Variable declaration and initialization in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a variable declaration in C++?

A variable declaration tells the computer to create a space in memory to store a value of a specific type, but it does not assign a value yet.

beginner

What does variable initialization mean?

Variable initialization means giving a variable its first value at the time it is created.

beginner

How do you declare and initialize an integer variable named age with the value 25 in C++?

You write: int age = 25; This creates the variable age and sets its value to 25.

intermediate

What happens if you declare a variable without initializing it?

The variable has an undefined value (garbage) until you assign a value to it. Using it before assignment can cause errors.

intermediate

Show two ways to initialize a variable in C++.

1. Using assignment: int x = 10;
2. Using constructor style: int x(10);

Which of the following is a correct variable declaration and initialization in C++?

A number = 5;

B int number = 5;

C int number;

D int = 5 number;

Option B declares an integer variable named 'number' and initializes it with 5. Option C declares but does not initialize. Option A is an assignment without declaration. Option D is invalid syntax.

What is the value of an uninitialized local variable in C++?

A Compiler error

B Zero

C Always one

D Undefined (garbage value)

Local variables not initialized have undefined values, which means they contain whatever was in memory before.

Which keyword is used to declare a variable in C++?

A int, float, char (type names)

B let

C declare

D var

In C++, variables are declared by specifying their type like int, float, char, etc.

What does this line do? double price(19.99);

A Declares and initializes price with 19.99

B Declares a double variable named price without value

C Assigns 19.99 to an existing variable price

D Syntax error

This is constructor-style initialization of the variable price with the value 19.99.

Which of these is NOT a valid variable name in C++?

A total_sum

B value3

C 2ndValue

D _count

Variable names cannot start with a number, so '2ndValue' is invalid.

Explain the difference between variable declaration and variable initialization in C++.

Think about creating a box versus putting something inside it.

Write an example of declaring and initializing a variable of type float with the value 3.14.

Use the syntax: type name = value;

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