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MathGroup Archive 2005

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Re: mathematica expressions in Compile

  • To: mathgroup at smc.vnet.net
  • Subject: [mg62006] Re: mathematica expressions in Compile
  • From: Peter Pein <petsie at dordos.net>
  • Date: Wed, 9 Nov 2005 03:45:20 -0500 (EST)
  • References: <dkpovc$reu$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

boyandshark schrieb:
> Hello -
> 
> I was wondering what is happening in the following example :
> 
> Compile[{}, Module[{i =  0, nums}, nums = {0, 0, 0}; i =
> First[Ordering[{1., 2., 3.}]]; nums[[i]]++; nums]];
> 
> I get an error when trying to evaluate this statement because
> Mathematica complains about assigning i the value of the
> First[Ordering[_]] expression - Compile seems to think
> First[Ordering[_]] in this example is a real number whereas I
> initialized i to be an integer. I can solve this simply by
> 
> Compile[{}, Module[{i =  0, nums}, nums = {0, 0, 0}; i =
> Round[First[Ordering[{1., 2., 3.}]]]; nums[[i]]++; nums]];
> 
> My question is - how come Compile knows that the value of Round is an
> integer, but does not know that the First[Ordering[_]] is an integer?
> 
> Thank you.
> 
Hi,

Compile *should* know the return type of Ordering, but obviously 
doesn't. You have to tell Compile about this:

Compile[{}, Module[{i = 0, nums = {0, 0, 0}},
   i = First[Ordering[{1., 2., 3.}]];
   nums[[i]]++; nums],
{{_Ordering, _Integer, 1}}][]

gives {1,0,0} without error- or warning-message.

Peter


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