1 do begin dy1 := dt * yDer(t[idx], y[idx]); dy2 := dt * yDer(t[idx] + dt / 2.0, y[idx] + dy1 / 2.0); dy3 := dt * yDer(t[idx] + dt / 2.0, y[idx] + dy2 / 2.0); dy4 := dt * yDer(t[idx] + dt, y[idx] + dy3); t[idx + 1] := t[idx] + dt; y[idx + 1] := y[idx] + (dy1 + 2.0 * (dy2 + dy3) + dy4) / 6.0; end;
end;
function CalcError(t, y : Real) : Real; var
trueVal : Real;
begin
trueVal := sqr(sqr(t) + 4.0) / 16.0; CalcError := abs(trueVal - y);
end;
procedure Print(t, y : array of Real;
modnum : Integer);
var
idx : Cardinal;
begin
for idx := Low(t) to High(t) do begin if idx mod modnum = 0 then begin WriteLn(Format('y(%4.1f) = %12.8f Error: %12.6e', [t[idx], y[idx], CalcError(t[idx], y[idx])])); end; end;
end;
function YPrime(t, y : Real) : Real; begin
YPrime := t * sqrt(y);
end;
const
dt = 0.10; N = 100;
var
tArr, yArr : array [0..N] of Real;
begin
tArr[0] := 0.0; yArr[0] := 1.0; RungeKutta(@YPrime, tArr, yArr, dt); Print(tArr, yArr, 10);
end.</lang>
- Output:
y( 0.0) = 1.00000000 Error: 0.00000E+000 y( 1.0) = 1.56249985 Error: 1.45722E-007 y( 2.0) = 3.99999908 Error: 9.19479E-007 y( 3.0) = 10.56249709 Error: 2.90956E-006 y( 4.0) = 24.99999377 Error: 6.23491E-006 y( 5.0) = 52.56248918 Error: 1.08197E-005 y( 6.0) = 99.99998341 Error: 1.65946E-005 y( 7.0) = 175.56247648 Error: 2.35177E-005 y( 8.0) = 288.99996843 Error: 3.15652E-005 y( 9.0) = 451.56245928 Error: 4.07232E-005 y(10.0) = 675.99994902 Error: 5.09833E-005
Perl
There are many ways of doing this. Here we define the runge_kutta function as a function of and , returning a closure which itself takes as argument and returns the next .
Notice how we have to use sprintf to deal with floating point rounding. See perlfaq4. <lang perl>sub runge_kutta {
my ($yp, $dt) = @_; sub {
my ($t, $y) = @_; my @dy = $dt * $yp->( $t , $y ); push @dy, $dt * $yp->( $t + $dt/2, $y + $dy[0]/2 ); push @dy, $dt * $yp->( $t + $dt/2, $y + $dy[1]/2 ); push @dy, $dt * $yp->( $t + $dt , $y + $dy[2] ); return $t + $dt, $y + ($dy[0] + 2*$dy[1] + 2*$dy[2] + $dy[3]) / 6;
}
}
my $RK = runge_kutta sub { $_[0] * sqrt $_[1] }, .1;
for(
my ($t, $y) = (0, 1); sprintf("%.0f", $t) <= 10; ($t, $y) = $RK->($t, $y)
) {
printf "y(%2.0f) = %12f ± %e\n", $t, $y, abs($y - ($t**2 + 4)**2 / 16) if sprintf("%.4f", $t) =~ /0000$/;
}</lang>
- Output:
y( 0) = 1.000000 ± 0.000000e+00 y( 1) = 1.562500 ± 1.457219e-07 y( 2) = 3.999999 ± 9.194792e-07 y( 3) = 10.562497 ± 2.909562e-06 y( 4) = 24.999994 ± 6.234909e-06 y( 5) = 52.562489 ± 1.081970e-05 y( 6) = 99.999983 ± 1.659460e-05 y( 7) = 175.562476 ± 2.351773e-05 y( 8) = 288.999968 ± 3.156520e-05 y( 9) = 451.562459 ± 4.072316e-05 y(10) = 675.999949 ± 5.098329e-05
Phix
<lang Phix>constant dt = 0.1 atom y = 1.0 printf(1," x true/actual y calculated y relative error\n") printf(1," --- ------------- ------------- --------------\n") for i=0 to 100 do
atom t = i*dt if integer(t) then atom act = power(t*t+4,2)/16 printf(1,"%4.1f %14.9f %14.9f %.9e\n",{t,act,y,abs(y-act)}) end if atom k1 = t*sqrt(y), k2 = (t+dt/2)*sqrt(y+dt/2*k1), k3 = (t+dt/2)*sqrt(y+dt/2*k2), k4 = (t+dt)*sqrt(y+dt*k3) y += dt*(k1+2*(k2+k3)+k4)/6
end for</lang>
- Output:
x true/actual y calculated y relative error --- ------------- ------------- -------------- 0.0 1.000000000 1.000000000 0.000000000e+0 1.0 1.562500000 1.562499854 1.457218921e-7 2.0 4.000000000 3.999999081 9.194791999e-7 3.0 10.562500000 10.562497090 2.909562447e-6 4.0 25.000000000 24.999993765 6.234909363e-6 5.0 52.562500000 52.562489180 1.081969741e-5 6.0 100.000000000 99.999983405 1.659459641e-5 7.0 175.562500000 175.562476482 2.351772874e-5 8.0 289.000000000 288.999968435 3.156520142e-5 9.0 451.562500000 451.562459277 4.072316033e-5 10.0 676.000000000 675.999949017 5.098329030e-5
PL/I
<lang PL/I> Runge_Kutta: procedure options (main); /* 10 March 2014 */
declare (y, dy1, dy2, dy3, dy4) float (18); declare t fixed decimal (10,1); declare dt float (18) static initial (0.1);
y = 1; do t = 0 to 10 by 0.1; dy1 = dt * ydash(t, y); dy2 = dt * ydash(t + dt/2, y + dy1/2); dy3 = dt * ydash(t + dt/2, y + dy2/2); dy4 = dt * ydash(t + dt, y + dy3);
if mod(t, 1.0) = 0 then put skip edit('y(', trim(t), ')=', y, ', error = ', abs(y - (t**2 + 4)**2 / 16 )) (3 a, column(9), f(16,10), a, f(13,10)); y = y + (dy1 + 2*dy2 + 2*dy3 + dy4)/6; end;
ydash: procedure (t, y) returns (float(18));
declare (t, y) float (18) nonassignable; return ( t*sqrt(y) );
end ydash;
end Runge_kutta; </lang>
- Output:
y(0.0)= 1.0000000000, error = 0.0000000000 y(1.0)= 1.5624998543, error = 0.0000001457 y(2.0)= 3.9999990805, error = 0.0000009195 y(3.0)= 10.5624970904, error = 0.0000029096 y(4.0)= 24.9999937651, error = 0.0000062349 y(5.0)= 52.5624891803, error = 0.0000108197 y(6.0)= 99.9999834054, error = 0.0000165946 y(7.0)= 175.5624764823, error = 0.0000235177 y(8.0)= 288.9999684348, error = 0.0000315652 y(9.0)= 451.5624592768, error = 0.0000407232 y(10.0)= 675.9999490167, error = 0.0000509833
PowerShell
<lang PowerShell> function Runge-Kutta (${function:F}, ${function:y}, $y0, $t0, $dt, $tEnd) {
function RK ($tn,$yn) { $y1 = $dt*(F -t $tn -y $yn) $y2 = $dt*(F -t ($tn + (1/2)*$dt) -y ($yn + (1/2)*$y1)) $y3 = $dt*(F -t ($tn + (1/2)*$dt) -y ($yn + (1/2)*$y2)) $y4 = $dt*(F -t ($tn + $dt) -y ($yn + $y3)) $yn + (1/6)*($y1 + 2*$y2 + 2*$y3 + $y4) } function time ($t0, $dt, $tEnd) { $end = [MATH]::Floor(($tEnd - $t0)/$dt) foreach ($_ in 0..$end) { $_*$dt + $t0 } } $time, $yn, $t = (time $t0 $dt $tEnd), $y0, 0 foreach ($tn in $time) { if($t -eq $tn) { [pscustomobject]@{ t = "$tn" y = "$yn" error = "$([MATH]::abs($yn - (y $tn)))" } $t += 1 } $yn = RK $tn $yn }
} function F ($t,$y) {
$t * [MATH]::Sqrt($y)
} function y ($t) {
(1/16) * [MATH]::Pow($t*$t + 4,2)
} $y0 = 1 $t0 = 0 $dt = 0.1 $tEnd = 10 Runge-Kutta F y $y0 $t0 $dt $tEnd </lang> Output:
t y error - - ----- 0 1 0 1 1.56249985427811 1.45721892108597E-07 2 3.9999990805208 9.19479200778284E-07 3 10.5624970904376 2.90956244874963E-06 4 24.9999937650906 6.23490936391136E-06 5 52.5624891803026 1.08196974153429E-05 6 99.9999834054036 1.65945964170078E-05 7 175.562476482271 2.35177287493116E-05 8 288.999968434799 3.156520142511E-05 9 451.56245927684 4.07231603389846E-05 10 675.99994901671 5.09832902935159E-05
PureBasic
<lang PureBasic>EnableExplicit Define.i i Define.d y=1.0, k1=0.0, k2=0.0, k3=0.0, k4=0.0, t=0.0
If OpenConsole()
For i=0 To 100 t=i/10 If Not i%10 PrintN("y("+RSet(StrF(t,0),2," ")+") ="+RSet(StrF(y,4),9," ")+#TAB$+"Error ="+RSet(StrF(Pow(Pow(t,2)+4,2)/16-y,10),14," ")) EndIf k1=t*Sqr(y) k2=(t+0.05)*Sqr(y+0.05*k1) k3=(t+0.05)*Sqr(y+0.05*k2) k4=(t+0.10)*Sqr(y+0.10*k3) y+0.1*(k1+2*(k2+k3)+k4)/6 Next Print("Press return to exit...") : Input()
EndIf End</lang>
- Output:
y( 0) = 1.0000 Error = 0.0000000000 y( 1) = 1.5625 Error = 0.0000001457 y( 2) = 4.0000 Error = 0.0000009195 y( 3) = 10.5625 Error = 0.0000029096 y( 4) = 25.0000 Error = 0.0000062349 y( 5) = 52.5625 Error = 0.0000108197 y( 6) = 100.0000 Error = 0.0000165946 y( 7) = 175.5625 Error = 0.0000235177 y( 8) = 289.0000 Error = 0.0000315652 y( 9) = 451.5625 Error = 0.0000407232 y(10) = 675.9999 Error = 0.0000509833 Press return to exit...
Python
using lambda
<lang Python>def RK4(f):
return lambda t, y, dt: ( lambda dy1: ( lambda dy2: ( lambda dy3: ( lambda dy4: (dy1 + 2*dy2 + 2*dy3 + dy4)/6 )( dt * f( t + dt , y + dy3 ) )
)( dt * f( t + dt/2, y + dy2/2 ) ) )( dt * f( t + dt/2, y + dy1/2 ) ) )( dt * f( t , y ) )
def theory(t): return (t**2 + 4)**2 /16
from math import sqrt dy = RK4(lambda t, y: t*sqrt(y))
t, y, dt = 0., 1., .1 while t <= 10:
if abs(round(t) - t) < 1e-5:
print("y(%2.1f)\t= %4.6f \t error: %4.6g" % ( t, y, abs(y - theory(t))))
t, y = t + dt, y + dy( t, y, dt )
</lang>
- Output:
y(0.0) = 1.000000 error: 0 y(1.0) = 1.562500 error: 1.45722e-07 y(2.0) = 3.999999 error: 9.19479e-07 y(3.0) = 10.562497 error: 2.90956e-06 y(4.0) = 24.999994 error: 6.23491e-06 y(5.0) = 52.562489 error: 1.08197e-05 y(6.0) = 99.999983 error: 1.65946e-05 y(7.0) = 175.562476 error: 2.35177e-05 y(8.0) = 288.999968 error: 3.15652e-05 y(9.0) = 451.562459 error: 4.07232e-05 y(10.0) = 675.999949 error: 5.09833e-05
Alternate solution
<lang python>from math import sqrt
def rk4(f, x0, y0, x1, n):
vx = [0] * (n + 1) vy = [0] * (n + 1) h = (x1 - x0) / float(n) vx[0] = x = x0 vy[0] = y = y0 for i in range(1, n + 1): k1 = h * f(x, y) k2 = h * f(x + 0.5 * h, y + 0.5 * k1) k3 = h * f(x + 0.5 * h, y + 0.5 * k2) k4 = h * f(x + h, y + k3) vx[i] = x = x0 + i * h vy[i] = y = y + (k1 + k2 + k2 + k3 + k3 + k4) / 6 return vx, vy
def f(x, y):
return x * sqrt(y)
vx, vy = rk4(f, 0, 1, 10, 100) for x, y in list(zip(vx, vy))[::10]:
print("%4.1f %10.5f %+12.4e" % (x, y, y - (4 + x * x)**2 / 16))
0.0 1.00000 +0.0000e+00 1.0 1.56250 -1.4572e-07 2.0 4.00000 -9.1948e-07 3.0 10.56250 -2.9096e-06 4.0 24.99999 -6.2349e-06 5.0 52.56249 -1.0820e-05 6.0 99.99998 -1.6595e-05 7.0 175.56248 -2.3518e-05 8.0 288.99997 -3.1565e-05 9.0 451.56246 -4.0723e-05
10.0 675.99995 -5.0983e-05</lang>
R
<lang r>rk4 <- function(f, x0, y0, x1, n) {
vx <- double(n + 1) vy <- double(n + 1) vx[1] <- x <- x0 vy[1] <- y <- y0 h <- (x1 - x0)/n for(i in 1:n) { k1 <- h*f(x, y) k2 <- h*f(x + 0.5*h, y + 0.5*k1) k3 <- h*f(x + 0.5*h, y + 0.5*k2) k4 <- h*f(x + h, y + k3) vx[i + 1] <- x <- x0 + i*h vy[i + 1] <- y <- y + (k1 + k2 + k2 + k3 + k3 + k4)/6 } cbind(vx, vy)
}
sol <- rk4(function(x, y) x*sqrt(y), 0, 1, 10, 100) cbind(sol, sol[, 2] - (4 + sol[, 1]^2)^2/16)[seq(1, 101, 10), ]
vx vy [1,] 0 1.000000 0.000000e+00 [2,] 1 1.562500 -1.457219e-07 [3,] 2 3.999999 -9.194792e-07 [4,] 3 10.562497 -2.909562e-06 [5,] 4 24.999994 -6.234909e-06 [6,] 5 52.562489 -1.081970e-05 [7,] 6 99.999983 -1.659460e-05 [8,] 7 175.562476 -2.351773e-05 [9,] 8 288.999968 -3.156520e-05
[10,] 9 451.562459 -4.072316e-05 [11,] 10 675.999949 -5.098329e-05</lang>
Racket
See Euler method#Racket for implementation of simple general ODE-solver.
The Runge-Kutta method <lang racket> (define (RK4 F δt)
(λ (t y) (define δy1 (* δt (F t y))) (define δy2 (* δt (F (+ t (* 1/2 δt)) (+ y (* 1/2 δy1))))) (define δy3 (* δt (F (+ t (* 1/2 δt)) (+ y (* 1/2 δy2))))) (define δy4 (* δt (F (+ t δt) (+ y δy1)))) (list (+ t δt) (+ y (* 1/6 (+ δy1 (* 2 δy2) (* 2 δy3) δy4))))))
</lang>
The method modifier which divides each time-step into n sub-steps: <lang racket> (define ((step-subdivision n method) F h)
(λ (x . y) (last (ODE-solve F (cons x y) #:x-max (+ x h) #:step (/ h n) #:method method))))
</lang>
Usage: <lang racket> (define (F t y) (* t (sqrt y)))
(define (exact-solution t) (* 1/16 (sqr (+ 4 (sqr t)))))
(define numeric-solution
(ODE-solve F '(0 1) #:x-max 10 #:step 1 #:method (step-subdivision 10 RK4)))
(for ([s numeric-solution])
(match-define (list t y) s) (printf "t=~a\ty=~a\terror=~a\n" t y (- y (exact-solution t))))
</lang>
- Output:
t=0 y=1 error=0 t=1 y=1.562499854278108 error=-1.4572189210859676e-07 t=2 y=3.999999080520799 error=-9.194792007782837e-07 t=3 y=10.562497090437551 error=-2.9095624487496252e-06 t=4 y=24.999993765090636 error=-6.234909363911356e-06 t=5 y=52.562489180302585 error=-1.0819697415342944e-05 t=6 y=99.99998340540358 error=-1.659459641700778e-05 t=7 y=175.56247648227125 error=-2.3517728749311573e-05 t=8 y=288.9999684347986 error=-3.156520142510999e-05 t=9 y=451.56245927683966 error=-4.07231603389846e-05 t=10 y=675.9999490167097 error=-5.098329029351589e-05
Graphical representation:
<lang racket> > (require plot) > (plot (list (function exact-solution 0 10 #:label "Exact solution")
(points numeric-solution #:label "Runge-Kutta method")) #:x-label "t" #:y-label "y(t)")
Raku
(formerly Perl 6)
<lang perl6>sub runge-kutta(&yp) {
return -> \t, \y, \δt { my $a = δt * yp( t, y ); my $b = δt * yp( t + δt/2, y + $a/2 ); my $c = δt * yp( t + δt/2, y + $b/2 ); my $d = δt * yp( t + δt, y + $c ); ($a + 2*($b + $c) + $d) / 6; }
}
constant δt = .1; my &δy = runge-kutta { $^t * sqrt($^y) };
loop (
my ($t, $y) = (0, 1); $t <= 10; ($t, $y) »+=« (δt, δy($t, $y, δt))
) {
printf "y(%2d) = %12f ± %e\n", $t, $y, abs($y - ($t**2 + 4)**2 / 16) if $t %% 1;
}</lang>
- Output:
y( 0) = 1.000000 ± 0.000000e+00 y( 1) = 1.562500 ± 1.457219e-07 y( 2) = 3.999999 ± 9.194792e-07 y( 3) = 10.562497 ± 2.909562e-06 y( 4) = 24.999994 ± 6.234909e-06 y( 5) = 52.562489 ± 1.081970e-05 y( 6) = 99.999983 ± 1.659460e-05 y( 7) = 175.562476 ± 2.351773e-05 y( 8) = 288.999968 ± 3.156520e-05 y( 9) = 451.562459 ± 4.072316e-05 y(10) = 675.999949 ± 5.098329e-05
REXX
The Runge─Kutta method is used to solve the following differential equation: ╔═══════════════╗ ______ ╔══ the exact solution: y(t)= (t²+4)²/16 ══╗ ╚═══════════════╝ y'(t)=t² √ y(t) ╚═══════════════════════════════════════════╝
<lang rexx>/*REXX program uses the Runge─Kutta method to solve the equation: y'(t) = t² √[y(t)] */ numeric digits 40; f= digits() % 4 /*use 40 decimal digs, but only show 10*/ x0= 0; x1= 10; dx= .1 /*define variables: X0 X1 DX */ n=1 + (x1-x0) / dx y.=1; do m=1 for n-1; p= m - 1; y.m= RK4(dx, x0 + dx*p, y.p)
end /*m*/ /* [↑] use 4th order Runge─Kutta. */
w= digits() % 2 /*W: width used for displaying numbers.*/ say center('X', f, "═") center('Y', w+2, "═") center("relative error", w+8, '═') /*hdr*/
do i=0 to n-1 by 10; x= (x0 + dx*i) / 1; $= y.i / (x*x/4+1)**2 - 1 say center(x, f) fmt(y.i) left(, 2 + ($>=0) ) fmt($) end /*i*/ /*└┴┴┴───◄─────── aligns positive #'s. */
exit /*stick a fork in it, we're all done. */ /*──────────────────────────────────────────────────────────────────────────────────────*/ fmt: parse arg z; z= right( format(z, w, f), w); hasE= pos('E', z)>0; has.= pos(., z)>0
jus= has. & \hasE; T= 'T'; if jus then z= left( strip( strip(z, T, 0), T, .), w) return translate( right(z, (z>=0) + w + 5*hasE + 2*(jus & (z<0) ) ), 'e', "E")
/*──────────────────────────────────────────────────────────────────────────────────────*/ RK4: procedure; parse arg dx,x,y; dxH= dx/2; k1= dx * (x ) * sqrt(y )
k2= dx * (x + dxH) * sqrt(y + k1/2) k3= dx * (x + dxH) * sqrt(y + k2/2) k4= dx * (x + dx ) * sqrt(y + k3 ) return y + (k1 + k2*2 + k3*2 + k4) / 6
/*──────────────────────────────────────────────────────────────────────────────────────*/ sqrt: procedure; parse arg x; if x=0 then return 0; d=digits(); m.=9; numeric form; h=d+6
numeric digits; parse value format(x,2,1,,0) 'E0' with g 'E' _ .; g=g * .5'e'_ % 2 do j=0 while h>9; m.j=h; h=h%2+1; end /*j*/ do k=j+5 to 0 by -1; numeric digits m.k; g=(g+x/g)*.5; end /*k*/; return g</lang>
Programming note: the fmt function is used to
align the output with attention paid to the different ways some
REXXes format numbers that are in floating point representation.
- output when using Regina REXX:
════X═════ ══════════Y═══════════ ═══════relative error═══════ 0 1 0 1 1.5624998543 -9.3262010935e-8 2 3.9999990805 -2.2986980019e-7 3 10.5624970904 -2.7546153356e-7 4 24.9999937651 -2.4939637459e-7 5 52.5624891803 -2.0584442174e-7 6 99.9999834054 -1.6594596403e-7 7 175.5624764823 -1.3395644713e-7 8 288.9999684348 -1.0922215040e-7 9 451.5624592768 -9.0182777476e-8 10 675.9999490167 -7.5419068846e-8
- output when using PC/REXX, Personal REXX, ROO, or R4 REXX:
════X═════ ══════════Y═══════════ ═══════relative error═══════ 0 1 0 1 1.5624998543 -0.0000000933 2 3.9999990805 -0.0000002299 3 10.5624970904 -0.0000002755 4 24.9999937651 -0.0000002494 5 52.5624891803 -0.0000002058 6 99.9999834054 -0.0000001659 7 175.5624764823 -0.000000134 8 288.9999684348 -0.0000001092 9 451.5624592768 -0.0000000902 10 675.9999490167 -0.0000000754
Ring
<lang ring> decimals(8) y = 1.0 for i = 0 to 100
t = i / 10 if t = floor(t) actual = (pow((pow(t,2) + 4),2)) / 16 see "y(" + t + ") = " + y + " error = " + (actual - y) + nl ok k1 = t * sqrt(y) k2 = (t + 0.05) * sqrt(y + 0.05 * k1) k3 = (t + 0.05) * sqrt(y + 0.05 * k2) k4 = (t + 0.10) * sqrt(y + 0.10 * k3) y += 0.1 * (k1 + 2 * (k2 + k3) + k4) / 6
next </lang>
Output:
y(0) = 1 error = 0 y(1) = 1.56249985 error = 0.00000015 y(2) = 3.99999908 error = 0.00000092 y(3) = 10.56249709 error = 0.00000291 y(4) = 24.99999377 error = 0.00000623 y(5) = 52.56248918 error = 0.00001082 y(6) = 99.99998341 error = 0.00001659 y(7) = 175.56247648 error = 0.00002352 y(8) = 288.99996843 error = 0.00003157 y(9) = 451.56245928 error = 0.00004072 y(10) = 675.99994902 error = 0.00005098
Ruby
<lang ruby>def calc_rk4(f)
return ->(t,y,dt){ ->(dy1 ){ ->(dy2 ){ ->(dy3 ){ ->(dy4 ){ ( dy1 + 2*dy2 + 2*dy3 + dy4 ) / 6 }.call( dt * f.call( t + dt , y + dy3 ))}.call( dt * f.call( t + dt/2, y + dy2/2 ))}.call( dt * f.call( t + dt/2, y + dy1/2 ))}.call( dt * f.call( t , y ))}
end
TIME_MAXIMUM, WHOLE_TOLERANCE = 10.0, 1.0e-5 T_START, Y_START, DT = 0.0, 1.0, 0.10
def my_diff_eqn(t,y) ; t * Math.sqrt(y) ; end def my_solution(t ) ; (t**2 + 4)**2 / 16 ; end def find_error(t,y) ; (y - my_solution(t)).abs ; end def is_whole?(t ) ; (t.round - t).abs < WHOLE_TOLERANCE ; end
dy = calc_rk4( ->(t,y){my_diff_eqn(t,y)} )
t, y = T_START, Y_START while t <= TIME_MAXIMUM
printf("y(%4.1f)\t= %12.6f \t error: %12.6e\n",t,y,find_error(t,y)) if is_whole?(t) t, y = t + DT, y + dy.call(t,y,DT)
end</lang>
- Output:
y( 0.0) = 1.000000 error: 0.000000e+00 y( 1.0) = 1.562500 error: 1.457219e-07 y( 2.0) = 3.999999 error: 9.194792e-07 y( 3.0) = 10.562497 error: 2.909562e-06 y( 4.0) = 24.999994 error: 6.234909e-06 y( 5.0) = 52.562489 error: 1.081970e-05 y( 6.0) = 99.999983 error: 1.659460e-05 y( 7.0) = 175.562476 error: 2.351773e-05 y( 8.0) = 288.999968 error: 3.156520e-05 y( 9.0) = 451.562459 error: 4.072316e-05 y(10.0) = 675.999949 error: 5.098329e-05
Run BASIC
<lang Runbasic>y = 1 while t <= 10
k1 = t * sqr(y) k2 = (t + .05) * sqr(y + .05 * k1) k3 = (t + .05) * sqr(y + .05 * k2) k4 = (t + .1) * sqr(y + .1 * k3)
if right$(using("##.#",t),1) = "0" then print "y(";using("##",t);") ="; using("####.#######", y);chr$(9);"Error ="; (((t^2 + 4)^2) /16) -y
y = y + .1 *(k1 + 2 * (k2 + k3) + k4) / 6 t = t + .1
wend end</lang>
- Output:
y( 0) = 1.0000000 Error =0 y( 1) = 1.5624999 Error =1.45721892e-7 y( 2) = 3.9999991 Error =9.19479203e-7 y( 3) = 10.5624971 Error =2.90956246e-6 y( 4) = 24.9999938 Error =6.23490939e-6 y( 5) = 52.5624892 Error =1.08196973e-5 y( 6) = 99.9999834 Error =1.65945961e-5 y( 7) = 175.5624765 Error =2.3517728e-5 y( 8) = 288.9999684 Error =3.15652e-5 y( 9) = 451.5624593 Error =4.07231581e-5 y(10) = 675.9999490 Error =5.09832864e-5
Rust
This is a translation of the javascript solution with some minor differences. <lang rust>fn runge_kutta4(fx: &Fn(f64, f64) -> f64, x: f64, y: f64, dx: f64) -> f64 {
let k1 = dx * fx(x, y); let k2 = dx * fx(x + dx / 2.0, y + k1 / 2.0); let k3 = dx * fx(x + dx / 2.0, y + k2 / 2.0); let k4 = dx * fx(x + dx, y + k3);
y + (k1 + 2.0 * k2 + 2.0 * k3 + k4) / 6.0
}
fn f(x: f64, y: f64) -> f64 {
x * y.sqrt()
}
fn actual(x: f64) -> f64 {
(1.0 / 16.0) * (x * x + 4.0).powi(2)
}
fn main() {
let mut y = 1.0; let mut x = 0.0; let step = 0.1; let max_steps = 101; let sample_every_n = 10;
for steps in 0..max_steps { if steps % sample_every_n == 0 { println!("y({}):\t{:.10}\t\t {:E}", x, y, actual(x) - y) }
y = runge_kutta4(&f, x, y, step);
x = ((x * 10.0) + (step * 10.0)) / 10.0; }
}</lang>
y(0): 1.0000000000 0E0 y(1): 1.5624998543 1.4572189210859676E-7 y(2): 3.9999990805 9.194792007782837E-7 y(3): 10.5624970904 2.9095624487496252E-6 y(4): 24.9999937651 6.234909363911356E-6 y(5): 52.5624891803 1.0819697415342944E-5 y(6): 99.9999834054 1.659459641700778E-5 y(7): 175.5624764823 2.3517728749311573E-5 y(8): 288.9999684348 3.156520142510999E-5 y(9): 451.5624592768 4.07231603389846E-5 y(10): 675.9999490167 5.098329029351589E-5
Scala
<lang scala>object Main extends App {
val f = (t: Double, y: Double) => t * Math.sqrt(y) // Runge-Kutta solution val g = (t: Double) => Math.pow(t * t + 4, 2) / 16 // Exact solution new Calculator(f, Some(g)).compute(100, 0, .1, 1)
}
class Calculator(f: (Double, Double) => Double, g: Option[Double => Double] = None) {
def compute(counter: Int, tn: Double, dt: Double, yn: Double): Unit = { if (counter % 10 == 0) { val c = (x: Double => Double) => (t: Double) => { val err = Math.abs(x(t) - yn) f" Error: $err%7.5e" } val s = g.map(c(_)).getOrElse((x: Double) => "") // If we don't have exact solution, just print nothing println(f"y($tn%4.1f) = $yn%12.8f${s(tn)}") // Else, print Error estimation here } if (counter > 0) { val dy1 = dt * f(tn, yn) val dy2 = dt * f(tn + dt / 2, yn + dy1 / 2) val dy3 = dt * f(tn + dt / 2, yn + dy2 / 2) val dy4 = dt * f(tn + dt, yn + dy3) val y = yn + (dy1 + 2 * dy2 + 2 * dy3 + dy4) / 6 val t = tn + dt compute(counter - 1, t, dt, y) } }
}</lang>
y( 0.0) = 1.00000000 Error: 0.00000e+00 y( 1.0) = 1.56249985 Error: 1.45722e-07 y( 2.0) = 3.99999908 Error: 9.19479e-07 y( 3.0) = 10.56249709 Error: 2.90956e-06 y( 4.0) = 24.99999377 Error: 6.23491e-06 y( 5.0) = 52.56248918 Error: 1.08197e-05 y( 6.0) = 99.99998341 Error: 1.65946e-05 y( 7.0) = 175.56247648 Error: 2.35177e-05 y( 8.0) = 288.99996843 Error: 3.15652e-05 y( 9.0) = 451.56245928 Error: 4.07232e-05 y(10.0) = 675.99994902 Error: 5.09833e-05
Sidef
<lang ruby>func runge_kutta(yp) {
func (t, y, δt) { var a = (δt * yp(t, y)); var b = (δt * yp(t + δt/2, y + a/2)); var c = (δt * yp(t + δt/2, y + b/2)); var d = (δt * yp(t + δt, y + c)); (a + 2*(b + c) + d) / 6; }
}
define δt = 0.1; var δy = runge_kutta(func(t, y) { t * y.sqrt });
var(t, y) = (0, 1); loop {
t.is_int && printf("y(%2d) = %12f ± %e\n", t, y, abs(y - ((t**2 + 4)**2 / 16))); t <= 10 || break; y += δy(t, y, δt); t += δt;
}</lang>
- Output:
y( 0) = 1.000000 ± 0.000000e+00 y( 1) = 1.562500 ± 1.457219e-07 y( 2) = 3.999999 ± 9.194792e-07 y( 3) = 10.562497 ± 2.909562e-06 y( 4) = 24.999994 ± 6.234909e-06 y( 5) = 52.562489 ± 1.081970e-05 y( 6) = 99.999983 ± 1.659460e-05 y( 7) = 175.562476 ± 2.351773e-05 y( 8) = 288.999968 ± 3.156520e-05 y( 9) = 451.562459 ± 4.072316e-05 y(10) = 675.999949 ± 5.098329e-05
Standard ML
<lang sml>fun step y' (tn,yn) dt =
let val dy1 = dt * y'(tn,yn) val dy2 = dt * y'(tn + 0.5 * dt, yn + 0.5 * dy1) val dy3 = dt * y'(tn + 0.5 * dt, yn + 0.5 * dy2) val dy4 = dt * y'(tn + dt, yn + dy3) in (tn + dt, yn + (1.0 / 6.0) * (dy1 + 2.0*dy2 + 2.0*dy3 + dy4)) end
(* Suggested test case *) fun testy' (t,y) =
t * Math.sqrt y
fun testy t =
(1.0 / 16.0) * Math.pow(Math.pow(t,2.0) + 4.0, 2.0)
(* Test-runner that iterates the step function and prints the results. *) fun test t0 y0 dt steps print_freq y y' =
let fun loop i (tn,yn) = if i = steps then () else let val (t1,y1) = step y' (tn,yn) dt val y1' = y tn val () = if i mod print_freq = 0 then (print ("Time: " ^ Real.toString tn ^ "\n"); print ("Exact: " ^ Real.toString y1' ^ "\n"); print ("Approx: " ^ Real.toString yn ^ "\n"); print ("Error: " ^ Real.toString (y1' - yn) ^ "\n\n")) else () in loop (i+1) (t1,y1) end in loop 0 (t0,y0) end
(* Run the suggested test case *) val () = test 0.0 1.0 0.1 101 10 testy testy'</lang>
- Output:
Time: 0.0 Exact: 1.0 Approx: 1.0 Error: ~1.11022302463E~16 Time: 1.0 Exact: 1.5625 Approx: 1.56249985428 Error: 1.45722452549E~07 Time: 2.0 Exact: 4.0 Approx: 3.99999908052 Error: 9.19479203443E~07 Time: 3.0 Exact: 10.5625 Approx: 10.5624970904 Error: 2.90956245586E~06 Time: 4.0 Exact: 25.0 Approx: 24.9999937651 Error: 6.23490938878E~06 Time: 5.0 Exact: 52.5625 Approx: 52.5624891803 Error: 1.08196973727E~05 Time: 6.0 Exact: 100.0 Approx: 99.9999834054 Error: 1.65945961186E~05 Time: 7.0 Exact: 175.5625 Approx: 175.562476482 Error: 2.35177280956E~05 Time: 8.0 Exact: 289.0 Approx: 288.999968435 Error: 3.15651997767E~05 Time: 9.0 Exact: 451.5625 Approx: 451.562459277 Error: 4.07231581221E~05 Time: 10.0 Exact: 676.0 Approx: 675.999949017 Error: 5.09832866555E~05
Stata
<lang stata>function rk4(f, t0, y0, t1, n) { h = (t1-t0)/(n-1) a = J(n, 2, 0) a[1, 1] = t = t0 a[1, 2] = y = y0 for (i=2; i<=n; i++) { k1 = h*(*f)(t, y) k2 = h*(*f)(t+0.5*h, y+0.5*k1) k3 = h*(*f)(t+0.5*h, y+0.5*k2) k4 = h*(*f)(t+h, y+k3) t = t+h y = y+(k1+2*k2+2*k3+k4)/6 a[i, 1] = t a[i, 2] = y } return(a) }
function f(t, y) { return(t*sqrt(y)) }
a = rk4(&f(), 0, 1, 10, 101) t = a[., 1] a = a, a[., 2]:-(t:^2:+4):^2:/16 a[range(1,101,10), .]
1 2 3 +----------------------------------------------+ 1 | 0 1 0 | 2 | 1 1.562499854 -1.45722e-07 | 3 | 2 3.999999081 -9.19479e-07 | 4 | 3 10.56249709 -2.90956e-06 | 5 | 4 24.99999377 -6.23491e-06 | 6 | 5 52.56248918 -.0000108197 | 7 | 6 99.99998341 -.0000165946 | 8 | 7 175.5624765 -.0000235177 | 9 | 8 288.9999684 -.0000315652 | 10 | 9 451.5624593 -.0000407232 | 11 | 10 675.999949 -.0000509833 | +----------------------------------------------+</lang>
Swift
<lang Swift>import Foundation
func rk4(dx: Double, x: Double, y: Double, f: (Double, Double) -> Double) -> Double {
let k1 = dx * f(x, y) let k2 = dx * f(x + dx / 2, y + k1 / 2) let k3 = dx * f(x + dx / 2, y + k2 / 2) let k4 = dx * f(x + dx, y + k3)
return y + (k1 + 2 * k2 + 2 * k3 + k4) / 6
}
var y = [Double]() var x: Double = 0.0 var y2: Double = 0.0
var x0: Double = 0.0 var x1: Double = 10.0 var dx: Double = 0.1
var i = 0 var n = Int(1 + (x1 - x0) / dx)
y.append(1) for i in 1..<n {
y.append(rk4(dx, x: x0 + dx * (Double(i) - 1), y: y[i - 1]) { (x: Double, y: Double) -> Double in return x * sqrt(y) })
}
print(" x y rel. err.") print("------------------------------")
for (var i = 0; i < n; i += 10) {
x = x0 + dx * Double(i) y2 = pow(x * x / 4 + 1, 2)
print(String(format: "%2g %11.6g %11.5g", x, y[i], y[i]/y2 - 1))
}</lang>
- Output:
x y rel. err. ------------------------------ 0 1 0 1 1.5625 -9.3262e-08 2 4 -2.2987e-07 3 10.5625 -2.7546e-07 4 25 -2.494e-07 5 52.5625 -2.0584e-07 6 100 -1.6595e-07 7 175.562 -1.3396e-07 8 289 -1.0922e-07 9 451.562 -9.0183e-08 10 676 -7.5419e-08
Tcl
<lang tcl>package require Tcl 8.5
- Hack to bring argument function into expression
proc tcl::mathfunc::dy {t y} {upvar 1 dyFn dyFn; $dyFn $t $y}
proc rk4step {dyFn y* t* dt} {
upvar 1 ${y*} y ${t*} t set dy1 [expr {$dt * dy($t, $y)}] set dy2 [expr {$dt * dy($t+$dt/2, $y+$dy1/2)}] set dy3 [expr {$dt * dy($t+$dt/2, $y+$dy2/2)}] set dy4 [expr {$dt * dy($t+$dt, $y+$dy3)}] set y [expr {$y + ($dy1 + 2*$dy2 + 2*$dy3 + $dy4)/6.0}] set t [expr {$t + $dt}]
}
proc y {t} {expr {($t**2 + 4)**2 / 16}} proc δy {t y} {expr {$t * sqrt($y)}}
proc printvals {t y} {
set err [expr {abs($y - [y $t])}] puts [format "y(%.1f) = %.8f\tError: %.8e" $t $y $err]
}
set t 0.0 set y 1.0 set dt 0.1 printvals $t $y for {set i 1} {$i <= 101} {incr i} {
rk4step δy y t $dt if {$i%10 == 0} {
printvals $t $y
}
}</lang>
- Output:
y(0.0) = 1.00000000 Error: 0.00000000e+00 y(1.0) = 1.56249985 Error: 1.45721892e-07 y(2.0) = 3.99999908 Error: 9.19479203e-07 y(3.0) = 10.56249709 Error: 2.90956245e-06 y(4.0) = 24.99999377 Error: 6.23490939e-06 y(5.0) = 52.56248918 Error: 1.08196973e-05 y(6.0) = 99.99998341 Error: 1.65945961e-05 y(7.0) = 175.56247648 Error: 2.35177280e-05 y(8.0) = 288.99996843 Error: 3.15652000e-05 y(9.0) = 451.56245928 Error: 4.07231581e-05 y(10.0) = 675.99994902 Error: 5.09832864e-05
Wren
<lang ecmascript>import "/fmt" for Fmt
var rungeKutta4 = Fn.new { |t0, tz, dt, y, yd|
var tn = t0 var yn = y.call(tn) var z = ((tz - t0)/dt).truncate for (i in 0..z) { if (i % 10 == 0) { var exact = y.call(tn) var error = yn - exact Fmt.print("$4.1f $10f $10f $9f", tn, yn, exact, error) } if (i == z) break var dy1 = dt * yd.call(tn, yn) var dy2 = dt * yd.call(tn + 0.5 * dt, yn + 0.5 * dy1) var dy3 = dt * yd.call(tn + 0.5 * dt, yn + 0.5 * dy2) var dy4 = dt * yd.call(tn + dt, yn + dy3) yn = yn + (dy1 + 2.0 * dy2 + 2.0 * dy3 + dy4) / 6.0 tn = tn + dt }
}
System.print(" T RK4 Exact Error") System.print("---- --------- ---------- ---------") var y = Fn.new { |t|
var x = t * t + 4.0 return x * x / 16.0
} var yd = Fn.new { |t, yt| t * yt.sqrt } rungeKutta4.call(0, 10, 0.1, y, yd)</lang>
- Output:
T RK4 Exact Error ---- --------- ---------- --------- 0.0 1.000000 1.000000 0.000000 1.0 1.562500 1.562500 -0.000000 2.0 3.999999 4.000000 -0.000001 3.0 10.562497 10.562500 -0.000003 4.0 24.999994 25.000000 -0.000006 5.0 52.562489 52.562500 -0.000011 6.0 99.999983 100.000000 -0.000017 7.0 175.562476 175.562500 -0.000024 8.0 288.999968 289.000000 -0.000032 9.0 451.562459 451.562500 -0.000041 10.0 675.999949 676.000000 -0.000051
zkl
<lang zkl>fcn yp(t,y) { t * y.sqrt() } fcn exact(t){ u:=0.25*t*t + 1.0; u*u }
fcn rk4_step([(y,t)],h){
k1:=h * yp(t,y); k2:=h * yp(t + 0.5*h, y + 0.5*k1); k3:=h * yp(t + 0.5*h, y + 0.5*k2); k4:=h * yp(t + h, y + k3); T(y + (k1+k4)/6.0 + (k2+k3)/3.0, t + h);
}
fcn loop(h,n,[(y,t)]){
if(n % 10 == 1) print("t = %f,\ty = %f,\terr = %g\n".fmt(t,y,(y - exact(t)).abs())); if(n < 102) return(loop(h,(n+1),rk4_step(T(y,t),h))) //tail recursion
}</lang>
- Output:
loop(0.1,1,T(1.0, 0.0)) t = 0.000000, y = 1.000000, err = 0 t = 1.000000, y = 1.562500, err = 1.45722e-07 t = 2.000000, y = 3.999999, err = 9.19479e-07 t = 3.000000, y = 10.562497, err = 2.90956e-06 t = 4.000000, y = 24.999994, err = 6.23491e-06 t = 5.000000, y = 52.562489, err = 1.08197e-05 t = 6.000000, y = 99.999983, err = 1.65946e-05 t = 7.000000, y = 175.562476, err = 2.35177e-05 t = 8.000000, y = 288.999968, err = 3.15652e-05 t = 9.000000, y = 451.562459, err = 4.07232e-05 t = 10.000000, y = 675.999949, err = 5.09833e-05
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