RE: AppendTo VERY slow
- To: mathgroup at smc.vnet.net
- Subject: [mg35315] RE: [mg35279] AppendTo VERY slow
- From: "tgarza01 at prodigy.net.mx" <tgarza01 at prodigy.net.mx>
- Date: Mon, 8 Jul 2002 03:16:09 -0400 (EDT)
- Reply-to: tgarza01 at prodigy.net.mx
- Sender: owner-wri-mathgroup at wolfram.com
Indeed, AppendTo is an awfully slow function. It takes exponentially longer
to execute with the list size. I would suggest a simple outlist = {outlist,
elem} with a Flatten and Partition at the end. For example,
In[1]:=
xlist=Table[Random[Integer,{0,9}],{10000}];
ylist=Table[Random[Integer,{0,9}],{10000}];
I define a function to do things with Append To:
In[2]:=
app[n_]:=(
outlist={};Do[elem={xlist[[i]],ylist[[i]]};
AppendTo[outlist,elem],{i,1,n}
])
In[3]:=
app[10000]//Timing
Out[3]=
{45.37 Second,Null}
It takes 45 seconds to process 10,000 elements. Now, with the approach I
suggest, I define the function f (in order to use Fold, a very powerful
function):
In[4]:=
f[x_,n_]:=outlist={outlist,{xlist[[n]],ylist[[n]]}}
In[5]:=
outlist={};Partition[Flatten[Fold[f,{},Range[10000]]],2];//Timing
Out[5]=
{0.49 Second,Null}
which produces the same result almost 100 times faster.
Tomas Garza
Mexico City
Original Message:
-----------------
From: M.P.Croucher at Sheffield.ac.uk (Mike)
To: mathgroup at smc.vnet.net
Subject: [mg35315] [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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