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MathGroup Archive 2005

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Re: Nested iterators in Compile

  • To: mathgroup at smc.vnet.net
  • Subject: [mg55045] Re: Nested iterators in Compile
  • From: Jon Harrop <usenet at jdh30.plus.com>
  • Date: Thu, 10 Mar 2005 05:25:06 -0500 (EST)
  • References: <d0mo9g$75h$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Maxim wrote:
> The fact that iterators work differently when used in Compile seems to
> lead to a serious limitation:

Yes, this may be a bug.

> The iterator {j, i} is evaluated before any value is assigned to i, and
> the evaluation of the compiled code fails (and if we add the
> initialization i=0 before Do, we'll only get an empty loop as the result,
> unlike the uncompiled version). So we have to resort to some workarounds
> such as changing the inner iterator to {j, n} and adding If[j>i,
> Continue[]] to the loop body, which of course decreases the performance.

You can nest your Do loops, giving a 21x performance improvement in this
case:

In[1] := Function[{n}, Module[{s = 0}, Do[s += j, {i, n}, {j, i}]; s]
[1000] // Timing
Out[1] = {2.33 Second, 167167000}

In[2] := Compile[{n}, Module[{s = 0}, Do[Do[s += j, {j, i}], {i, n}]; s]]
[1000] // Timing
Out[2] = {0.11 Second, 167167000}

-- 
Dr Jon D Harrop, Flying Frog Consultancy
http://www.ffconsultancy.com


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