Why dynamic memory is needed in C++ - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to allocate an integer dynamically.

C++

int* ptr = [1] int;

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

Aauto

Bmalloc

Cdelete

Dnew

Attempts:

3 left

💡 Hint

Common Mistakes

Using malloc instead of new in C++.

Using delete to allocate memory.

In C++, new is used to allocate memory dynamically.

2fill in blank

medium

Complete the code to free the dynamically allocated memory.

C++

delete [1];

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

Aptr

B*ptr

Cptr()

D&ptr

Attempts:

3 left

💡 Hint

Common Mistakes

Using delete *ptr which deletes the value, not the memory.

Using delete ptr() which is invalid syntax.

To free memory allocated with new, use delete followed by the pointer variable.

3fill in blank

hard

Fix the error in the code to allocate an array dynamically.

C++

int* arr = new int[1]10;

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

A{}

B()

C[]

D<>

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of square brackets for array size.

Using angle brackets which are for templates.

To allocate an array dynamically, use square brackets [] with the size.

4fill in blank

hard

Fill both blanks to create a dynamic array and delete it properly.

C++

int* arr = new int[1]5;
[2] arr;

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

A[]

Bdelete[]

Cdelete

D()

Attempts:

3 left

💡 Hint

Common Mistakes

Using delete instead of delete[] for arrays.

Using parentheses instead of square brackets for allocation.

Use [] to allocate an array and delete[] to free it.

5fill in blank

hard

Fill all three blanks to allocate, use, and free dynamic memory correctly.

C++

int* [1] = new int[2]3;
[1][0] = 10;
[3] [1];

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

Aarr

B[]

Cdelete[]

Ddelete

Attempts:

3 left

💡 Hint

Common Mistakes

Using delete instead of delete[] for arrays.

Inconsistent pointer variable names.

Declare pointer arr, allocate array with [], and free with delete[].

Practice

1. Why do we need dynamic memory in C++ programs?

easy

A. To write code without any variables

B. To allocate memory during program execution when size is unknown

C. To avoid using any memory at all

D. To make the program run faster by using fixed memory

Solution

1. Step 1: Understand memory allocation timing

   Static memory is fixed before running, but dynamic memory is allocated while the program runs.

2. Step 2: Recognize the need for unknown sizes

   When the amount of data is not known before running, dynamic memory allows flexible allocation.

3. Final Answer:

   To allocate memory during program execution when size is unknown -> Option B

4. Quick Check:

   Dynamic memory = flexible size allocation [OK]

Hint: Dynamic memory = memory decided during program run [OK]

Common Mistakes:

• Thinking dynamic memory is for speed only
• Confusing dynamic with static memory
• Believing dynamic memory avoids variables

2. Which of the following is the correct syntax to allocate an integer dynamically in C++?

easy

A. int *p = new int;

B. int p = new int;

C. int *p = int new;

D. new int = int *p;

Solution

1. Step 1: Recall dynamic allocation syntax

   In C++, dynamic memory for an int is allocated using 'new int' and assigned to an int pointer.

2. Step 2: Check pointer declaration

   The pointer must be declared as 'int *p' to hold the address of the allocated int.

3. Final Answer:

   int *p = new int; -> Option A

4. Quick Check:

   Pointer = new type; correct syntax [OK]

Hint: Use 'int *p = new int;' for dynamic int allocation [OK]

Common Mistakes:

• Missing pointer * in declaration
• Wrong order of new and type
• Assigning new int to non-pointer variable

3. What will be the output of this C++ code snippet?

int *arr = new int[3];
arr[0] = 10;
arr[1] = 20;
arr[2] = 30;
std::cout << arr[1] << std::endl;
delete[] arr;

medium

A. 10

B. 30

C. Compilation error

D. 20

Solution

1. Step 1: Understand array allocation and assignment

   Dynamic array of size 3 is created, and values 10, 20, 30 are assigned to indices 0, 1, 2 respectively.

2. Step 2: Identify output statement

   The code prints arr[1], which holds the value 20.

3. Final Answer:

   20 -> Option D

4. Quick Check:

   arr[1] = 20 printed [OK]

Hint: Index 1 of array holds 20, so output is 20 [OK]

Common Mistakes:

• Confusing index positions
• Forgetting to use delete[] for arrays
• Assuming compilation error due to dynamic array

4. Identify the error in this C++ code snippet:

int *p;
p = new int(5);
std::cout << *p << std::endl;
// missing delete statement

medium

A. Missing delete to free allocated memory

B. Syntax error in new int(5)

C. Pointer p is not declared

D. Cannot print value using *p

Solution

1. Step 1: Check dynamic allocation correctness

   Pointer p is declared and assigned new int(5), which is correct syntax.

2. Step 2: Identify missing memory management

   Allocated memory is not freed with delete, causing a memory leak.

3. Final Answer:

   Missing delete to free allocated memory -> Option A

4. Quick Check:

   Always delete after new to avoid leaks [OK]

Hint: Always pair new with delete to free memory [OK]

Common Mistakes:

• Ignoring memory leaks
• Thinking new int(5) is wrong syntax
• Assuming pointer must be initialized differently

5. You want to store user input numbers but don't know how many will be entered. Which approach using dynamic memory is best?

hard

A. Use only static variables to store numbers

B. Use a fixed-size array of 100 elements

C. Use a dynamically allocated array and resize it as needed

D. Use a pointer without allocating memory

Solution

1. Step 1: Understand the problem of unknown input size

   Since the number of inputs is unknown, fixed-size arrays may waste memory or be too small.

2. Step 2: Choose flexible memory allocation

   Dynamically allocated arrays can be resized (e.g., by allocating new larger arrays and copying data) to fit input size.

3. Final Answer:

   Use a dynamically allocated array and resize it as needed -> Option C

4. Quick Check:

   Dynamic resizing fits unknown input sizes [OK]

Hint: Resize dynamic array to fit unknown input size [OK]

Common Mistakes:

• Using fixed arrays that may overflow
• Not allocating memory before use
• Ignoring memory management when resizing