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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: [mg72576] Re: Call-by-reference from inside a function
  • From: Peter Pein <petsie at dordos.net>
  • Date: Wed, 10 Jan 2007 02:50:59 -0500 (EST)
  • References: <eo012n$kp1$1@smc.vnet.net>

zac schrieb:
> 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]
> 

Hi Istvan,

use the attribute HoldFirst for your function and change "list_List" to simply
"list_".


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

lst = {a, b, c, d, e};
RandomChoseElem[lst]
--> a

lst
--> {b, c, d, e}

Hope that helps,
 Peter


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