MATLAB: How to calculate conditional sum of a part of vector/array based on a 2nd one

Question

Hello,

I want to get the conditional sum of part of a vector based on a second vector.

For example:

A = [143 14 -4 299 -29 83 151 190 178 -61 -109 ]

and

B = [1 1 -1 1 -1 1 1 1 1 -1 -1 ]

then I want to get the output:

C=[0 157 -4 299 -29 0 0 0 602 -170 ]

and

D=[0 2 1 1 1 0 0 0 4 0 2 ]

The idea is, when successive B's = 1, I add the corresponding values from A and store in C. Similarly if successive B's = -1, I add the values up and store in C. If more than 1 value in A is added, I fill the gaps in C with 0's. It would also be great if I could get a seperate array D mentioning how many values in A have been added to produce the value in C!

Please help! Thanks a lot!

Answer 1

Just for comparison, here is a FOR loop solution. FOR loops can actually be very fast, even for large arrays, especially when you are doing simple operations like this.

C = zeros(size(A));
D = zeros(size(A));
previous = B(1);
runningSum = A(1);
numValues = 1;
for n = 2:numel(B)
    if B(n) == previous
        runningSum = runningSum + A(n);
        numValues = numValues+1;
    else
        C(n-1) = runningSum;
        D(n-1) = numValues;
        runningSum = A(n);
        numValues = 1;
        previous = B(n);
    end
end
C(end) = runningSum;
D(end) = numValues;