Pointer arithmetic - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to access the next element using pointer arithmetic.

C

int arr[] = {10, 20, 30};
int *ptr = arr;
int value = *(ptr [1] 1);

Drag options to blanks, or click blank then click option'

A/

B-

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' moves the pointer backward, which is incorrect here.

Adding 1 to the pointer moves it to the next integer element in the array.

2fill in blank

medium

Complete the code to calculate the number of elements between two pointers.

C

int arr[] = {5, 10, 15, 20};
int *start = &arr[0];
int *end = &arr[3];
int diff = end [1] start;

Drag options to blanks, or click blank then click option'

A-

B+

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' will not give the distance between pointers.

Subtracting two pointers gives the number of elements between them.

3fill in blank

hard

Fix the error in the pointer arithmetic expression to access the third element.

C

int nums[] = {1, 2, 3, 4};
int *p = nums;
int val = *(p[1] 2);

Drag options to blanks, or click blank then click option'

A+

B*

C-

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' or '/' causes syntax errors.

To access the third element, add 2 to the pointer before dereferencing.

4fill in blank

hard

Fill both blanks to create a pointer that points to the second element and then access it.

C

int data[] = {7, 14, 21};
int *ptr = data [1] 1;
int val = [2]ptr;

Drag options to blanks, or click blank then click option'

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' moves pointer backward incorrectly.

Using '/' or other operators causes errors.

Adding 1 moves the pointer to the second element. Dereferencing with '*' accesses the value.

5fill in blank

hard

Fill all three blanks to create a pointer to the last element, subtract 2 to get the first, and dereference it.

C

int vals[] = {3, 6, 9};
int *p = vals [1] 2;
int *q = p [2] 2;
int result = [3]q;

Drag options to blanks, or click blank then click option'

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing '+' and '-' incorrectly.

Using '/' causes errors.

Adding 2 moves pointer to last element, subtracting 2 moves back to first, dereferencing with '*' gets the value.

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