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Re: Call-by-reference from inside a function


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]
>  
>
Give RandomChoseElem the attribute HoldAll. Here is a modified version 
of your function:

SetAttributes[RandomChoseElem,HoldAll]

RandomChoseElem[list_]:=Module[
    {pos,elem},
    pos=Random[Integer,{1,Length[list]}];
    elem=list[[pos]];
    (*This is to be solved inside:*)
    list=Drop[list,{pos}];
    elem
]/;MatchQ[list,_List]

I eliminated the superfluous Return statement, and moved the List 
checking into a condition since the HoldAll attribute will interfere 
with the pattern matching approach. Testing:

set={1,2,3,4}
{1,2,3,4}

RandomChoseElem[set]
1

set
{2,3,4}

Carl Woll
Wolfram Research


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