MATLAB: How to sum over rows within specific ranges without for loop

intervalsmatrixrangessumvectorization

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!

Best Answer

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).')
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