Re: AppendTo VERY slow
- To: mathgroup at smc.vnet.net
- Subject: [mg35323] Re: [mg35279] AppendTo VERY slow
- From: Andrzej Kozlowski <andrzej at platon.c.u-tokyo.ac.jp>
- Date: Mon, 8 Jul 2002 03:17:36 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
The answer is to learn to program in "Mathematica style", which
manipulates lists as wholes and in particular avoids commands that force
them to be re-computed. Append is one of these commands to be avoided.
Compare the timings of the following two programs:
In[2]:=
n=1000;xlist=Table[Random[],{n}];ylist=Table[Random[],{n}];outlist={};
your version:
In[3]:=
Do[ elem = {xlist[[count]], ylist[[count]]};
AppendTo[outlist, elem];
, {count, 1, n}
];//Timing
Out[3]=
{0.42 Second,Null}
The "Mathematica style" approach:
In[4]:=
s=Transpose[{xlist,ylist}];//Timing
Out[4]=
{0. Second,Null}
In[5]:=
s==outlist
Out[5]=
True
Andrzej Kozlowski
Toyama International University
JAPAN
http://platon.c.u-tokyo.ac.jp/andrzej/
On Saturday, July 6, 2002, at 06:44 PM, Mike wrote:
> I use lists a lot in mathematica and tend to use AppendTo[] a lot in
> my programs. Recently I wrote a function that i call over and over
> again and found that the results were coming very slowly and i MEAN
> slowly. I was doing Fourier Transforms and all kinds of stuff so I
> put it down to those at first but I have just put in a load of Print
> statements just after each part of the function to see what was taking
> so long.
>
> I was amazed to see that the Fourier Transforms were so quick and what
> was actually taking the time was a part of my function that collected
> the results togther in the form I wanted and outputted the result. It
> looks like this
>
> Do[
> elem = {xlist[[count]], ylist[[count]]]};
> AppendTo[outlist, elem];
> , {count, 1, number}
> ];
>
> It seems that as the list grows it gets slower and slower. Any tips
> on a way around this would be greatly appreciated (would speed my life
> up no end)
>
>
> Thank
>
> Mike
>
>
>