2D Arrays

Declaration and indexing

int grid[3][4];                    // 3 rows, 4 columns
grid[1][2] = 7;                    // row 1, column 2
int diag[2][2] = {{1, 0}, {0, 1}};

A 2D array is one contiguous block, stored row-major: row 0's four ints, then row 1's, then row 2's. [r][c] is the element at row r, column c.

The nested-loop walk

int grid[3][4] = {0};
for (int r = 0; r < 3; r++) {
    for (int c = 0; c < 4; c++) {
        grid[r][c] = r * 10 + c;
    }
}

Outer loop = rows, inner loop = columns. Total elements = 3 × 4 = 12.

Row totals

int row_sum(const int g[][4], int rows) {   // column count is part of the type
    int total = 0;
    for (int r = 0; r < rows; r++)
        for (int c = 0; c < 4; c++)
            total += g[r][c];
    return total;
}

Note the parameter shape: the first bracket may be empty, the second may not.