Multidim array pointer

Learn to pass multidimensional arrays as pointers.

Multidimensional arrays

In C a two-dimensional array is an array of arrays. int m[2][3] is a table with two rows and three columns. In memory the rows sit side by side.

int m[2][3] = {{1, 2, 3},
               {4, 5, 6}};
printf("%d", m[1][2]);  /* 6 */

The first index selects the row, the second the column. m[1][2] is the third column of the second row.

Nested loop traversal

To walk a two-dimensional array you use two nested for loops. The outer loop walks rows, the inner loop walks columns.

int m[2][3] = {{1, 2, 3}, {4, 5, 6}};
for (int i = 0; i < 2; i++) {
    for (int j = 0; j < 3; j++) {
        printf("%d ", m[i][j]);
    }
    printf("\n");
}

This pattern writes each row to one output line. A newline is added at the end of each row.

Exercise Print a two-dimensional array

Define int m[2][3] = {{1, 2, 3}, {4, 5, 6}};. With a for loop print each row as printf("%d %d %d\n", m[i][0], m[i][1], m[i][2]);. The loop runs while i < 2. Output should be 1 2 3 and 4 5 6.

Show example solution

#include <stdio.h>

int main(void) {
    int m[2][3] = {{1, 2, 3}, {4, 5, 6}};
    for (int i = 0; i < 2; i++) {
        printf("%d %d %d\n", m[i][0], m[i][1], m[i][2]);
    }
    return 0;
}

Test yourself

Second row, third column

int m[2][3] = {{1,2,3},{4,5,6}};
printf("%d", m[____][2]);  /* prints 6 */

Show answer

int m[2][3] = {{1,2,3},{4,5,6}};
printf("%d", m[1][2]);  /* prints 6 */

int m[2][3]; m[1][1] is which?
- First element of first row
- Second element of second row
- Second element of third row
- First element of second row
Show answer

Second element of second row — m[1][1] is the second column (index 1) of the second row (index 1).
Match indices

m[0][0]

first row first column
m[1][2]

second row third column
m[i][j]

row i column j
int m[2][3]

two-dimensional array

Next lesson: Summary - Arrays and pointers

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