Very slow for loop
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Hi, I am new to Matlab, i have more experience with C/C++
I am trying run this simple loop:
x=zeros([10001,10001,2]);
for t=1:2
for z =1:10000
for i =1:10000
x(z,i,t)=i+z+t+2;
end
end
end
x(5,5,1)
But is taking a while..imagine many of then inside a single algorithm. in C++ this loop runs in a few seconds...
Is there any other way to run this loop in Matlab faster???
Sorry if my question is silly, first time running Matlab. I tried some vectorization but for nested loops things get really complicated, especially with different loop sizes.
Thanks
3 commentaires
Daniel Shub
le 31 Déc 2011
Why should that be any faster/slower than 1:10000? The variable x is already intialized to a size that handles either case.
Andrew Newell
le 31 Déc 2011
The point of my question is that the values in each row and column increase until they suddenly drop to zero at the end. I'm guessing that is not what Rafael intended.
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Daniel Shub
le 31 Déc 2011
You might be able to get a slight performance boost by changing how the memory is acessed ...
Basically x(z,i,t) might not be the same as x(t,z,i) or x(i,z,t).
Plus de réponses (3)
the cyclist
le 1 Jan 2012
x = bsxfun(@plus,bsxfun(@plus,1:10000,(1:10000)'),permute(1:2,[1 3 2]))+2;
Andrew Newell
le 31 Déc 2011
For a problem where the numbers count upward monotonically in each direction, here is a compact and fast solution:
y = repmat(2:10002,10001,1);
x = cat(3,y+y',y+y'+1);
On my computer, your code takes about 25 seconds and mine takes 6 seconds.
2 commentaires
Andrew Newell
le 1 Jan 2012
I think that you mostly can leave loops in because MATLAB uses just-in-time compiling on loops. However, as far as I know this compiling isn't done on arrays of dimension greater than 2.
Jan
le 1 Jan 2012
If the shown code is your original problem, and not a simplification, I'm in doubt, that it is an efficient approach: You occupy 1.6GB memory with values, which are very easy to calculate dynamically.
Note: The reference "x(z,i,t)" requires two multiplications, while "i+z+t+2" uses 3 additions only.
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