MATLAB: Create a vector summing different elements from another vector

Question

Hi,

I'm trying to do the following. Suppose you have a vector A=ones(10,1) and another vector id=[1 1 1 2 2 3 3 3 3 3]'. I would like to create a vector C that should sum the elements of A according to the vector id. The dimmensions of C should be 10 times 1, so C should be C=[3 3 3 2 2 5 5 5 5 5]'. With C=accumarray(id,A) I get C=[3 2 5]', but I don't know how to expand it to get the write dimmensions. This is just an example. In the real case the dimmensions of A are 40000 times 1 and id takes values from 1 to 12000. I was able to do some manipulations with repmat that implied generating a matrix to use id, but in my computer this only works up to 20000 rows (at that point the matrix is of dimmensions 20000 times 12000) and I get a message saying "??? Out of memory", so I need to stay in the vector case. Thanks,

Fernando

Answer 1

Index the result of accumarray with the id in order to get the extended vector you are looking for.