Constants and literals in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of const and literal usage

What is the output of this C++ code snippet?

C++

#include <iostream>
int main() {
    const int x = 5;
    constexpr int y = 10;
    int z = x + y;
    std::cout << z << std::endl;
    return 0;
}

A Undefined behavior

B 510

C Compilation error

D 15

💡 Hint

Remember that const and constexpr values can be used in expressions.

The const int x and constexpr int y are both constants. Adding them results in 15, which is printed.

❓ Predict Output

intermediate

Integer literal suffix effect

What is the output of this C++ code?

C++

#include <iostream>
int main() {
    auto a = 10u;
    auto b = 5;
    std::cout << (a - b) << std::endl;
    return 0;
}

A Compilation error

B Unsigned integer wrap-around value

C -5

D 5

💡 Hint

Check the types of a and b and how subtraction works.

a is unsigned int (10u), b is int (5). Subtracting 5 from 10u results in unsigned arithmetic, which can cause wrap-around if b is larger. Here, 10u - 5 is 5, so the output is 5.

❓ Predict Output

advanced

Floating-point literal precision

What is the output of this C++ code snippet?

C++

#include <iostream>
#include <iomanip>
int main() {
    float f = 1.23456789f;
    double d = 1.23456789;
    std::cout << std::setprecision(9) << f << " " << d << std::endl;
    return 0;
}

A 1.23456788 1.23456789

B Compilation error

C 1.23456789 1.23456788

D 1.23456789 1.23456789

💡 Hint

Float has less precision than double.

The float variable loses precision and prints 1.23456788, while double keeps 1.23456789.

❓ Predict Output

advanced

Character literal and integer conversion

What is the output of this C++ code?

C++

#include <iostream>
int main() {
    char c = 'A';
    int i = c + 1;
    std::cout << i << std::endl;
    return 0;
}

A A1

B Compilation error

C 66

D 65

💡 Hint

Character literals are stored as integer ASCII codes.

'A' has ASCII code 65, adding 1 results in 66 printed as integer.

🧠 Conceptual

expert

Constant expression evaluation

Which of the following declarations is NOT a valid constant expression in C++?

A constexpr int z = rand();

B const int y = 10 * 2;

C constexpr int x = 5 + 3;

D constexpr int w = 7 - 4;

💡 Hint

Constant expressions must be evaluable at compile time.

rand() is a runtime function, so constexpr int z = rand(); is invalid as a constant expression.

Practice

1. Which of the following best describes a constant in C++?

easy

A. A fixed value that cannot be changed after initialization

B. A variable that can change its value anytime

C. A function that returns a fixed value

D. A pointer to a variable

Solution

1. Step 1: Understand the meaning of constant

   A constant in C++ is a value that once set cannot be changed during program execution.

2. Step 2: Compare options with definition

   A fixed value that cannot be changed after initialization matches this definition exactly, while others describe variables or functions.

3. Final Answer:

   A fixed value that cannot be changed after initialization -> Option A

4. Quick Check:

   Constant = fixed value [OK]

Hint: Constants never change value after being set [OK]

Common Mistakes:

• Confusing constants with variables
• Thinking constants are functions
• Assuming pointers are constants

2. Which of the following is the correct way to declare a constant integer named MAX_SIZE with value 100 in C++?

easy

A. int MAX_SIZE = 100;

B. constant int MAX_SIZE = 100;

C. int const MAX_SIZE;

D. const int MAX_SIZE = 100;

Solution

1. Step 1: Recall syntax for constant declaration

   In C++, the keyword const before the type declares a constant.

2. Step 2: Check each option

   const int MAX_SIZE = 100; uses const int MAX_SIZE = 100; which is correct. int MAX_SIZE = 100; misses const, C is incomplete without initialization, D uses invalid keyword.

3. Final Answer:

   const int MAX_SIZE = 100; -> Option D

4. Quick Check:

   Use const before type for constants [OK]

Hint: Use 'const' keyword before type to declare constants [OK]

Common Mistakes:

• Omitting 'const' keyword
• Using 'constant' instead of 'const'
• Not initializing the constant

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

const int x = 5;
int y = 10;
std::cout << x + y << std::endl;

medium

A. Runtime error

B. 15

C. Syntax error

D. 510

Solution

1. Step 1: Understand the code variables

   Variable x is a constant with value 5, y is 10.

2. Step 2: Calculate the expression output

   The expression x + y equals 5 + 10 = 15, which is printed.

3. Final Answer:

   15 -> Option B

4. Quick Check:

   5 + 10 = 15 [OK]

Hint: Add constant and variable values normally [OK]

Common Mistakes:

• Thinking constants cause errors
• Concatenating numbers as strings
• Expecting runtime errors

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

const int a = 10;
a = 20;
std::cout << a << std::endl;

medium

A. Incorrect use of std::cout

B. Missing semicolon after declaration

C. Cannot assign a new value to a constant variable

D. Variable 'a' not declared

Solution

1. Step 1: Analyze constant assignment

   The variable a is declared as constant with value 10.

2. Step 2: Check reassignment

   Trying to assign 20 to a is illegal because constants cannot change value.

3. Final Answer:

   Cannot assign a new value to a constant variable -> Option C

4. Quick Check:

   Constants cannot be reassigned [OK]

Hint: Constants cannot be changed after initialization [OK]

Common Mistakes:

• Trying to reassign constants
• Ignoring compiler error messages
• Confusing constants with variables

5. You want to define a constant literal for the value of pi (3.14159) that cannot be changed and can be used throughout your program. Which of the following is the best way to do this in C++?

hard

A. const double pi = 3.14159;

B. #define pi 3.14159

C. float pi = 3.14159;

D. double const pi;

Solution

1. Step 1: Understand constant literal requirements

   The value should be fixed, unchangeable, and precise (double is better than float).

2. Step 2: Evaluate options

   const double pi = 3.14159; declares a constant double with initialization, which is best practice. float pi = 3.14159; is not constant, C uses macro which is less safe, D misses initialization.

3. Final Answer:

   const double pi = 3.14159; -> Option A

4. Quick Check:

   Use 'const double' with initialization for constants [OK]

Hint: Use 'const double' with value for precise constants [OK]

Common Mistakes:

• Using non-const variables for constants
• Using #define instead of const
• Not initializing constants