Re: Call-by-reference from inside a function
- To: mathgroup at smc.vnet.net
- Subject: [mg72580] Re: Call-by-reference from inside a function
- From: albert <awnl at arcor.de>
- Date: Wed, 10 Jan 2007 03:07:03 -0500 (EST)
- References: <eo012n$kp1$1@smc.vnet.net>
Hi Istvan,
> 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?
What you are probably looking for is the attribute HoldAll or HoldFirst. Is
the following doing what you want and user-friendly enough?
SetAttributes[RandomChoseElem, HoldFirst];
RandomChoseElem[list_?(MatchQ[#, _List] &)] := Module[{
pos, elem
},
pos = Random[Integer, {1, Length[list]}];
elem = list[[pos]];
list = Delete[list, pos];
Return[elem]
];
you probably want to define a reasonable return value for the empty list...
hth,
albert