Re: Call-by-reference from inside a function
- To: mathgroup at smc.vnet.net
- Subject: [mg72577] Re: Call-by-reference from inside a function
- From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
- Date: Wed, 10 Jan 2007 02:55:00 -0500 (EST)
- Organization: The Open University, Milton Keynes, UK
- References: <eo012n$kp1$1@smc.vnet.net>
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]
>
Setting the attribute HoldFirst to your function will do the trick. For
a nice, detailed, and clear explanation see Daniel Lichtblau answer to
the thread "is there really no efficient way to delete an element from a
list??" (Mathgroup, June 28 2000)
http://groups-beta.google.com/group/comp.soft-sys.math.mathematica/browse_thread/thread/a7762aea066f29c0/8436f232fc060841?lnk=gst&q=call+reference&rnum=8&hl=en#8436f232fc060841
Here is your function (with some minor modification):
In[1]:=
Remove[RandomChoseElem];
SetAttributes[RandomChoseElem, HoldFirst];
RandomChoseElem[list_]:=Module[{pos,elem},
pos=Random[Integer,{1,Length[list]}];
elem=list[[pos]];
list=Drop[list,{pos}];
elem (* Return is not needed here *)
];
set={1,2,3,4};
RandomChoseElem[set]
set
Out[5]=
3
Out[6]=
{1,2,4}
Regards,
Jean-Marc