Improving code in c (sparse matrix representation)

Question

I made this piece of code written in C which takes too long to run. Is there any way to improve it? What i want to do is to sum the values of each row and save the value in a vector. In this code, i1 is the value containing the row locations of a matrix, the columns and the associated value. i1 is not sorted.

while(a < 2*var)
{
    for (int c=0; c < 2*var; c++)
    {
        if (i1[c][0] == a)
        {
            diag[b] += i1[c][2];
        }
    }
    a = a+1;
    b = b+1;
}

Any idea or suggestion will be greatly appreciated. Thank you.

Answer 1

As b = a + const, you could simply use diag[i1[c][0] + const] += i1[c][2] and reduce the complexity from O(N²) to O(N).