Arrays (1D and Multidimensional)

// Declares a one-dimensional array.
type arrayName[size];

// Declares and initializes an array at the same time.
type arrayName[size] = {value1, value2, value3};

// Omits the size and lets the compiler determine it from the initializer.
type arrayName[] = {value1, value2, value3};

// Accesses an element (indexes are zero-based).
arrayName[index];

// Declares and initializes a two-dimensional array.
type arrayName[rows][cols] = {{value, value}, {value, value}};

sample_array.c

#include <stdio.h>

int main(void) {
    // Declares and initializes a one-dimensional array.
    int scores[5] = {80, 75, 90, 65, 88};

    // Accesses individual elements by index.
    printf("First element: %d\n", scores[0]); // Outputs '80'.
    printf("Last element: %d\n", scores[4]); // Outputs '88'.

    // Iterates over all elements with a for loop.
    int sum = 0;
    for (int i = 0; i < 5; i++) {
        sum += scores[i];
    }
    printf("Sum: %d\n", sum); // Outputs 'Sum: 398'.
    printf("Average: %.1f\n", (double)sum / 5); // Outputs 'Average: 79.6'.

    // Uses sizeof to get the number of elements.
    int count = sizeof(scores) / sizeof(scores[0]);
    printf("Count: %d\n", count); // Outputs 'Count: 5'.

    // Represents a string as a char array.
    char name[] = "Yagami Iori";
    printf("Name: %s\n", name); // Outputs 'Name: Yagami Iori'.
    printf("Length: %zu\n", sizeof(name) - 1); // Length excluding the null character.

    // Declares and initializes a two-dimensional array.
    int matrix[2][3] = {
        {1, 2, 3},
        {4, 5, 6}
    };
    // Displays all elements using nested loops.
    for (int i = 0; i < 2; i++) {
        for (int j = 0; j < 3; j++) {
            printf("%d ", matrix[i][j]);
        }
        printf("\n");
    }
    // Output: 1 2 3 / 4 5 6

    return 0;
}

Run the following command:

gcc array.c -o array
./array
First element: 80
Last element: 88
Sum: 398
Average: 79.6
Count: 5
Name: Yagami Iori
Length: 11
1 2 3
4 5 6

Common Mistake: Out-of-Bounds Access

Accessing an array out of bounds causes undefined behavior. C does not perform boundary checking.

array_oob_ng.c

#include <stdio.h>

int main(void) {
    int arr[5] = {1, 2, 3, 4, 5};
    printf("%d\n", arr[5]); /* index 5 is out of bounds for a 5-element array */
    return 0;
}

Run the following command:

gcc array_oob_ng.c -o array_oob_ng
./array_oob_ng
-1735289080

Reading beyond the array boundary returns an indeterminate value. Always keep indexes within the range of 0 to less than the element count.

array_oob_ok.c

#include <stdio.h>

int main(void) {
    int arr[5] = {1, 2, 3, 4, 5};
    int n = sizeof(arr) / sizeof(arr[0]);

    for (int i = 0; i < n; i++) {
        printf("%d\n", arr[i]);
    }
    return 0;
}

Run the following command:

gcc array_oob_ok.c -o array_oob_ok
./array_oob_ok
1
2
3
4
5

C arrays do not perform bounds checking. Accessing an array out of bounds causes undefined behavior — the program may crash or corrupt data in another memory region. Always ensure indexes stay within the valid range.

Arrays and pointers are closely related. scores[i] is equivalent to *(scores + i). For details on how arrays are handled when passed to functions, see Pointers and Arrays.

To determine the size at runtime, use dynamic memory allocation (malloc()). Variable-length arrays (VLAs) are also available since C99, but avoid them for large sizes due to the risk of stack overflow.