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I am trying to apply InversePermutation on a list of sublists of selected permutation elements. First I am trying to see if I am able to reach every element,

temp = {{Cycles[{}]}, {Cycles[{}], Cycles[(12)]}, {Cycles[{}], 

So the code I have tried to print all is

k = 1; i = 1; Do[Do[Apply[Print, i], {i, k}], {k, temp}]

which gives $\{\}\{\}\{1 2\}\{\}\{1 3\}....$ it pics only content of Cycle. So, I changed it a bit as,

k = 1; i = 1; Do[Do[Apply[Print, {i}], {i, k}], {k, temp}]

Surprisingly it started to take all the contents with Cycle prefix along, giving

$\{\text{Cycles}[\{\}]\}\\ \left\{\text{Cycles}[\{\}],\text{Cycles}\left[\left( \begin{array}{cc} 1 & 2 \\ \end{array} \right)\right]\right\}....$

What I could not understand is that putting {i} instead of i changed what ?
Edit: I am not able to replace this Print with InversePermutation.It doesn't respond if I replace Print with it. Can someone suggest what I am doing wrong?

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2 Answers 2

up vote 5 down vote accepted

Note that your definitions k=1 and i=1 make little difference, as i and k are local in the do loops. To see how the code behaves we actually have to look at the range specifications for k first.

temp is a list, so values for k be taken from this list. But the values for k are then always lists as well, as temp is a list of lists. So also in the inner loop we have for every value of k a Do loop of the form Do[expr, {i, list}]. So i takes every element of this list as a value.

I am unsure if you meant this to happen. At least you are finding all the Cycles :). But it confuses me that you would set i and k to 1 then..



Clear[i, k]
Do[Print[i], {k, temp}, {i, k}]

Cycles[{}], Cycles[{}], Cycles[12]...

You see that you need no definitions for i and k? And that you can do with only one Do loop? And that you don't need Apply?

Also evaluate

Map[Print, Flatten[temp]];

Cycles[{}], Cycles[{}], Cycles[12]...

Further note

Apply is a function that changes the head of an expression. Apply[Print, i] is generally not good code in a Do loop where i is a variable, as in case i=1 for example, i has no proper head, as the integer 1 is a so called atom of Mathematica.

Apply[Print, {1}] is "good code", as the head of {1} is List and {1}.

Anyway, as it turns out, i is never 1 inside the Do loops so I guess this doesn't matter.

In your example, if you use Apply[Print, Cycles[{}]], that becomes Print[{}]. If you do Apply[Print, {Cycles[{}]}], that becomes Print[Cycles[{}]].

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your explanation is very informative, thanks a lot. –  Rorschach Jun 1 '13 at 18:40
@rafiki Okido, no problem :) –  Jacob Akkerboom Jun 1 '13 at 18:43

Looking at your Do loops, you can combine them into a single Do expression; I will replace Do with Table to return all evaluations in nested lists, and I will use an arbitrary head label to show which parts are assigned to i in each loop:

Table[label[i], {k, temp}, {i, k}]
{{label[Cycles[{}]]}, {label[Cycles[{}]], label[Cycles[12]]},
 {label[Cycles[{}]], label[Cycles[13]]}}

Note that the same expression can be produced with:

Map[label, temp, {2}]
{{label[Cycles[{}]]}, {label[Cycles[{}]], label[Cycles[12]]},
 {label[Cycles[{}]], label[Cycles[13]]}}

Now we can address this part of your question:

What I could not understand is that putting {i} instead of i changed what?

The operation Apply[ff, expr] replaces the head of expr with ff. In the case of {stuff} the head is List, so you simply get ff[stuff]. Combined with the above, and using Scan in place of Map (which has similar syntax), with {i} you are therefore just doing the equivalent of:

Scan[Print, temp, {2}]

Depending on what you want to do you may find value in Level:

Level[temp, {2}]
{Cycles[{}], Cycles[{}], Cycles[12], Cycles[{}], Cycles[13]}
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ya yesterday night after reading your other post on loops, I tried the same and could do it. Thanks for posting here in more refined way. –  Rorschach Jun 3 '13 at 4:01

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