MATLAB: Fastest way to substitute elements in a matrix at given positions

Question

Good evening,

suppose I have an array A with size (n,m) and an array B with size (n,l), with l<m.

Suppose elements of array B point to elements of array A in the following fashion:

• B(x,y) –> A(x, B(x,y))

What is the fastest way to substitute all elements of A that are pointed by elements of B with a given value? At present I am using for loops but I guess they are not very efficient.

Also, would it be different if matrix A would be generated by repetition of the same known row n times?

Thanks a lot for any help or hint you are willing to give!!

Answer 1

idx = sub2ind([n,m], repmat((1:n).',1,l)  ,B);
A(idx)=value;