Pointers to pointers in C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of pointer to pointer dereferencing

What is the output of this C program?

C

#include <stdio.h>

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

A 10

B Address of p

C Address of x

D Compilation error

💡 Hint

Remember that *p points to x, so **pp dereferences p to get x's value.

The variable pp is a pointer to p, which itself points to x. So **pp accesses the value of x, which is 10.

❓ Predict Output

intermediate

Value after modifying through pointer to pointer

What is the output of this program?

C

#include <stdio.h>

int main() {
    int a = 5;
    int *p = &a;
    int **pp = &p;
    **pp = 20;
    printf("%d\n", a);
    return 0;
}

A 5

B 20

C Address of a

D Compilation error

💡 Hint

Changing **pp changes the value of a through the pointers.

Since **pp points to a, assigning 20 to it changes a to 20. So printing a outputs 20.

🔧 Debug

advanced

Identify the error in pointer to pointer usage

What error does this code produce when compiled?

C

#include <stdio.h>

int main() {
    int x = 100;
    int *p = &x;
    int **pp = p;
    printf("%d\n", **pp);
    return 0;
}

A Segmentation fault at runtime

B Compilation error: missing semicolon

C Prints 100

D Compilation error: incompatible types in assignment

💡 Hint

Check the type of the right side and left side in the assignment to pp.

The variable pp is declared as int ** but assigned p which is int *. This causes a type mismatch error during compilation.

🧠 Conceptual

advanced

Number of indirections in pointer to pointer

Given int ***ppp;, how many times must you dereference ppp to get the integer value it ultimately points to?

A 3

B 2

C 4

D 1

💡 Hint

Each * removes one level of pointer indirection.

ppp is a pointer to a pointer to a pointer to an int. So you must dereference it three times (***ppp) to get the int value.

❓ Predict Output

expert

Output of complex pointer to pointer manipulation

What is the output of this program?

C

#include <stdio.h>

int main() {
    int val = 50;
    int *p1 = &val;
    int **p2 = &p1;
    int ***p3 = &p2;

    ***p3 = 100;
    printf("%d\n", val);

    **p2 = 200;
    printf("%d\n", val);

    *p1 = 300;
    printf("%d\n", val);

    return 0;
}

A 100\n100\n100

B 50\n200\n300

C 100\n200\n300

D Compilation error

💡 Hint

Each assignment changes the value of val through different levels of pointers.

All three assignments modify val through different pointer levels. So the printed values are 100, then 200, then 300.

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