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MathGroup Archive 2002

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RE: AppendTo VERY slow

  • To: mathgroup at smc.vnet.net
  • Subject: [mg35303] RE: [mg35279] AppendTo VERY slow
  • From: "Harvey P. Dale" <hpd1 at nyu.edu>
  • Date: Mon, 8 Jul 2002 03:15:29 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Mike:
	This is a well-known problem.  David Wagner wrote about it under a
heading, "Avoid Append and Prepend," in Power Programming with Mathematica:
The Kernel.  He also suggested several ways around the problem.  For your
purposes, his suggestion of using a linked-list structure is probably best.
To do that, make each existing list the first element and each new value the
second element of the new list.  For example, if the values are 1, 2, 3, 4,
5, and 6, make the first list {1}, the second list {{1},2}, the third list
{{{1},2},3}, and so forth up to {{{{{{1},2},3},4},5},6}.  Then, when you're
done, Flatten the final list.
	Hope that helps.
	Best,
	Harvey
Harvey P. Dale
University Professor of Philanthropy and the Law
Director, National Center on Philanthropy and the Law
New York University School of Law
Room 206A
110 West 3rd Street
New York, N.Y. 10012-1074

-----Original Message-----
From: M.P.Croucher at Sheffield.ac.uk [mailto:M.P.Croucher at Sheffield.ac.uk] 
To: mathgroup at smc.vnet.net
Subject: [mg35303] [mg35279] AppendTo VERY slow

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

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