Problem 876. Sum the entries of each column of a matrix which satisfy a logical condition.

Given a numeric matrix A and a logical array L of the same size as A, return a row vector S containing the columnwise sums of the entries of A which satisfy L (i.e. where there is a true value in the logical array L). If L contains a column of false values, then the corresponding entry of S should be zero.

Examples:

1) If A = [1, 2; 3, 4] and L = [true false; false true], then S should be the row vector [1 4].

2) If A = triu(ones(3)) and L = logical(A), then S should be the row vector [1 2 3].

3) If A = repmat(1:3, 3, 1) and L = [false(3,1), true(3, 2)], then S should be the row vector [0 6 9].