make efficient the copy of symmetric matrix in c#

Question

I want to store in an array a symmetric matrix

for a matrix I was doing this

    double[,] mat = new double[size,size];
    for (int i = 0; i < s         


        

Answer 1

Regarding the selected answer, unless I am being a complete idiot the code isn't correct:

Try this:

i = 2, j = 1

therefore we use:

k(i,j) = j*N-j*(j-1)/2+i

to find the index k, solving:

k(i,j) = 1*5 - 1*(1-1)/2 + 2

k(i,j) = 5 - 0 + 2 = 7

From the matrix in the selected answer we see that (2,1) is not 7, it seems to be 6. In fact (since this seems to be 0-base), 7 occurs at (3,1) or (1,3). The second formula for i > j seems to be inaccurate unless I am missing something.

UPDATE:

This seems to work if you alter the i > j formula to:

k(i,j) = j*(N-1)-j*(j-1)/2+i