MATLAB: Vector confusion

Question

I have a vector A whose values are indices for vector P.

Say A = 2 occurs 10 times.

If I do:

P(A) = P(A) + 1

P(2) = 1 % not 10

I believe I understand why this is happening. Is there a solution without going into FOR loops? A could be quite large.

Answer 1

Especially if you are starting with an all-zero array, use

P = accumarray(A(:),1);