int m[NROWS][NCOLS];
int row, col;

//row-by-row
for (row = 0; row < NROWS; row++) {
    for (col = 0; col < NCOLS; col++) {
        … m[row][col] …
    }
}
// colum-by-column
for (col = 0; col < NCOLS; col++) {
    for (row = 0; row < NROWS; row++) {
        … m[row][col] …
    }
}