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Possible Duplicate:
Creating a table/Matrix during a For loop

Yet another simple question.

For[i = 1, i < 3, i++,
 For[j = 1, j < 3, j++,
   Print[{i, j}]

I want to assign the output to a list, for example like:

tts = {{1, 1}, {1, 2}, {2, 1}, {2, 2}}

All my efforts did not gave me the desired result, but I am sure it is possible. Any suggestions are welcome.

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marked as duplicate by Mr.Wizard Jan 23 '13 at 14:03

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

Have a look at Table. – b.gatessucks Jan 23 '13 at 9:23
... and at the tutorial Loops and control structures – kglr Jan 23 '13 at 9:26
if you must use For loops: try tts={};For[..., For[..., Print[{i,j}];AppendTo[tts,{i,j}]]]. – kglr Jan 23 '13 at 10:31
up vote 1 down vote accepted

When you have some procedural style loop where you want to output things, you can often get quite good results using Sow and Reap:

 For[i = 1, i < 3, i++,
   For[j = 1, j < 3, j++,
     Sow[{i, j}]
 (* {Null, {{{1, 1}, {1, 2}, {2, 1}, {2, 2}}}} *)

Sow stores the expression it's given and when Reap is called, it will return a list where the first element is the result of the expression it was given, and the second argument is a collection of all the elements that where sowed during evaluation of that expression. Much more complex interaction is available if you look it up the doc, and often you can solve problems in an entirely procedural style and then just reap the results at the end, and perhaps reformat it if that is desired.

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thx jVincent, is there anyway to get rid of Null in the code, without doing it manually – Darwin Jan 23 '13 at 13:41
@Darwin I'm not sure what you mean with manually. Normally if you are doing something where you only care about the sowed elements, you use code//Reap//Last. – jVincent Jan 23 '13 at 14:01
i needed exactly that Last function! thx – Darwin Jan 23 '13 at 15:48

Mathematica tends to use print only for side effects. Also try to avoid the use of For.

You can just use the result of table to the variable.


If you want to look at the contents of tts just enter it into a cell and press shift+return.

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hi, @ b.gatessucks and image_doctor , i would like to use Table but the code i have is bit complex, i am not sure if it is also possible with Tabel my For-Loop looks like this For[p = 1, p < Length[lss] - 20, q = p; dx = lss[[q, 2]]; ds = lss[[q, 3]]; For[r = 1, ds <= dx, ds = ds + lss[[++q, 3]], r++] Print[{lss[[p, 1]], lss[[q, 1]], 10*(lss[[p, 1]] - lss[[q, 1]])}] p++] – Darwin Jan 23 '13 at 9:53
@Darwin You should have started with your real problem / code, as long as it is not too long. – Leonid Shifrin Jan 23 '13 at 11:22

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