Talk:Pascal's triangle: Difference between revisions
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== Right Output Format? ==
The task bdescription says the
<pre> 1
1 1
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Revision as of 14:58, 11 June 2010
Right Output Format?
The task bdescription says the triangle looks like this:
1 1 1 1 2 1 1 3 3 1
And yet, some examples are showing the easier to construct:
1 1 1 1 2 1 1 3 3 1
I think that maybe all example output should follow the task description format of an isosceles triangle. --Paddy3118 08:59, 27 December 2009 (UTC)
- That's not always easy to do. I think the important part of the task is the generation of each row. We don't need to complicate it with output formatting that isn't important to the theory involved. --Mwn3d 18:37, 27 December 2009 (UTC)
J Explanation
<lang j>!/~@i. N</lang>
The triangle itself is simply the table of number-of-combinations, for the first N non-negative integers.
<lang J> !/~i.5 1 1 1 1 1 0 1 2 3 4 0 0 1 3 6 0 0 0 1 4 0 0 0 0 1</lang>
That is, C(n,k) for all n,k in [0 .. n). J's notation for C(n,k) is k ! n
(mnemonic: combinations are closely related to factorials, which are denoted by ! in math).
So, for example, the number of ways to choose a poker hand (5 cards from the deck of 52): <lang j> 5!52 2598960</lang>
So !
is the mathematical choose function. As for /~@i.
, the /~
would be "table of" and i.
"the first N non-negative integers (i.e. 0 .. N-1)". (And @
is just glue.)
But, formatting the thing takes a bit more effort:
<lang j> (-@|. |."_1 [: ;:inv [: ":@-.&0&.>@|: !/~)@i. 5
1 1 1 1 2 1 1 3 3 1 1 4 6 4 1</lang>
Here, we have refactored the operation slightly:
<lang j>(stuff)@i.5</lang> makes the sequence 0 1 2 3 4 available every odd verb (counting from the right) in stuff
The "stuff" are (from right to left) <lang j>!/~</lang> build our table of combinations <lang j>":@-.&0&.>@|:</lang> transpose that array, and for each number remove it if it is zero, format it as a string and put it in a box <lang j>[:</lang> placeholder meaning "no verb here" the verb to the right does not get a left argument <lang j>;:inv</lang> combine the strings in rows of boxes as word (with spaces between them <lang j>[:</lang> placeholder meaning "no verb here" the verb to the right does not get a left argument <lang j>|."_1</lang> rotate each item (each row of characters) by the indicated amount (rotation would be moving contents left except that the amounts are negative). <lang j>-@|.</lang> reverse and negate the argument (resulting in the negative sequence _4 _3 _2 _1 0)