Pointer arithmetic

Introduction

Pointer arithmetic lets you move through memory locations easily by adding or subtracting numbers to pointers.

When you want to access elements in an array using pointers.

When you need to loop through a block of memory efficiently.

When working with strings as arrays of characters.

When manipulating data structures like linked lists using pointers.

When you want to calculate the distance between two memory addresses.

Syntax

C

pointer + n  // moves pointer forward by n elements
pointer - n  // moves pointer backward by n elements
pointer++     // moves pointer to next element
pointer--     // moves pointer to previous element
pointer1 - pointer2  // gives number of elements between two pointers

Pointer arithmetic moves in units of the size of the data type the pointer points to.

Adding 1 to a pointer moves it to the next element, not just the next byte.

Examples

Moves pointer p two integers ahead in the array.

C

int *p = arr;
p = p + 2;

Moves pointer c to the next character in the string.

C

char *c = str;
c++;

Calculates how many elements are between pointers p1 and p2.

C

int diff = p2 - p1;

Sample Program

This program shows how to move a pointer through an integer array and access different elements using pointer arithmetic.

C

#include <stdio.h>

int main() {
    int arr[] = {10, 20, 30, 40, 50};
    int *p = arr;  // points to first element

    printf("First element: %d\n", *p);

    p = p + 3;  // move pointer 3 elements ahead
    printf("Fourth element: %d\n", *p);

    p--;
    printf("Third element after moving back: %d\n", *p);

    int diff = p - arr;
    printf("Pointer is %d elements away from start\n", diff);

    return 0;
}

Important Notes

Pointer arithmetic only works correctly with pointers pointing inside the same array or one past the end.

Be careful not to move pointers outside valid memory, or your program may crash.

Summary

Pointer arithmetic lets you move pointers by elements, not bytes.

You can add, subtract, increment, and decrement pointers.

Subtracting two pointers gives the number of elements between them.

Practice

1. What does adding 1 to a pointer in C do?
int *p;

easy

A. Causes a syntax error

B. Moves the pointer by 1 byte

C. Moves the pointer to the previous element

D. Moves the pointer to the next element of the type it points to

Solution

1. Step 1: Understand pointer arithmetic basics

   In C, adding 1 to a pointer moves it to the next element of the type it points to, not just 1 byte.

2. Step 2: Apply to the given pointer

   Since p is an int*, p + 1 points to the next int in memory.

3. Final Answer:

   Moves the pointer to the next element of the type it points to -> Option D

4. Quick Check:

   Pointer + 1 moves by element size [OK]

Hint: Pointer + 1 moves to next element, not next byte [OK]

Common Mistakes:

• Thinking pointer arithmetic moves by 1 byte
• Confusing pointer increment with value increment
• Assuming pointer addition causes errors

2. Which of the following pointer arithmetic expressions is syntactically correct in C?

int arr[5];
int *p = arr;

easy

A. p + 2

B. p ++ 2

C. p + ++ 2

D. p +* 2

Solution

1. Step 1: Review pointer arithmetic syntax

   Valid pointer arithmetic uses p + n or p - n where n is an integer.

2. Step 2: Check each option

   p + 2 is valid syntax. Options A, C, and D contain invalid operators or spacing causing syntax errors.

3. Final Answer:

   p + 2 -> Option A

4. Quick Check:

   Correct pointer addition syntax is p + n [OK]

Hint: Pointer arithmetic uses '+' or '-' with integers only [OK]

Common Mistakes:

• Using invalid operators like '++' incorrectly
• Adding spaces that change meaning
• Confusing pointer increment with addition

3. What is the output of this code?

int arr[] = {10, 20, 30, 40};
int *p = arr;
printf("%d", *(p + 2));

medium

A. 10

B. 20

C. 30

D. 40

Solution

1. Step 1: Understand pointer and array relation

   Pointer p points to arr[0], which is 10.

2. Step 2: Calculate pointer offset

   *(p + 2) accesses the element 2 positions after arr[0], which is arr[2] = 30.

3. Final Answer:

   30 -> Option C

4. Quick Check:

   Pointer + 2 points to third element = 30 [OK]

Hint: *(p + n) accesses nth element after pointer [OK]

Common Mistakes:

• Confusing index starting at 1 instead of 0
• Dereferencing pointer without offset
• Mixing pointer arithmetic with array indexing

4. Identify the error in this code snippet:

int arr[3] = {1, 2, 3};
int *p = arr + 3;
printf("%d", *p);

medium

A. Syntax error in pointer declaration

B. Pointer points beyond array bounds causing undefined behavior

C. Missing semicolon after array initialization

D. Incorrect printf format specifier

Solution

1. Step 1: Analyze pointer assignment

   p = arr + 3 points to one element past the last valid element (index 3 in a 3-element array).

2. Step 2: Understand dereferencing invalid pointer

   Dereferencing *p here causes undefined behavior because it accesses memory outside the array.

3. Final Answer:

   Pointer points beyond array bounds causing undefined behavior -> Option B

4. Quick Check:

   Pointer must not dereference beyond array end [OK]

Hint: Pointer + array size points just past array, not valid to dereference [OK]

Common Mistakes:

• Assuming pointer can safely dereference past array end
• Confusing pointer arithmetic with array length
• Ignoring undefined behavior risks

5. Given two pointers int *p1 and int *p2 pointing inside the same array, what does p2 - p1 represent?

hard

A. The number of elements between the two pointers

B. The difference in memory addresses as a raw number

C. The number of bytes between the two pointers

D. A syntax error because pointers cannot be subtracted

Solution

1. Step 1: Understand pointer subtraction in C

   Subtracting two pointers gives the number of elements of the pointer type between them, not bytes.

2. Step 2: Apply to pointers inside the same array

   Since p1 and p2 point inside the same array, p2 - p1 counts how many elements separate them.

3. Final Answer:

   The number of elements between the two pointers -> Option A

4. Quick Check:

   Pointer subtraction = element count difference [OK]

Hint: Pointer subtraction counts elements, not bytes [OK]

Common Mistakes:

• Thinking subtraction returns byte difference
• Assuming pointer subtraction is invalid
• Confusing pointer subtraction with pointer addition