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Re: Re: fast way of appending x zeros to a list?

• To: mathgroup at smc.vnet.net
• Subject: [mg79178] Re: [mg79106] Re: fast way of appending x zeros to a list?
• From: Murray Eisenberg <murray at math.umass.edu>
• Date: Fri, 20 Jul 2007 03:29:24 -0400 (EDT)
• Organization: Mathematics & Statistics, Univ. of Mass./Amherst
• References: <f7ke8n$508$1@smc.vnet.net> <200707190721.DAA01175@smc.vnet.net>
• Reply-to: murray at math.umass.edu

Why not use Join or PadRight instead of Do?  They seem to be faster. 
Here are timings in Mathematica 6.0.1 (under Windows XP):

   data1=data2=data3=data=Table[RandomReal[],{100000}];
   k=100000;
   (Do[data1={0,data1},{k}];Flatten[data1];)//Timing
   data2=Join[data,Table[0,{k}]];//Timing
   data3=PadRight[data,k];//Timing
{0.156,Null}
{0.031,Null}
{0.016,Null}


Peter Breitfeld wrote:
> kristoph schrieb:
>> Dear all,
>>
>> I'm looking for a fast way to append 0's to a list. Unfortunately, I
>> append a variable number of 0's to a list which is generated by a
>> For[...]-loop. Therefore, PadLeft[...] is not variable enough to
>> append exactly x zeros to the list.
>>
>> At the moment I use: Do[PrependTo[data,0],{K}] where the K depends on
>> certain factors.
> 
> AppendTo and PrependTo are known to be very slow. It's better to
> build up a nested list an Flatten it:
> 
> Do[data={0,data},{K}]; Flatten[data]
> 
> Gruss Peter

-- 
Murray Eisenberg                     murray at math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower      phone 413 549-1020 (H)
University of Massachusetts                413 545-2859 (W)
710 North Pleasant Street            fax   413 545-1801
Amherst, MA 01003-9305

• References:
  • Re: fast way of appending x zeros to a list?
    • From: Peter Breitfeld <phbrf@t-online.de>