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Re: pass by reference

  • To: mathgroup at smc.vnet.net
  • Subject: [mg93735] Re: pass by reference
  • From: Albert Retey <awnl at gmx-topmail.de>
  • Date: Mon, 24 Nov 2008 04:14:37 -0500 (EST)
  • References: <gg8p9s$jsp$1@smc.vnet.net>

Hi,

> How do you pass by reference in Mathematica?

What you can do is to prevent arguments from being evaluated with using
one of the Hold attributes for the functions. So you can pass symbols
instead of the expression they evaluate to, which effectively is like
pass by reference. These are the definitions you could make for your
example:

ClearAll[Push, Pop]

SetAttributes[{Push, Pop}, HoldFirst]

Push[stack_, val_] := PrependTo[stack, val]

Pop[stack_] :=
  Module[{r = First[stack]}, stack = Rest[stack]; r];

And this is how you could use these definitions:

stack = {1, 2, 3}

Push[stack, 5]

Pop[stack]

stack


You still could give any kind of expression as first argument, but then
the evaluations of the RHS will fail, e.g.:

Push[{1,3,4}+a^5,5]

so it makes sense to restrict the function definitions to only accept
symbols as arguments:

Push[stack_Symbol, val_] := PrependTo[stack, val]

Pop[stack_Symbol] :=
  Module[{r = First[stack]}, stack = Rest[stack]; r];

hth,

albert


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