Why is vertcat slowing things down?
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In the code below, 10,000 repetitions takes 2 seconds... and 100,000 repetitions takes 60 seconds! (I was expecting 20 seconds or less)...
What is going on? by a process of elimination, I suspect "vertcat" is slowing things down...
(Please see how "tic toc" behaves with repetitions = 10,000 and 100,000)
Is this a known issue with vertcat? If so, what are my other options?
(Note: looping the 10,000 algorithm 10 times would be much faster... but surely there has to be a more elegant way to do 100,000 repetitions in 20 seconds)
Thanks
Doron
*********
tic
repetitions = 10000
llim = -1;
ulim = 3;
xstack = [];
xlifestack = [];
for i = 1:repetitions
x = 0;
xlife = 0;
while ulim > x && x > llim
x = x + normrnd(0,1);
xlife = xlife + 1; % "...and the walk lives one more step"
end
if x > llim
x = ulim;
else x = llim;
end
xstack = vertcat(xstack, x);
xlifestack = vertcat(xlifestack, xlife);
end
xbar = mean(xstack);
xlifebar = mean(xlifestack);
toc
0 Comments
Accepted Answer
Honglei Chen
on 23 Feb 2012
My 2 cents::
1. You may want to preallocate the memory. The in each iteration, you just put the new data into corresponding slots.
2. If your entire purpose is to compute the mean, then why not just record the sum and number of samples and update them for each iteration. These are just scalars.
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