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.