Pointers & Arrays

Pointer and Array Relationship

In C, arrays and pointers are closely related. The array name usually represents the address of the first element. Because of this, array elements can be accessed using both index notation and pointer notation.

Array Name as Pointer

#include <stdio.h>

int main() {
    int arr[5] = {10, 20, 30, 40, 50};

    printf("Address of first element = %p\n", (void*)&arr[0]);
    printf("Array name address       = %p\n", (void*)arr);
    printf("First element            = %d\n", *arr);
    printf("Third element            = %d\n", *(arr + 2));

    return 0;
}

Pointer Arithmetic with Arrays

When an integer pointer is increased by 1, it moves to the next integer element, not the next byte. The compiler automatically uses the size of the data type.

#include <stdio.h>

int main() {
    int marks[] = {85, 90, 78, 92};
    int *p = marks;

    for(int i = 0; i < 4; i++) {
        printf("marks[%d] = %d\n", i, *(p + i));
    }

    return 0;
}

Passing Arrays to Functions

When an array is passed to a function, actually the address of its first element is passed. Therefore, changes made inside the function can affect the original array.

#include <stdio.h>

void update(int *a, int n) {
    for(int i = 0; i < n; i++) {
        a[i] = a[i] + 5;
    }
}

void display(int a[], int n) {
    for(int i = 0; i < n; i++) {
        printf("%d ", *(a + i));
    }
}

int main() {
    int arr[] = {10, 20, 30};
    update(arr, 3);
    display(arr, 3);
    return 0;
}

Summary

• arr normally gives the base address of the array
• arr[i] and *(arr+i) access the same value
• Pointer arithmetic moves according to data type size
• Arrays passed to functions can be handled using pointers
• Array name is not a modifiable pointer, so arr++ is invalid