Variable declaration and initialization in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an integer variable named age and initialize it with 25.

C++

int [1] = 25;

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

Aage

Bnumber

Cvalue

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that is not descriptive.

Forgetting to initialize the variable.

The variable name age is used to declare and initialize the integer variable.

2fill in blank

medium

Complete the code to declare a floating-point variable named price and initialize it with 19.99.

C++

float [1] = 19.99f;

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

Avalue

Bprice

Camount

Dcost

Attempts:

3 left

💡 Hint

Common Mistakes

Using an integer type instead of float.

Forgetting the 'f' suffix for float literals.

The variable price is declared as a float and initialized with 19.99.

3fill in blank

hard

Fix the error in the code by completing the variable declaration for a character named grade initialized with 'A'.

C++

char [1] = 'A';

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

Aletter

Bscore

Cmark

Dgrade

Attempts:

3 left

💡 Hint

Common Mistakes

Using double quotes instead of single quotes for char.

Using an incorrect variable name.

The variable grade is declared as a char and initialized with the character 'A'.

4fill in blank

hard

Fill both blanks to declare a boolean variable named isOpen and initialize it with true.

C++

bool [1] = [2];

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

AisOpen

Bfalse

Ctrue

Dopen

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect variable names.

Assigning a string instead of a boolean value.

The variable isOpen is declared as a bool and initialized with true.

5fill in blank

hard

Fill both blanks to declare a constant integer named MAX_USERS initialized with 100.

C++

const int [1] = [2];

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

AMAX_USERS

B100

DmaxUsers

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase variable names for constants.

Adding unnecessary suffixes to integer literals.

The constant integer MAX_USERS is declared and initialized with 100. The third blank is empty because no suffix is needed.

Practice

1. Which of the following correctly declares and initializes an integer variable named age with the value 25 in C++?

easy

A. int age = 25;

B. age int = 25;

C. int = 25 age;

D. int age;

Solution

1. Step 1: Understand variable declaration syntax

   In C++, the type comes first, then the variable name, followed by an optional initialization with '='.

2. Step 2: Check each option

   int age = 25; follows the correct syntax: int age = 25;. Others have wrong order or missing initialization.

3. Final Answer:

   int age = 25; -> Option A

4. Quick Check:

   Type + name + = value = correct declaration [OK]

Hint: Type first, then name, then = value for initialization [OK]

Common Mistakes:

• Putting variable name before type
• Missing semicolon
• Trying to assign value before declaring type

2. Which of the following is a syntax error in C++ variable declaration?

easy

A. float price = 9.99;

B. char letter = 'A';

C. int 2ndNumber = 10;

D. bool isReady = true;

Solution

1. Step 1: Identify valid variable names

   Variable names cannot start with a digit in C++.

2. Step 2: Check each option

   int 2ndNumber = 10; uses 2ndNumber which starts with '2', invalid. Others are correct declarations.

3. Final Answer:

   int 2ndNumber = 10; -> Option C

4. Quick Check:

   Variable names cannot start with digits [OK]

Hint: Variable names must start with letter or underscore [OK]

Common Mistakes:

• Starting variable name with a number
• Using spaces in variable names
• Missing quotes for char values

3. What is the output of the following C++ code?

int x = 5;
int y = x + 3;
std::cout << y << std::endl;

medium

A. 3

B. 8

C. 5

D. Compilation error

Solution

1. Step 1: Understand variable initialization and addition

   Variable x is 5, y is initialized as x + 3, so 5 + 3 = 8.

2. Step 2: Output the value of y

   The code prints y, which is 8.

3. Final Answer:

   8 -> Option B

4. Quick Check:

   5 + 3 = 8 [OK]

Hint: Add values assigned to variables before printing [OK]

Common Mistakes:

• Confusing variable names
• Forgetting to add values
• Assuming uninitialized variables

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

int number;
number = 10
std::cout << number << std::endl;

medium

A. Missing semicolon after assignment

B. Variable 'number' not declared

C. Cannot assign value after declaration

D. std::cout syntax is incorrect

Solution

1. Step 1: Check syntax after assignment

   In C++, each statement must end with a semicolon. The line number = 10 is missing a semicolon.

2. Step 2: Verify other parts

   Variable is declared correctly, assignment after declaration is allowed, and std::cout syntax is correct.

3. Final Answer:

   Missing semicolon after assignment -> Option A

4. Quick Check:

   Statements must end with semicolon [OK]

Hint: Check for missing semicolons after each statement [OK]

Common Mistakes:

• Forgetting semicolon after assignment
• Assuming declaration and assignment must be together
• Misusing std::cout syntax

5. You want to declare and initialize three variables in one line: an integer a with 10, a float b with 5.5, and a boolean c as true. Which of the following is correct in C++?

hard

A. int a = 10, float b = 5.5, bool c = true;

B. int a = 10; int b = 5.5; int c = true;

C. int a = 10, b = 5.5, c = true;

D. int a = 10; float b = 5.5; bool c = true;

Solution

1. Step 1: Understand type declarations in one line

   In C++, variables of different types cannot be declared in a single statement separated by commas. Each type needs its own statement.

2. Step 2: Check each option

   int a = 10; float b = 5.5; bool c = true; declares each variable separately with correct types and initialization. int a = 10, float b = 5.5, bool c = true; tries to mix types in one statement, which is invalid. int a = 10, b = 5.5, c = true; declares all as int, which is wrong for float and bool. int a = 10; int b = 5.5; int c = true; declares all as int, which is incorrect types.

3. Final Answer:

   int a = 10; float b = 5.5; bool c = true; -> Option D

4. Quick Check:

   Different types need separate declarations [OK]

Hint: Declare different types in separate statements [OK]

Common Mistakes:

• Mixing types in one declaration line
• Assigning float or bool to int variables
• Missing semicolons between declarations