Pointers to pointers in C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a pointer to a pointer to an integer.

C

int x = 10;
int *p = &x;
int [1] = &p;

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

A**pp

B*pp

Cpp

D&pp

Attempts:

3 left

💡 Hint

Common Mistakes

Using only one asterisk (*) instead of two.

Forgetting to use a pointer type for pp.

The declaration int **pp means pp is a pointer to a pointer to an integer.

2fill in blank

medium

Complete the code to assign the address of pointer p to pointer to pointer pp.

C

int x = 5;
int *p = &x;
int **pp;
pp = [1];

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

Ap

B&p

C*p

Dx

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning pp = p; which is incorrect type.

Assigning pp = *p; which dereferences p.

To make pp point to p, assign pp = &p; because pp is a pointer to a pointer.

3fill in blank

hard

Fix the error in the code to correctly print the value of x using pointer to pointer pp.

C

int x = 20;
int *p = &x;
int **pp = &p;
printf("%d\n", [1]);

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

A*pp

Bpp

C**pp

D*p

Attempts:

3 left

💡 Hint

Common Mistakes

Using only one dereference *pp which gives pointer p, not value.

Using pp directly which is a pointer to pointer.

To get the value of x through pp, you need to dereference twice: **pp.

4fill in blank

hard

Fill both blanks to correctly change the value of x to 50 using pointer to pointer pp.

C

int x = 10;
int *p = &x;
int **pp = &p;
**pp = [1];
printf("%d\n", [2]);

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

A50

Bx

C**pp

D*p

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to assign value to *pp instead of **pp.

Printing **pp instead of *p.

Assigning **pp = 50; changes x to 50. Printing *p shows the updated value.

5fill in blank

hard

Fill all three blanks to create a pointer to pointer, assign it, and print the value of x.

C

int x = 100;
int *p = &x;
int [1] = &p;
printf("%d\n", [2]);

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

A**pp

B*p

Dpp

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing pointer declaration syntax.

Using wrong dereference level when printing.

Declare pp as int **pp, print value with *p or **pp. Here, pp is the pointer to pointer.

Practice

1. What does a pointer to a pointer store in C?

easy

A. The size of a variable

B. The value of an integer

C. The address of another pointer

D. The address of a function

Solution

1. Step 1: Understand pointer basics

   A pointer stores the address of a variable.

2. Step 2: Understand pointer to pointer

   A pointer to pointer stores the address of another pointer, not a direct value.

3. Final Answer:

   The address of another pointer -> Option C

4. Quick Check:

   Pointer to pointer = address of pointer [OK]

Hint: Pointer to pointer holds address of pointer, not value [OK]

Common Mistakes:

• Confusing pointer to pointer with pointer to value
• Thinking it stores the value directly
• Mixing it with function pointers

2. Which of the following is the correct way to declare a pointer to a pointer to an int in C?

easy

A. int **ptr;

B. int ptr*;

C. int *ptr*;

D. int &ptr*;

Solution

1. Step 1: Recall pointer declaration syntax

   Single pointer to int is declared as int *ptr;.

2. Step 2: Extend to pointer to pointer

   Pointer to pointer uses two asterisks: int **ptr;.

3. Final Answer:

   int **ptr; -> Option A

4. Quick Check:

   Pointer to pointer declaration uses two * [OK]

Hint: Use two * for pointer to pointer declaration [OK]

Common Mistakes:

• Placing * incorrectly in declaration
• Using & instead of *
• Confusing pointer syntax with references

3. What is the output of the following C code?

int x = 10;
int *p = &x;
int **pp = &p;
printf("%d", **pp);

medium

A. Address of x

B. Compilation error

C. Address of p

D. 10

Solution

1. Step 1: Understand variable assignments

   x = 10, p points to x, pp points to p.

2. Step 2: Evaluate **pp

   *pp gives p (pointer to x), **pp gives value at x, which is 10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   **pp dereferences twice to get x's value [OK]

Hint: Double dereference **pp gets original value [OK]

Common Mistakes:

• Printing pointer address instead of value
• Using single * instead of **
• Confusing *p and **pp

4. Identify the error in the following code snippet:

int x = 5;
int *p = &x;
int **pp;
*pp = p;
printf("%d", **pp);

medium

A. Syntax error in printf

B. pp is not initialized before dereferencing

C. x is not initialized

D. p is not assigned correctly

Solution

1. Step 1: Check pointer initialization

   pp is declared but not initialized to point anywhere.

2. Step 2: Analyze dereferencing *pp = p;

   Dereferencing uninitialized pp causes undefined behavior or crash.

3. Final Answer:

   pp is not initialized before dereferencing -> Option B

4. Quick Check:

   Dereferencing uninitialized pointer causes error [OK]

Hint: Always initialize pointers before dereferencing [OK]

Common Mistakes:

• Assuming pointer auto-initializes
• Ignoring uninitialized pointer usage
• Confusing pointer assignment with value assignment

5. Given the code below, what will be the output?

int a = 3, b = 7;
int *p1 = &a;
int *p2 = &b;
int **pp = &p1;
*pp = p2;
printf("%d %d", *p1, *p2);

hard

A. 7 7

B. 3 7

C. 3 3

D. Compilation error

Solution

1. Step 1: Initial pointer assignments

   p1 points to a (3), p2 points to b (7), pp points to p1.

2. Step 2: Change p1 via pointer to pointer

   *pp = p2 changes p1 to point to b (7).

3. Step 3: Print values

   *p1 now points to b (7), *p2 is 7, so output is "7 7".

4. Final Answer:

   7 7 -> Option A

5. Quick Check:

   Pointer to pointer changed p1 to point to b [OK]

Hint: Pointer to pointer can change pointer target [OK]

Common Mistakes:

• Assuming p1 still points to a
• Confusing *pp and **pp
• Ignoring pointer reassignment effect