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MathGroup Archive 2006

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Re: Help with functions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg65645] Re: [mg65612] Help with functions
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Wed, 12 Apr 2006 05:59:55 -0400 (EDT)
  • References: <200604110804.EAA11186@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

On 11 Apr 2006, at 17:04, IronBone wrote:

> I have written a simmple function which (in my opinion) should always
> return False. But it returns True.
>
> It seems that Return[False] breaks the Do loop but not returns from  
> the
> function.
> I thing I don't understand,  what is defining of functions. Can  
> somebody
> explain me this.
>
> fun[x_] := Module[{i},
>      Do[If[i == 2, Return[False]], {i, 5}];
>      Return[True];
>      ]
>
>

You need a double Return in the case of a DO loop:


fun[] := Module[{i},
      Do[If[i == 2, Return[Return[False]]], {i, 5}];
True;
      ]


fun[]

False


A single Return will work in the case of a While loop.

fun1[] := Module[{i = 1}, While[i < Infinity, i++; If[PrimeQ[i],  
Return[i]]]]


fun1[]


2

The reason for this different behaviour is that Do and While loops  
are used for different purposes and both kinds of behaviour can be  
convenient. But in general it is much better to use Throw and Catch;  
you never really  need to use Return in Mathematica.

(And Return[True] at the end of your code is probably borrowed from  
another language like Fortran or C; it is not needed in Mathematica  
and only makes the code look unnecessarily cluttered).

Andrzej Kozlowski
Tokyo, Japan




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