Vectorizing a simple for loop

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Karim Alame
Karim Alame 2011-12-3
Hi everyone, I'm not sure if people asked this before but I tried googling my question before posting.
so i know for this type of loop for example:
for i = 2:n-1
for j = 2:n-1
q(i,j) = a*u(i,j)
end
end
can be simply expressed as: q((2:n-1),(2:n-1)) = a*u((2:n-1),(2:n-1));
what if my expression was as follows: for i = 2:n-1 for j = 2:n-1 q(i,j) = a*u(i+1,j) end end
how could I incorporate the "i+1" expression into vectorizing.
Thank you for your help

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Jan
Jan 2011-12-3
q(2:n-1, 2:n-1) = a*u(3:n, 2:n-1);
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Karim Alame
Karim Alame 2011-12-5
@Jan: Thank you , I ran it again this time and it worked. Interesting thing is I ran it for a 501x501 nodes and the for loop was quicker. The code I posted above is within a larger k loop that updates X=Y.
Thanks again for your help.
Jan
Jan 2011-12-5
The vectorization suffers from the need of large temporary arrays. Allocating memory for "q(2:(nx-1),2:(ny-1))" etc is very expensive, while the multiplication by a scalar is very cheap. Therefore the loop will be more efficient for large inputs.

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