Matrix Loop Techniques

Lets image we have in input a Matrix (m,n) rows and columns  in the format of a 2D array.

int matrix[m][n] = { 
                    { 1, 3, 7, 0, 4 },
                    { 9, 2, 5, 2, 8 },
                    { 6, 2, 3, 4, 1 },
                    { 1, 2, 3, 4, 5 },
                    { 8, 3, 7, 4, 5 }
                 };

Simple Loop

A simple row by row loop

for (int r = 0; r < rows; r++)
    {
        for (int c = 0; c < clms; c++)
        {
            // DO SOMETHING ON THE ELEMENT (i,j)
            matrix[r][c];
        }
    }

Columns Loop

    //  Traverse through all columns
    for (int c = 0; c < rows; c++)
    {
        //iterate over the column incrementig r
        for (int r = 0; r < clms; r++)
        {
            // DO SOMETHING
            matrix[r][c]; //c is fixed during this loop
        }
    }

Snake Loop

  for (int r = 0; i < rows; r++)
    {
        // If current row is even, go from left to right
        if (r % 2 == 0)
        {
            for (int c = 0; c < clms; c++)
            {
                // DO SOMETHING
            }
        }
        else // If current row is odd, go from right to left
        {
            for (int c = clms - 1; c >= 0; c--)
            {
                // DO SOMETHING
            }
        }
    }

By reversing the two innermost loops, the mirror pattern can easily be obtained.