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Mathematica

Break nested loops

Al Guy
Posted 10 years ago

How can I exit multiple nested loops? Break[] exits nearest loop only.

For[j = 1, j <= 10, j++,
  For[i = 1, i <= 10, i++,
   If[i == 3 && j == 4, Break[]]
   ]];
Print[i, j]
POSTED BY: Al Guy
7 Replies
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Kuba Podkalicki
Kuba Podkalicki
Posted 10 years ago

(how to delete the post? :))
POSTED BY: Kuba Podkalicki
Erik Mahieu
Erik Mahieu
Posted 10 years ago

I had the same problem coming from basic and Fortran. That was procedural programming, so I was told... For loops: use Table, Nest, NestWhile, Fold, etc instead of For-Next. This is functional programming. Interrupt your "loops" by enclosing your code in a Catch and exit with Throw. It is a new habit but you learn it fast, it is faster and clear. Here is an example based on yr old style code:

In[9]:= Catch[
 Table[Table[{i, j}; 
   If[i == 3 && j == 4, Throw[{i, j}]], {i, 10}], {j, 10}]]

Out[9]= {3, 4}
POSTED BY: Erik Mahieu

It is not that those commands are not good but those commands were put in Mathematica version one so that people coming from a strictly procedural background dating all the way back to the beginning of Fortran would have a simple entry point into Mathematica and then move on to the more powerful constructs of the wolfram language. In the Mathematica there is always a much clearer and more powerful approach using functional and rule-based programming.
POSTED BY: David Reiss
Al Guy
Al Guy
Posted 10 years ago

Thanks. What's the reason the commands you mentioned aren't as good as Throw Catch?
POSTED BY: Al Guy

Throw and Catch.

But perhaps you would like to move away from ancient procedural programming constructs. And things like Break, Return, GoTo, and so on are the stuff of original Fortran and Basic.

POSTED BY: David Reiss
Al Guy
Al Guy
Posted 10 years ago

That will terminate whole computation, not just exit the nested loop.
POSTED BY: Al Guy

Abort?
POSTED BY: Isaac Abraham
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