How to vectorize for loop with custom index

Question

I'm new to Matlab, so I'm not sure if this is possible. I have a simple for-loop:

for i=1:n
    B.x(indexB(i)) += A.x(i);
end

Where A.x and B.x are two vectors of length n, and indexB is a vector of length n that contains the appropriate mapping from elements in A.x to B.x.

Is it possible to vectorize this loop?

Maurits · Accepted Answer · 2012-10-02 22:30:09Z

8

I think so, following this example:

a = [1 2 3 4 5];
b = a;
idx = [5 4 3 2 1];
a(idx)  = a(idx) + b(1:5);

Which should give:

a =

 6     6     6     6     6

So in your case, if indexB has size n you can write:

B.x(indexB) = B.x(indexB) + A.x(1:n);

And otherwise:

B.x(indexB(1:n)) = B.x(indexB(1:n)) + A.x(1:n);

edited Oct 2, 2012 at 22:30

answered Oct 2, 2012 at 22:00

Maurits

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2 Comments

Wesley Tansey Over a year ago

Would this still work if both A and B were not the same length?

Maurits Over a year ago

Yes, but there has to be an one-to-one mapping between the indices left and right, or in this case, the index vector indexB used for B.x and the index vector [1:n] used for A.x. And obviously, the elements the indices refer to, need to exist.

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