efficient way to represent a lower/upper triangular matrix

I am working on my data in a C/C++ program, which is 2 dimensional. Here my value is calculated for pair-wise and here values would be same for foo[i][j] and foo[j][i].

Thus if I implement it by using a simple 2 dimensional array, half of my space would be wasted. So what would be best data structure to represent this lower/upper triangular matrix.

Regards,