Pointer arithmetic - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of pointer increment on array

What is the output of this C code snippet?

C

#include <stdio.h>

int main() {
    int arr[] = {10, 20, 30, 40};
    int *p = arr;
    p++;
    printf("%d\n", *p);
    return 0;
}

A 20

B 10

C 30

D 40

💡 Hint

Incrementing a pointer moves it to the next element in the array.

The pointer p initially points to arr[0] which is 10. After p++, it points to arr[1], which is 20. So, *p prints 20.

❓ Predict Output

intermediate

Pointer subtraction result

What is the output of this C code?

C

#include <stdio.h>

int main() {
    int arr[] = {5, 10, 15, 20, 25};
    int *p1 = &arr[4];
    int *p2 = &arr[1];
    printf("%td\n", p1 - p2);
    return 0;
}

A 3

B 6

C 5

D 4

💡 Hint

Pointer subtraction gives the number of elements between them.

p1 points to arr[4], p2 points to arr[1]. The difference is 4 - 1 = 3 elements.

🔧 Debug

advanced

Identify the error in pointer arithmetic

What error will this C code produce when compiled or run?

C

#include <stdio.h>

int main() {
    int x = 10;
    int *p = &x;
    p = p + 2;
    printf("%d\n", *p);
    return 0;
}

A Compilation error: invalid pointer arithmetic

B Output: 10

C Runtime error: segmentation fault or undefined behavior

D Output: 0

💡 Hint

Pointer arithmetic beyond allocated memory is unsafe.

Adding 2 to pointer p moves it beyond the single int variable x. Dereferencing it causes undefined behavior, often a segmentation fault.

📝 Syntax

advanced

Which pointer arithmetic expression is valid?

Which of the following pointer arithmetic expressions is valid in C?

A p / 2

B p - p

C p * 2

D p + p

💡 Hint

Pointer subtraction between two pointers to the same array is allowed.

You can subtract two pointers (p - p) to get the number of elements between them. Adding two pointers or multiplying/dividing pointers is invalid.

🚀 Application

expert

Calculate sum using pointer arithmetic

What is the output of this C program that sums array elements using pointer arithmetic?

C

#include <stdio.h>

int main() {
    int arr[] = {1, 2, 3, 4, 5};
    int *p = arr;
    int *end = arr + 5;
    int sum = 0;
    while (p < end) {
        sum += *p++;
    }
    printf("%d\n", sum);
    return 0;
}

A 0

B 10

C 5

D 15

💡 Hint

Pointer p moves from start to end of array, adding each element.

The loop adds all elements: 1+2+3+4+5 = 15. Pointer arithmetic is used to traverse the array.

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