Variable declaration and initialization in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of uninitialized local variable

What is the output of this C++ code?

C++

#include <iostream>
int main() {
    int x;
    std::cout << x << std::endl;
    return 0;
}

A Garbage value (undefined behavior)

B 1

C Compilation error

D 0

💡 Hint

Local variables without initialization contain whatever was in memory before.

Local variables in C++ are not automatically initialized. Printing an uninitialized int gives an unpredictable value (garbage).

❓ Predict Output

intermediate

Value of initialized variable with brace syntax

What is the output of this code snippet?

C++

#include <iostream>
int main() {
    int a{5};
    std::cout << a << std::endl;
    return 0;
}

A 0

B 5

C Compilation error

D Garbage value

💡 Hint

Brace initialization sets the variable to the given value.

Using brace initialization int a{5}; sets a to 5.

❓ Predict Output

advanced

Output of const variable initialization

What will this program print?

C++

#include <iostream>
int main() {
    const int x = 10;
    // x = 20; // Uncommenting this line causes error
    std::cout << x << std::endl;
    return 0;
}

A Compilation error

B 20

C Undefined behavior

D 10

💡 Hint

Const variables cannot be changed after initialization.

The const int x is initialized to 10 and cannot be changed. The program prints 10.

❓ Predict Output

advanced

Output of auto type deduction with initialization

What is the output of this code?

C++

#include <iostream>
#include <typeinfo>
int main() {
    auto val = 3.14;
    std::cout << typeid(val).name() << std::endl;
    return 0;
}

A f

B i

C d

D c

💡 Hint

auto deduces the type from the initializer.

auto val = 3.14; deduces val as double. typeid(val).name() returns 'd' for double on most compilers.

🧠 Conceptual

expert

Behavior of static variable initialization

Consider this code snippet. What is the output after running main() twice?

#include 
void func() {
    static int count = 0;
    count++;
    std::cout << count << std::endl;
}

int main() {
    func();
    func();
    return 0;
}

A 1 2

B 0 1

C 1 1

D 2 3

💡 Hint

Static variables keep their value between function calls.

The static variable count starts at 0, increments each call. First call prints 1, second prints 2.

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