Permutations: Difference between revisions
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=={{header|Quackery}}==
===General Solution===
The word ''perms'' solves a more general task; generate permutations of between ''a'' and ''b'' items (inclusive) from the specified nest.
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kucq kuca kaq kaqu kaqc kau kauq kauc kac kacq
kacu kcq kcqu kcqa kcu kcuq kcua kca kcaq kcau</pre>
===An Uncommon Ordering===
The central idea is that given a list of the permutations of say 3 items, each permutation can be used to generate 4 of the permutations of 4 items, so for example, from <code>[ 2 0 1 ]</code> we can generate
::<code>[ 3 2 0 1 ]</code>
::<code>[ 2 3 0 1 ]</code>
::<code>[ 2 0 3 1 ]</code>
::<code>[ 2 0 1 3 ]</code>
by stuffing the 3 into each of the 4 possible positions that it could go.
The code start with a nest of all the permutations of 0 items <code>[ [ ] ]</code>, and each time though the outer <code>times</code> loop (i.e. 4 times in the example) it takes each of the permutations generated so far (this is the <code>witheach</code> loop) and applies the central idea descried above (that is the inner <code>times</code> loop.)
'''Some aids to reading the code.'''
Quackery is a stack based language. If you are unfamiliar the with words <code>swap</code>, <code>rot</code>, <code>dup</code>, <code>2dup</code>, <code>dip</code>, <code>unrot</code> or <code>drop</code> they can be skimmed over as "noise" to get a gist of the process.
<code>[]</code> creates an empty nest <code>[ ]</code>.
<code>times</code> indicates that the word or nest following it is to be repeated a specified number of times. (The specified number is on the top of the stack, so <code>4 times [ ... ]</code>repeats some arbitrary code 4 times.
<code>i</code> returns the number of times a <code>times</code> loop has left to repeat. It counts down to zero.
<code>i^</code> returns the number of times a <code>times</code> loop has been repeated. It counts up from zero.
<code>size</code> returns the number of items (words, numbers, nests) in a nest.
<code>witheach</code> indicates that the word or nest following it is to be repeated once for each item in a specified nest, with successive items from the nest available on the top of stack on each repetition.
<code>999 ' [ 10 11 12 13 ] 3 stuff</code> will return <code>[ 10 11 12 99 13 ]</code>by stuffing the number 99 into the 3rd position in the nest. (The start of a nest is the zeroth position, the end of this nest is the 5th position.)
<code>nested join</code> adds a nest to the end of a nest as its last item.
=={{header|R}}==
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