MATLAB: Sum of specific elements in a column matrix

summation

Say I have a column matrix [1;2;3;4;5;6;7;8;9]. I want to add the elements from row 3 to row 9.

So the result will come as 42

a=[1;2;3;4;5;6;7;8;9];
sum(a(3:size(a,1)))     % size(a,1) can be whichever element you wish

command window:

ans =
    42

>> fliplr(cumsum(fliplr(A)))
ans =
   9   8   8   7   6   6   5   5   4   4   4   3   2   1

Try this:

rowsToSum = A(:, 2) < 0;
theSum = sum(A(rowsToSum, 1)); % Sum first column where second column is negative.