Pointers and arrays in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of pointer arithmetic with arrays

What is the output of this C++ code snippet?

C++

#include <iostream>
int main() {
    int arr[] = {10, 20, 30, 40};
    int *p = arr;
    std::cout << *(p + 2) << std::endl;
    return 0;
}

A 20

B 40

C 10

D 30

💡 Hint

Remember that pointer arithmetic moves by the size of the data type.

The pointer p points to the first element of arr. Adding 2 moves it to the third element, which is 30.

❓ Predict Output

intermediate

Value of pointer after increment

What will be the value printed by this program?

C++

#include <iostream>
int main() {
    int arr[3] = {5, 10, 15};
    int *ptr = arr + 1;
    std::cout << *ptr << std::endl;
    ptr++;
    std::cout << *ptr << std::endl;
    return 0;
}

A 10\n15

B 5\n10

C 15\n10

D 10\n10

💡 Hint

Pointer starts at second element, then moves to third.

ptr initially points to arr[1] which is 10, then after ptr++ it points to arr[2] which is 15.

🔧 Debug

advanced

Identify the error in pointer usage

What error will this code produce when compiled or run?

C++

#include <iostream>
int main() {
    int *p;
    *p = 10;
    std::cout << *p << std::endl;
    return 0;
}

A Compilation error: uninitialized pointer

B Runtime error: segmentation fault

C Compilation error: missing semicolon

D Outputs 10

💡 Hint

Think about what happens when you dereference a pointer that has not been assigned a valid address.

Pointer p is uninitialized and points to an unknown memory location. Dereferencing it causes a runtime segmentation fault.

🧠 Conceptual

advanced

Size of array vs pointer

Given the code below, what will be the output?

C++

#include <iostream>
int main() {
    int arr[5] = {1,2,3,4,5};
    int *p = arr;
    std::cout << sizeof(arr) << " " << sizeof(p) << std::endl;
    return 0;
}

A 20 8

B 20 20

C 5 8

D 5 5

💡 Hint

sizeof(arr) gives total bytes of all elements, sizeof(p) gives size of pointer variable.

arr is an array of 5 ints, each 4 bytes, so 20 bytes total. p is a pointer, typically 8 bytes on 64-bit systems.

📝 Syntax

expert

Correct pointer declaration and initialization

Which option correctly declares and initializes a pointer to the first element of an integer array named 'data'?

A int ptr = &data[0];

B int ptr* = data;

C int *ptr = &data[0];

D int *ptr = data[0];

💡 Hint

Pointer must be declared with * and initialized with address of an element.

Option C correctly declares a pointer to int and initializes it with the address of the first element of the array.

Practice

1. What does the name of an array represent in C++?

easy

A. A copy of the entire array

B. The total size of the array

C. The last element of the array

D. A pointer to the first element of the array

Solution

1. Step 1: Understand array name behavior

   In C++, the name of an array acts as a pointer to its first element, not the whole array or size.

2. Step 2: Confirm pointer usage

   This pointer can be used to access elements by pointer arithmetic or indexing.

3. Final Answer:

   A pointer to the first element of the array -> Option D

4. Quick Check:

   Array name = pointer to first element [OK]

Hint: Remember array name points to element zero [OK]

Common Mistakes:

• Thinking array name stores size
• Confusing array name with last element
• Assuming array name copies data

2. Which of the following is the correct way to declare a pointer to an integer array named arr?

easy

A. int *ptr = &arr;

B. int ptr = &arr;

C. int *ptr = arr;

D. int ptr = arr;

Solution

1. Step 1: Understand pointer declaration

   To declare a pointer to an integer, use int *ptr. The array name arr acts as a pointer to its first element.

2. Step 2: Assign pointer correctly

   Assigning ptr = arr; sets the pointer to the first element's address. Using &arr gives the address of the whole array, which has a different type.

3. Final Answer:

   int *ptr = arr; -> Option C

4. Quick Check:

   Pointer to first element = int *ptr = arr [OK]

Hint: Use int *ptr = arr; to point to array start [OK]

Common Mistakes:

• Using &arr instead of arr for pointer
• Declaring pointer as int instead of int*
• Assigning pointer without *

3. What is the output of the following code?

int arr[] = {10, 20, 30};
int *p = arr;
std::cout << *(p + 1) << std::endl;

medium

A. 20

B. 10

C. 30

D. Compilation error

Solution

1. Step 1: Understand pointer arithmetic

   Pointer p points to arr[0]. Adding 1 moves pointer to arr[1].

2. Step 2: Dereference pointer at new position

   Dereferencing *(p + 1) gives the value at arr[1], which is 20.

3. Final Answer:

   20 -> Option A

4. Quick Check:

   *(p + 1) = arr[1] = 20 [OK]

Hint: Pointer + 1 points to next array element [OK]

Common Mistakes:

• Confusing pointer arithmetic with addition of values
• Dereferencing pointer without offset
• Expecting compilation error

4. Identify the error in the following code snippet:

int arr[3] = {1, 2, 3};
int *p = &arr;
std::cout << *(p + 2) << std::endl;

medium

A. Pointer declaration is correct, no error

B. Incorrect pointer assignment: &arr should be arr

C. Dereferencing pointer is invalid syntax

D. Array size is too small for pointer arithmetic

Solution

1. Step 1: Analyze pointer assignment

   arr is an array name acting as pointer to first element. &arr is address of whole array, type int (*)[3], incompatible with int *.

2. Step 2: Correct pointer assignment

   Assigning int *p = arr; is correct to point to first element. Using &arr causes type mismatch and incorrect pointer arithmetic.

3. Final Answer:

   Incorrect pointer assignment: &arr should be arr -> Option B

4. Quick Check:

   Use arr, not &arr, for int* pointer [OK]

Hint: Use arr, not &arr, when assigning int* pointer [OK]

Common Mistakes:

• Using &arr instead of arr for int*
• Ignoring pointer type mismatch warnings
• Assuming &arr points to first element

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

int arr[] = {5, 10, 15, 20};
int *p = arr + 1;
std::cout << p[2] << std::endl;

hard

A. 20

B. 15

C. Undefined behavior

D. 10

Solution

1. Step 1: Understand pointer initialization

   p = arr + 1; points to arr[1], which is 10.

2. Step 2: Access element using pointer indexing

   p[2] means *(p + 2), which points to arr[3] (1 + 2 = 3), value 20.

3. Final Answer:

   20 -> Option A

4. Quick Check:

   p[2] = arr[3] = 20 [OK]

Hint: Pointer[index] = *(pointer + index) [OK]

Common Mistakes:

• Confusing p[2] with arr[2]
• Forgetting pointer offset starts at arr + 1
• Assuming p[2] is out of bounds