Re: Replace, ReplaceAll and If time performace comparition

• To: mathgroup at smc.vnet.net
• Subject: [mg127025] Re: Replace, ReplaceAll and If time performace comparition
• From: Andrzej Kozlowski <akozlowski at gmail.com>
• Date: Mon, 25 Jun 2012 04:01:00 -0400 (EDT)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <201206240826.EAA08423@smc.vnet.net>

```On 24 Jun 2012, at 10:26, Murta wrote:

> HI all
> I was working in some simulations with random numbers and get this example of performance comparition.
>
> randomList = RandomInteger[{-100, 100}, 10 10^6];
>
> (randomList /. (x_ /; x < 0 -> 0)); // AbsoluteTiming
> {5.747133, Null}
>
> Replace[randomList, (x_ /; x < 0 -> 0), 1]; // AbsoluteTiming
> {4.758984, Null}
>
> (If[# < 0, 0, #] & /@ randomList); // AbsoluteTiming
> {0.572200, Null}
>
> I personally prefer work with patterns because they are more compact and functional.
> Someone knows why patter is one magnitude order slow?? There is some trick to make it faster?
>
> tks
> Murta
>

Purpose built-in functions will always be much faster than general methods (such as pattern matching). However, in this particular your fastest method is actually far from optional:

In[1]:= randomList=RandomInteger[{-100,100},10 10^6];
In[2]:= (a=If[#<0,0,#]&/@randomList);//AbsoluteTiming
Out[2]= {0.874206,Null}
In[3]:= (b=Clip[randomList,{0,Infinity},{0,1}]);//AbsoluteTiming
Out[3]= {0.064867,Null}
In[4]:= a==b
Out[4]= True

There is no way to speed up pattern matching to give comparable performance.

Andrzej Kozlowski

```

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