Re: Better Way of Testing and Replacing List Elements?

• To: mathgroup at smc.vnet.net
• Subject: [mg104039] Re: Better Way of Testing and Replacing List Elements?
• From: "E. Martin-Serrano" <eMartinSerrano at telefonica.net>
• Date: Fri, 16 Oct 2009 07:21:14 -0400 (EDT)

```Hi,

After several tries the faster seems to be xxx*UnitStep[xxx]

See:

In[1]:= xxx = RandomChoice[{-1, 1}, 1000000];

In[2]:= Length@Select[xxx, (# == -1) &]

Out[2]= 500002

In[3]:= Timing[ ReplacePart[xxx, Position[Map[Negative, xxx], True] -> 0];]

Out[3]= {2.672, Null}

In[4]:= Timing[(xxx /. _?Negative -> 0);]

Out[4]= {0.796, Null}

In[5]:= Timing[(xxx*UnitStep[xxx]);]

Out[5]= {0.063, Null}

In[6]:= Timing[Clip[xxx, {0, Infinity}];]

Out[6]= {0.171, Null}

In[7]:= Timing[(Max[#, 0] & /@ xxx);]

Out[7]= {1.406, Null}

In[8]:= Timing[(xxx /. (x_ /; x < 0) -> 0);]

Out[8]= {0.859, Null}

In[9]:= Timing[(xxx Unitize[Sign[xxx] + 1]);]

Out[9]= {0.078, Null}

Emilio

-----Original Message-----
From: Bill Rowe [mailto:readnews at sbcglobal.net]
Sent: Thursday, October 15, 2009 12:18 PM
To: mathgroup at smc.vnet.net
Subject: [mg104039] [mg104006] Re: Better Way of Testing and Replacing List Elements?

On 10/13/09 at 11:19 PM, careysub at gmail.com (careysub) wrote:

>The code below replaces the negative values in a list with zero, and
>is an example of a type of operation I use a lot:

>xxx = {1, 2, 3, 4, 5, -6, -7, 8, 9, 10, -1, 11, 12};
>ReplacePart[xxx, Position[Map[Negative, xxx], True] -> 0]

>Is there a "better" way of doing this (fewer function calls, more
>efficient)?

>My feeling is that I'm doing this in an awkward way.

Other ways would be:

In[1]:= xxx = {1, 2, 3, 4, 5, -6, -7, 8, 9, 10, -1, 11, 12};

In[2]:= Clip[xxx, {0, Infinity}]

Out[2]= {1,2,3,4,5,0,0,8,9,10,0,11,12}

In[3]:= xxx /. a_?Negative :> 0

Out[3]= {1,2,3,4,5,0,0,8,9,10,0,11,12}

In[4]:= xxx Unitize[Sign[xxx] + 1]

Out[4]= {1,2,3,4,5,0,0,8,9,10,0,11,12}

=46or small data sets, the timing of these won't be much
different. For large data sets, Clip is probably the fastest.

```

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