Pointer Arithmetic

# CHAPTER 13

Pointer Arithmetic #

1. Introduction #

2. Learning Objectives #

• Explain the relationship between arrays and pointers.

• Perform pointer addition and subtraction.

• Traverse arrays using pointer arithmetic instead of indices.

• Understand how data types affect pointer math.

3. Arrays are Pointers! #

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

printf("Address of arr[0]: %p\n", &arr[0]);
printf("Value of arr:      %p\n", arr); // Identical to &arr[0]!

4. Pointer Addition and Subtraction #

If an int is 4 bytes, and ptr points to address 1000:

• ptr + 1 does NOT equal 1001.

• ptr + 1 jumps ahead by one integer (4 bytes), so it equals 1004.

int arr[3] = {10, 20, 30};
int *ptr = arr; // ptr points to arr[0]

printf("Value: %d\n", *ptr);       // Prints 10
printf("Value: %d\n", *(ptr + 1)); // Prints 20
printf("Value: %d\n", *(ptr + 2)); // Prints 30

5. Array Traversal using Pointers #

#include <stdio.h>

int main() {
    int arr[] = {100, 200, 300, 400};
    int *ptr = arr;
    int size = 4;

    for (int i = 0; i < size; i++) {
        // ptr++ moves the pointer to the next memory address
        printf("Element %d: %d\n", i, *ptr);
        ptr++; 
    }
    return 0;
}

6. Visualizing Pointer Arithmetic #

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

Memory Address:   [1000]   [1004]   [1008]
Data:             [ 10 ]   [ 20 ]   [ 30 ]

ptr       == 1000  => *ptr       == 10
ptr + 1   == 1004  => *(ptr + 1) == 20
ptr + 2   == 1008  => *(ptr + 2) == 30

7. Comparing Pointers #

int arr[3] = {10, 20, 30};
int *ptr = &arr[0];
int *end = &arr[2];

while (ptr <= end) {
    printf("%d ", *ptr);
    ptr++;
}
// Outputs: 10 20 30

8. Common Mistakes #

• Writing *ptr++ vs (*ptr)++:

• *ptr++ : Returns the value, THEN moves the pointer to the next address.

• (*ptr)++ : Increments the VALUE at that memory address, the pointer does not move.

• Out-of-bounds pointer math: C will happily let you do ptr + 100. If you dereference it, your program will likely crash.

• Adding two pointers: You can subtract pointers (gives the distance between them), but adding two pointers (ptr1 + ptr2) is illegal and makes no logical sense.

9. Exercises #

1. 1. Write a program to print elements of an array in reverse using a pointer.

1. 2. Calculate the length of a string using pointer arithmetic (subtracting the end pointer from the start pointer).

1. 3. Write a function that accepts an array via a pointer and sums its elements.

10. MCQ Quiz with Answers #

11. Interview Questions #

• Q: Explain why arr[2] and *(arr + 2) do the exact same thing in C.

• Q: What is the difference between *ptr++ and (*ptr)++?

• Q: Can you perform pointer arithmetic on a void pointer? (No, because the compiler doesn't know the size of the data type).

12. Summary #

13. Next Chapter Recommendation #