How to Iterate over a Symmetric Table?

By Kardi Teknomo

This is a very simple tutorial on programming for fresh programmers. The problem goes like this:

You have a symmetric table or matrix size by and you want to do something (i.e. evaluate or simply print) based on the content of the matrix. Because the matrix is symmetric, you do not need to iterate over the whole matrix. You can iterate over upper triangular part only or lower triangular part only and get the same results. How to do that in the program?

Solution:

To be precise, you have square symmetric matrix

and you want to iterate over …

•  Upper triangular matrix without diagonal, or

•  Upper triangular matrix with diagonal, or

•  Lower triangular matrix without diagonal, or

•  Lower triangular matrix with diagonal

Pseudo Code: 

For row = 1 to (q-1) 

     For col = (row + 1) to q 

          Print A (row, col) ‘// do something 

     Next col 

Next row 

Pseudo Code: 

For row = 1 to q 

     For col = row to q 

          Print A (row, col) ‘// do something 

     Next col 

Next row 

Pseudo Code: 

For col = 1 to (q-1) 

     For row = (col + 1) to q 

          Print A (row, col) ‘// do something 

     Next col 

Next row 

Pseudo Code: 

For col = 1 to q 

     For row = col to q 

          Print A (row, col) ‘// do something 

     Next col 

Next row 

See Also: VB Tutorial