Two-Dimensional Arrays

A two-dimensional array can represent a table, matrix, board, or map. It is an “array of arrays”: the first index selects a row and the second selects a column in that row. Both start at 0.

Creating and accessing

int grid[3][4] = {
    {1, 2, 3, 4},
    {5, 6, 7, 8},
    {9, 10, 11, 12}
};

printf("%d\n", grid[0][0]); // 1
printf("%d\n", grid[1][2]); // 7

Rows and columns

Every row of a built-in two-dimensional array has the same column count. This example stores the dimensions in rows and cols; ragged structures require pointers or containers introduced later.

int triangle[3][3] = {
    {1, 0, 0},
    {1, 2, 0},
    {1, 2, 3}
};

int rows = sizeof triangle / sizeof triangle[0];
int cols = sizeof triangle[0] / sizeof triangle[0][0];

Nested loops

The outer loop controls rows and the inner loop controls columns. Use cols as the inner bound so the dimension is maintained in one place.

for (int row = 0; row < 3; row++) {
    for (int col = 0; col < 4; col++) {
        printf("%4d", grid[row][col]);
    }

    printf("\n");
}

Matrix addition

Matrix addition and subtraction require matching shapes. The result at position i,j uses only the elements at that same position in the two inputs.

int a[3][2] = {{1, 3}, {1, 0}, {1, 2}};
int b[3][2] = {{0, 0}, {7, 5}, {2, 1}};
int result[3][2] = {0};

for (int row = 0; row < 3; row++) {
    for (int col = 0; col < 2; col++) {
        result[row][col] = a[row][col] + b[row][col];
    }
}

Statistics over a 2D array

First decide the scope: one row, one column, or the whole table. This code averages one row; converting to double avoids integer division.

int row = 1;
int sum = 0;

for (int col = 0; col < 4; col++) {
    sum += grid[row][col];
}

double average = (double) sum / 4;