How to sum over rows within specific ranges without for loop?

I would like to sum up rows over specific ranges specified by an array. I do not know if there is a vectorised solution to it.

I have a matrix A say

A = [1,0,0,2;
     0,3,1,0;
     1,2,3,4;
     0,1,0,1;
     1,0,0,1];
 

I have an array B specifying the ranges of rows I want to sum up:

B = [2;
     1;
     2];

B shows that I would like to sum up the first 2 rows, and then the next 1 row, and then the next 2 rows. sum(B) equals to the total number of rows in A.

The output C has the same number of rows as B and the same number of columns as A:

C = [1, 3, 1, 2;
     1, 2, 3, 4;
     1, 1, 0, 2];
 

Many thanks!

Answer 1

[Fabio Freschi]

Not sure if it is the best way, but this seems to work

 index = [0; cumsum(B)];
 C = cell2mat(arrayfun(@(i)sum(A(index(i)+1:index(i+1),:),1),1:length(B),'UniformOutput',false).')

[Andy Wu]

I see. Thank you, Fabio. Wondering if there is better ways to do it. The run time based on a 1e5 * 100 matrix is similar with using loops.