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MathGroup Archive 2007

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Re: Call-by-reference from inside a function

  • To: mathgroup at smc.vnet.net
  • Subject: [mg72598] [mg72598] Re: Call-by-reference from inside a function
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Thu, 11 Jan 2007 03:48:47 -0500 (EST)
  • Organization: Uni Leipzig
  • References: <eo012n$kp1$1@smc.vnet.net>
  • Reply-to: kuska at informatik.uni-leipzig.de

Hi,

and

SetAttributes[RandomChoseElem, HoldAll]

RandomChoseElem[list_] := Module[{pos, elem},
   pos = Random[Integer, {1, Length[list]}];
   elem = list[[pos]];
   list = Drop[list, {pos}];
   elem
  ]

does what you want.

Regards
   Jens

zac wrote:
> Dear Group,
> 
> I'm in need of a function, which is called with one argument, a list.
> The function is to chose one element of the list randomly, AND modify
> the original list by removing the chosen element from it. My problem is
> that I cannot modify the global variable from inside the function. It
> is not a solution to return with a list such as:
> Return[{chosen_element, modified_local_list}], and than make an
> assignment outside the function.
> I'm thinking on some call-by-reference method, but as I've learned so
> far, there is no user-friendly way to do this, just some workarounds
> (which I don't know). Any suggestions?
> 
> Istvan
> 
> example code below:
> 
> RandomChoseElem[list_List] := Module[
>       {pos, elem},
>       pos = Random[Integer, {1, Length[list]}];
>       elem = list[[pos]];
>       (* This is to be solved inside : *)
>       (* global`list = Drop[global`list, {pos}]; *)
>       Return[elem]
>       ];
> 
> set = {1, 2, 3, 4};
> 
> RandomChoseElem[set]
> 


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