Pointers and arrays in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a pointer to an integer.

C++

int* [1];

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

Aptr

Bpointer

Cp

DintPtr

Attempts:

3 left

💡 Hint

Common Mistakes

Using a type name instead of a variable name.

Forgetting the asterisk (*) to declare a pointer.

The code declares a pointer variable named ptr that can hold the address of an integer.

2fill in blank

medium

Complete the code to assign the address of variable num to pointer ptr.

C++

ptr = [1]num;

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

A#

B*

C&&

D&

Attempts:

3 left

💡 Hint

Common Mistakes

Using the dereference operator * instead of the address-of operator.

Using double ampersand && which is incorrect here.

The ampersand & operator gives the address of the variable num, which is assigned to the pointer ptr.

3fill in blank

hard

Fix the error in the code to access the first element of the array using a pointer.

C++

int arr[3] = {1, 2, 3};
int* ptr = arr;
int first = [1]ptr;

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

Aptr

B&

C*

D->

Attempts:

3 left

💡 Hint

Common Mistakes

Using the address-of operator & instead of dereference.

Using the arrow operator -> which is for pointers to structs or classes.

The dereference operator * is used to access the value pointed to by ptr, which is the first element of the array.

4fill in blank

hard

Fill both blanks to create a pointer that points to the third element of the array.

C++

int arr[5] = {10, 20, 30, 40, 50};
int* ptr = [1] + [2];

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

Aarr

B2

C3

D&arr

Attempts:

3 left

💡 Hint

Common Mistakes

Using &arr which is a pointer to the whole array, not the first element.

Adding 3 instead of 2 to point to the third element (index 2).

The pointer ptr is set to point to the element at index 2 (third element) by adding 2 to the array name arr.

5fill in blank

hard

Fill all three blanks to create a pointer to an array element and access its value.

C++

int nums[4] = {5, 10, 15, 20};
int* ptr = [1] + [2];
int val = *[3];

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

Anums

B1

Cptr

Dnums + 2

Attempts:

3 left

💡 Hint

Common Mistakes

Using nums + 2 for the first blank which is incorrect here.

Dereferencing nums instead of the pointer ptr.

The pointer ptr points to the second element (index 1) of the array nums. Then *ptr accesses the value at that position.

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