Access 2D matrix using 2 arrays of coords

hi, i'm having troubles working with indexes. Suppose i have this matrix (wich represent a 25x25):

matrix =

     (4,6)      -0.0000
     (6,8)      -1.0000
     (2,9)      -1.0000
     (4,9)      -1.0000
     (2,10)     -0.0000
     (1,11)     -0.0000
     (5,11)     -1.0000
     (7,11)     -1.0000
     (4,12)     -1.0000
     (8,12)     -0.0000
     (3,13)     -0.0000
     (6,13)     -0.0000
     (7,13)     -1.0000
     (2,14)     -0.0000
    (11,14)     -0.0000
     (1,15)     -1.0000
     (3,15)     -1.0000
     (7,15)     -0.0000
     (9,16)     -0.0000
    (10,16)     -1.0000
     (2,17)     -1.0000
    (10,18)     -1.0000
    (17,18)     -0.0000
     (4,19)     -0.0000
    (16,19)     -1.0000
     (3,20)     -1.0000
     (8,21)     -0.0000
    (13,21)     -1.0000
    (20,21)     -0.0000
     (5,22)     -0.0000
    (14,22)     -1.0000
    (17,22)     -0.0000
     (6,23)     -1.0000
     (7,23)     -0.0000
     (9,23)     -0.0000
    (14,23)     -1.0000
    (12,24)     -0.0000
    (19,24)     -1.0000
     (1,25)     -1.0000
     (5,25)     -0.0000

now let's say i want to create the diagonal as the sum of each row so that matrix[1][1] = sum(matrix(1,:)), matrix[2][2] = sum(matrix(2,:)) etc... problem is: I'm using an upper triangular matrix to act like a simmetric one ( space is of the essence). In this condition, the use of sum as before, takes only half the values i need to be added. My idea to work around this problem was to switch (i,j) -> (j,i) (wich outside of matlab would be a piece of cake), but had no luck with my trials so far.

the optimum will be something like:

for i=0 : rows_of_my_matrix
matrix[i][i] = sum(matrix(i,:)); %sums the [i][j] non zero elements of the row i in  [i][i]
% something that sums the [i][j] non zero elements of the row i in [j][j]
end

can someone help my with this ? seems quite simple but i can't wrap my head around it (yes, i'm quite new to matlab)

Answer 1

[Walter Roberson]

for K = 1 : number_of_rows
   total_right = sum(matrix(K,K:end));
   total_left = sum(matrix(1:K-1,K));
   matrix(K,K) = total_left + total_right;
end