How to vectorize this loop ?

I am a fortran user and i am currently switching to matlab. I read that to be efficient, it is better to vectorize than using for loops.

How would you vectorize this loop ?

for i=init:fina
 a(i,i+nf)=b(i)*c(i);
end

I tried this but it does not work as "a" is a matrix and "b" and "c" are two vectors ; how to include a vector which is not a row or a column in a matrix ?

i=init:fina;
a(i,i+nf)=b(i).*c(i);

And I have also the inverse problem...how to extract a vector from a matrix a(i,i+nf) to be computed with another one like this :

b(i)=a(i,i+nf).*c(i);

Last but not least, the vectorization in these cases will be more efficient than the for loop ?

Kind regards.

FP

[Jos (10584)]

• What are at the inputs (init, fina, nf, b, c)?
• Does a exist already? or is it created on the spot?
• Perhaps you can give a small example of the inputs and the expected output?

Answer 1

[Andrei Bobrov]

first

a = zeros(fina,fina + nf);
ii = init:fina;
a(sub2ind([fina,fina + nf], ii,ii + nf)) = b(ii).*c(ii);

second

d = diag(a,nf-1);
b(ii) = d(ii)./c(ii);

[frenchprion]

Great ! sub2ind and diag are the solutions !

Thanks !

FP