MATLAB: Array indexing question for vectorization

Question

I have three arrays A, B, and IND, with A containing some values that I want to update, B containing the values that I want to use to update A, and IND containing the indeces to update in A. IND may reference the same index in A several times. For example, I may have:

A = ones(1, 10);
B = 1:10;
IND = [1 1 2 5 5 5 5 6 8 9];

and the update I want to do is multiplication. I can achieve this by doing

for ii = 1:10
  A(IND(ii)) = A(IND(ii)) * B(ii);
end

which gives me my desired answer of

A =
   2     3     1     1   840     8     1     9    10     1

Is there a way I can do this in a vectorized operation, avoiding the for loop (I'm potentially doing this kind of operation thousands of times on large arrays)? Doing

A(IND) = A(IND) .* B

results in

A =
   2     3     1     1     7     8     1     9    10     1

Any tips greatly appreciated!

Answer 1

You can try the following trick

c = unique(IND)
d = accumarray(IND',B',[],@prod)
A(c) = A(c).*d(c)'