How to vectorise this simple assignment loop in matlab

Question

I have a vector v of values and a vector r of indices. I want to store the values in a matrix m as follows:

for i = 1:length(v)
    m(i, r(i)) = v(i);
end

What is the fastest way to do this in a vectorized way?

Aabaz · Accepted Answer · 2012-01-09 12:41:06Z

2

I do not know if it is faster, I suppose so but the difference might be very small, but here is one way:

m(sub2ind(size(m),1:length(v),r(1:length(v))))=v;

If r is a column vector then sub2ind will complain about vectors size, you can just take its transpose and it will solve this.

answered Jan 9, 2012 at 12:30

Aabaz

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1 Comment

If r and v are the same length, then r(1:length(v)) simplifies to just r.