Any way for matlab to sum an array/matrix rows according to specified bins NOT by "for" iteration? Best if there is buildin function for this

For example, if

A = [7,8,1,1,2,2,2]; % the bins
B = [2,1,1,1,1,1,2]; % the array

then the desired function "binsum" has two outputs, one is the bins, and the other is the sum. It is just adding values in B according to subscripts in A. For example, for 2, the sum is 1 + 1 + 2 = 4, for 1 it is 1 + 1 = 2. So in output, 1 in "bins" corresponds to 2 in "sum", and 2 in "bins" corresponds to 4 in "sums".

[bins, sums] = binsum(A,B);

bins = [1,2,7,8]
sums = [2,4,2,1]

The elements in "bins" need not be ordered but must correspond to elements in "sums". This can surely be done by "for" iterations, but "for" iteration is not desired, because there is a performance concern. It is best if there is a build in function for this.

More generally, if

    A = [7,8,1,1,2,2,2]; % the bins or subscripts
    B = [2,1; ...
         1,1; ...
         1,1; ...
         2,0; ...
         3,1; ...
         0,2; ...
         2,4]; % the matrix

then the desired function "binsum" has two outputs, one is the bins, and the other is the accumulated row vectors. It is adding rows in B according to subscripts in A. For example, for 2, the sum is [3,1] + [0,2] + [2,4] = [5,6], for 1 it is [1,1] + [2,0] = [3,1].

   [bins, sums] = binsum(A,B);

    bins = [1,2,7,8]
    sums = [2,1;
            1,1;
            3,1;
            5,6]

The first method [accumarray][1] says its "val" argument can only be a scalar or vector. The second method spare seems not to accept a vector as the value "v" for each tuple (i,j) neither. So I have to post for help again, and it is still not desired to use iterations to go over the columns of B to do this.

Thanks a lot!

Answer 1

[Walter Roberson]

unique() the bin numbers and use the third output of that as the first input to accumarray with the second input being B(:). Final output is the first output of the unique() as the bin numbers and the output of accumarray as the counts.