MATLAB: Matrix linear indexing: how to add more than once to the same index

Question

Here is my question. Let's imagine we have this matrix:

A = [1 2 ; 3 4];

So if I use linear indexing to add 1 to, for instance, positions 1 and 3:

A([1 3]) = A([1 3]) + 1
A =
       2     3
       3     4

Now, let's imagine that, having the initial A matrix, I have an index vector in which some indices are repeated. For example, I need to add twice 1 to the first position, and once to the index 3:

A([1 3 1])=A([1 3 1])+1
A =
       2     3
       3     4

And surprisingly I get A(1,1)=2, instead of A(1,1)=3. How can I make it work?

The first idea was to put it inside a loop, but the matrix is normally big and this operation runs many times, and it becomes very slow. Any ideas?

Thank you very much in advance!

Answer 1

Another option, possibly faster than a loop or unique + histc:

A = [10 20;30 40];
idx = [1 3 1];
A(:) = A(:) + accumarray(idx(:), 1, [numel(A), 1]);