C Pointer vs Array – Key Differences and Similarities in C

Introduction – Are Pointers and Arrays the Same in C?

In C programming, pointers and arrays appear similar, especially when accessing elements using the [] operator. However, they have important differences in memory, behavior, and usage. Understanding these distinctions is vital for writing efficient and error-free code.

In this guide, you’ll learn:

• How arrays and pointers are related and how they differ
• Memory behavior of both
• Access techniques and syntax
• Best practices with side-by-side comparisons

Core Concept – The Relationship

Similarities:

• The array name often acts like a pointer to the first element (arr == &arr[0])
• Both allow indexed access: arr[i] is equivalent to *(arr + i)

Differences:

• Arrays have fixed size, pointers can be reassigned
• sizeof(arr) ≠ sizeof(ptr)
• Arrays are compile-time constructs, pointers are runtime variables

Code Examples – Compare Side-by-Side

Example 1: Access via Pointer and Array

#include <stdio.h>

int main() {
    int arr[3] = {10, 20, 30};
    int *ptr = arr;

    printf("%d %d\n", arr[1], *(ptr + 1));  // Output: 20 20
    return 0;
}

Example 2: sizeof Array vs Pointer

#include <stdio.h>

int main() {
    int arr[5];
    int *ptr = arr;

    printf("sizeof(arr) = %lu\n", sizeof(arr));  // Typically 20 bytes
    printf("sizeof(ptr) = %lu\n", sizeof(ptr));  // Typically 8 bytes (on 64-bit)

    return 0;
}

Shows that arr is not just a pointer—it includes size information at compile time.

Example 3: Pointer Reassignment vs Array Name

#include <stdio.h>

int main() {
    int x[] = {1, 2};
    int y[] = {3, 4};
    int *p = x;
    p = y;    //  Valid: pointer can point elsewhere

    // x = y;  Error: array names are fixed and not assignable
    return 0;
}

Comparison Table – Pointer vs Array in C

Best Practices & Tips

Best Practice:
Use arrays for fixed-size data and pointers for dynamic or flexible memory operations.

Tip:
When passing arrays to functions, they decay into pointers. Always pass the size separately.

Pitfall:
Confusing sizeof on arrays vs pointers can lead to bugs in loops or memory operations.

Real-World Applications

• Arrays are used for constant-length data like days of the week, fixed-size buffers
• Pointers are used in linked lists, file buffers, dynamic matrices
• Function parameters often receive arrays as pointers (int *arr)

Summary – Recap & Next Steps

Though arrays and pointers are closely related in syntax, they behave differently under the hood. Understanding these differences helps in memory management and function design.

Key Takeaways:

• Arrays are not pointers, but often behave like them
• Arrays have fixed size and memory, pointers are flexible
• arr[i] == *(arr + i) is true for both
• Be cautious with sizeof and reassignment

Real-World Relevance:

Understanding pointer vs array differences is crucial in systems programming, embedded C, dynamic memory management, and library design.

Frequently Asked Questions (FAQ)

Is an array a pointer in C?

No. An array name often behaves like a pointer, but it’s not a pointer variable.

Can I assign one array to another?

No. Arrays cannot be reassigned like pointers. Use a loop or memcpy.

What happens when I pass an array to a function?

The array decays into a pointer to its first element. Size info is lost unless passed explicitly.

Can I use malloc() with an array?

Not directly. You can allocate memory and assign it to a pointer, not an array name.

Is sizeof(arr) always equal to sizeof(ptr)?

No. sizeof(arr) gives total size of the array; sizeof(ptr) gives the size of a pointer.