Talk:Sorting algorithms/Pancake sort: Difference between revisions
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Once again, I think to myself, "I should really do '''''X'''''," and someone else goes and does it almost the instant I think of it. This time it's the C implementation here. -- [[User:Eriksiers|Erik Siers]] 20:46, 10 May 2010 (UTC)
== J implementation ==
<lang J>flip=: C.~ C.@i.@-
unsorted=: #~ 1 , [: >./\. 2 >/\ ]
FlDown=: flip 1 + (i. >./)@unsorted
FlipUp=: flip [:+/<./\@(<: {.)
pancake=: FlipUp@FlDown^:_</lang>
<code>flip</code> reverses the first N items in a list
<lang J> 2 3 4 5 6 7 flip 3
4 3 2 5 6 7
</lang>
<code>unsorted</code> finds the largest prefix where a number is less than a number to its left.
<lang J> unsorted 3 2 1 2 3
3 2 1</lang>
<code>FlDown</code> finds the largest element in that unsorted prefix and brings it to the front.
<lang J> FlDown 2 5 3 6 4 7 1 0 8
7 4 6 3 5 2 1 0 8</lang>
<code>FlipUp</code> flips the first element of the list as far right as possible
<lang J> FlipUp 7 4 6 3 5 2 1 0 8
0 1 2 5 3 6 4 7 8</lang>
<code>pancake</code> repeatedly uses FlipDown and then FlipUp until the result stops changing.
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