Compile time initialization of arrays

Compile time initialisation of a 2D array

The values are given in the declaration, so the compiler stores them when the program is compiled/loaded, and no input is needed at run time.

int m[2][3] = {{1, 2, 3}, {4, 5, 6}};

Each inner brace is one row. m[0][1] is 2 and m[1][2] is 6. It can also be written in one list: int m[2][3] = {1, 2, 3, 4, 5, 6};, filled row by row. The row size may be omitted, but the column size is compulsory: int n[][2] = {{1, 2}, {3, 4}, {5, 6}}; gives 3 rows.

How the compiler handles the initialiser count and array size (1D)

1. Size given and initialisers equal: all elements get the given values.
2. Fewer initialisers than size: the given values go to the first elements and the remaining elements are set to 0, e.g. int b[6] = {1, 2}; gives 1 2 0 0 0 0.
3. Size omitted: the compiler counts the initialisers and sets the size, e.g. int a[] = {5, 10, 15, 20}; creates 4 elements.
4. More initialisers than size: error/warning (excess elements); not allowed.

#include <stdio.h>

int main(void)
{
    int m[2][3] = {{1, 2, 3}, {4, 5, 6}};   /* 2D, fully initialised */
    int a[]  = {5, 10, 15, 20};             /* size omitted -> 4 */
    int b[6] = {1, 2};                      /* rest become 0 */
    int n[][2] = {{1, 2}, {3, 4}, {5, 6}};  /* only first size may be omitted */

    printf("sizeof a = %zu bytes (%zu elements)\n", sizeof a, sizeof a / sizeof a[0]);
    printf("b: ");
    for (int i = 0; i < 6; i++)
        printf("%d ", b[i]);
    printf("\nm[1][2] = %d, rows of n = %zu\n", m[1][2], sizeof n / sizeof n[0]);
    return 0;
}

Output:

sizeof a = 16 bytes (4 elements)
b: 1 2 0 0 0 0 
m[1][2] = 6, rows of n = 3