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Re: Pass by reference for compiled functions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg114281] Re: Pass by reference for compiled functions
  • From: Daniel Lichtblau <danl at wolfram.com>
  • Date: Tue, 30 Nov 2010 04:01:43 -0500 (EST)

Eric Michielssen wrote:
> For noncompiled functions, one can pass arguments "by reference" and then
> modify them inside the routine by assigning the function the attribute
> HoldAll.
> 
> This does not work for compiled functions: it does not appear possible to
> modify arguments to compiled functions, even if they are passed as "global
> variables". That is, Mathematica responds to
> 
> Myfunction = Compile[
> {{var1,_Real}},
> Module[{}, 
> Modify var 1 or var2;
> Return[]], {var2,_Real}];
> 
> With
> 
> Compile::argset: The assignment to var1 is illegal; it is not valid to
> assign a value to an argument. >>  And something similar for var2.
> 
> HoldAll does not seem to apply here...
> 
> This makes it memory-inefficient to modify large arrays in compiled
> functions, as one would have to operate on a copy of the array.
> 
> Is there a workaround in Mathematica 8?
> 
> Eric Michielssen

Not to my knowledge. I agree this mutability would be a really nice 
capability to have. I have wanted it myself since almost forever. Well, 
at least since the time I posted the message at the URL below.

http://forums.wolfram.com/mathgroup/archive/2001/Feb/msg00195.html

We have discussed the desire for call-by-reference, or something else 
that might allow for mutability of arrays, within Compile. People here 
seem toa gree it could be very useful. But I do not know whether we will 
attempt to go in this direction, or what might be the design or 
implementation issues (I suspect neither is trivial).

One thing that might be new to version 8 is an ability to pass large 
arrays in Compile and not have them automatically copied. It is only 
when one needs to alter them that a copy is needed. So it is possible to 
invoke what amount to subroutines (that is, Compile calling functions 
that are also defined via Compile), using parge arrays as arguments, and 
not suffering a time or memory penalty. But again, this only holds if 
the arrays are not to be modified. I'm told some code along these lines 
might appear in an upcoming Wolfram blog.


Daniel Lichtbau
Wolfram Research




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