Pointers and Arrays

// An array name can be used directly as a pointer to the first element.
int arr[5] = {10, 20, 30, 40, 50};
int *p = arr; // Same as &arr[0].

// Index notation and pointer notation are equivalent.
arr[i]      // Index notation
*(arr + i)  // Pointer notation

// Incrementing a pointer advances to the next element.
p++;     // Advances the address by sizeof(int) bytes.
p += n;  // Advances the address by n elements.

// Passing an array to a function via a pointer.
void func(int *arr, int size);
void func(int arr[], int size); // Same meaning as above.

#include <stdio.h>

// A function that receives an array as a pointer and returns the sum.
int sum(int *arr, int size) {
    int total = 0;
    for (int i = 0; i < size; i++) {
        total += arr[i]; // Same as *(arr + i).
    }
    return total;
}

// An alternative way to loop using pointer arithmetic.
void print_array(const int *p, int size) {
    const int *end = p + size; // One past the last element.
    while (p < end) {
        printf("%d ", *p);
        p++; // Advance to the next element.
    }
    printf("\n");
}

int main(void) {
    int scores[] = {80, 75, 90, 65, 88};
    int n = sizeof(scores) / sizeof(scores[0]);

    // Index notation and pointer notation are equivalent.
    printf("scores[2] = %d\n", scores[2]);     // Prints "90".
    printf("*(scores+2) = %d\n", *(scores+2)); // Prints "90".

    // Using a pointer variable to work with the array.
    int *p = scores;
    printf("*p = %d\n", *p);    // First element: prints "80".
    p++;
    printf("*p = %d\n", *p);    // Next element: prints "75".

    // Passing the array to functions.
    printf("Sum: %d\n", sum(scores, n)); // Prints "Sum: 398".
    print_array(scores, n);              // Prints "80 75 90 65 88".

    // Relationship between 2D arrays and pointers.
    int matrix[2][3] = {{1, 2, 3}, {4, 5, 6}};
    // matrix[i][j] is the same as *(*(matrix + i) + j).
    printf("matrix[1][2] = %d\n", *(*(matrix + 1) + 2)); // Prints "6".

    return 0;
}

Although an array name can be used like a pointer, there is a fundamental difference. An array name is itself an address and cannot be assigned a different address. A pointer variable, on the other hand, can hold any address and supports incrementing and reassignment.

When you pass an array to a function, size information is lost. Using sizeof(arr) inside the function returns the size of the pointer, not the size of the entire array. Always pass the array size as a separate argument.

For the basics of pointers, see Pointer Basics. The relationship between strings (char arrays) and pointers is covered in Pointers and Strings.