Pointer arithmetic - Mini Project: Build & Apply

Pointer Arithmetic in C

📖 Scenario: You are working with a list of numbers stored in an array in C. You want to use pointers to access and manipulate these numbers by moving through the array step-by-step.

🎯 Goal: Learn how to use pointer arithmetic to access elements in an array by moving the pointer to different positions.

📋 What You'll Learn

Create an integer array with specific values

Create a pointer to the start of the array

Use pointer arithmetic to access the third element in the array

Print the value pointed to by the pointer after moving it

💡 Why This Matters

🌍 Real World

Pointer arithmetic is used in low-level programming to efficiently access and manipulate data stored in arrays or buffers.

💼 Career

Understanding pointers and pointer arithmetic is essential for systems programming, embedded development, and performance-critical applications.

1

Create an integer array

Create an integer array called numbers with these exact values: 10, 20, 30, 40, 50.

C

# Create the integer array 'numbers' with values 10, 20, 30, 40, 50
// Your code here

Hint

Use curly braces to list the values inside the array.

2

Create a pointer to the array

Create an integer pointer called ptr and set it to point to the first element of the numbers array.

C

int numbers[] = {10, 20, 30, 40, 50};
// Create an integer pointer 'ptr' pointing to 'numbers'
// Your code here

Hint

Set the pointer to the array name, which points to the first element.

3

Use pointer arithmetic to access the third element

Move the pointer ptr two positions forward using pointer arithmetic to point to the third element in the array.

C

int numbers[] = {10, 20, 30, 40, 50};
int *ptr = numbers;
// Move 'ptr' two positions forward
// Your code here

Hint

Adding 2 to the pointer moves it two elements ahead.

4

Print the value pointed to by the pointer

Print the value pointed to by ptr using printf. The output should be the third element's value.

C

int numbers[] = {10, 20, 30, 40, 50};
int *ptr = numbers;
ptr = ptr + 2;
// Print the value pointed to by 'ptr'
// Your code here

Hint

Use *ptr to get the value the pointer points to.

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