Re: Nested iterators in Compile
- To: mathgroup at smc.vnet.net
- Subject: [mg55027] Re: [mg55010] Nested iterators in Compile
- From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
- Date: Thu, 10 Mar 2005 05:24:26 -0500 (EST)
- References: <200503091134.GAA07027@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
On 9 Mar 2005, at 12:34, Maxim wrote:
> The fact that iterators work differently when used in Compile seems to
> lead to a serious limitation:
>
> In[1]:=
> Compile[{n},
> Module[{s = 0},
> Do[s += j, {i, n}, {j, i}];
> s
> ]][10]
>
> CompiledFunction::cfse: Compiled expression i should be a machine-size
> real number.
>
> CompiledFunction::cfex: External evaluation error at instruction 4;
> proceeding with uncompiled evaluation.
>
> Out[1]=
> 220
>
> 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.
>
> Maxim Rytin
> m.r at inbox.ru
>
>
I am not sure how to tell if performance is slower than a hypothetical
(i.e. non-working) alternative but:
n = 1000;
Module[{s = 0},
Do[s += j, {i, n}, {j, i}];
s
]//Timing
{3.81 Second,167167000}
vs
Compile[{n},
Module[{s = 0},
Do[ Do[s += j, {j, i}],{i,n}];
s
]][1000]//Timing
{0.2 Second,167167000}
suggests to me the the limitation you are referring to may not be that
bad.
Andrzej Kozlowski
Chiba, Japan
http://www.akikoz.net/andrzej/index.html
http://www.mimuw.edu.pl/~akoz/
- References:
- Nested iterators in Compile
- From: Maxim <ab_def@prontomail.com>
- Nested iterators in Compile