How to add repeated elements to a vector without looping?

I have the following code:

a = zeros(1, 4);
a([1 2 4 3 2 1 1]) = a([1 2 4 3 2 1 1]) + [1 1 1 1 1 1 1];

This gives a == [1 1 1 1]. What I would want is a similar operation that gives a = [3 2 1 1], basically add 1 to each element of the selection, everytime it is in the selection. This would be easy to do in a for loop:

a = zeros(1, 4);
selection = [1 2 4 3 2 1 1];
addVec = [1 1 1 1 1 1 1];
for i = 1:length(selection)
    a(selection(i)) = a(selection(i)) + addVec(i);
end

But this is not efficient. I'm looking for a vectorized version of this.

Thank you

[Stephen23]

Your first attempt is described here:

https://blogs.mathworks.com/loren/2007/07/26/assignment-with-repeated-indices/

This blog describes the three ways to perform accumulation like you want (loop, sparse array, and accumarray):

https://blogs.mathworks.com/loren/2020/01/31/repeated-indexing-in-matlab/

Answer 1

[Bruno Luong]

accumarray([1 2 4 3 2 1 1]', 1)'

Put [1 1 1 1 1 1 1]' as second argument if the values to accumulate are not uniform.

(important the quotes, especially the inner one)

ans =

     3     2     1     1

[Alexander Winter]

Thank you very much!