C++ Switch Case

Introduction

When writing programs, you'll often need to make decisions based on the value of a variable. While if-else statements can handle such decisions, they can become unwieldy when dealing with multiple conditions. The switch statement provides a cleaner alternative for handling multi-way decisions based on a single expression.

The switch statement evaluates an expression once and compares it against multiple case values to find a match and execute the corresponding code block. It's particularly useful when you need to select one option from many alternatives.

Basic Syntax

The general syntax of a switch statement in C++ is:

cpp

switch (expression) {
    case constant1:
        // code to be executed if expression equals constant1
        break;
    case constant2:
        // code to be executed if expression equals constant2
        break;
    // you can have any number of case statements
    default:
        // code to be executed if expression doesn't match any constants
}

Let's break down the components:

• switch: The keyword that starts the statement
• expression: The value to be compared (usually a variable)
• case: Defines a specific value to compare against
• constant: A literal value (like 1, 2, 'A', etc.)
• break: Exits the switch statement
• default: Optional section that executes if no case matches

How Switch Case Works

flowchart TD A[Start] --> B[Evaluate expression] B --> C[Compare with case values] C -->|case 1| D[Execute case 1 code] C -->|case 2| E[Execute case 2 code] C -->|case 3| F[Execute case 3 code] C -->|No match| G[Execute default code] D --> H[break?] E --> H F --> H G --> I[End] H -->|Yes| I H -->|No| J[Continue to next case] J --> H

Basic Examples

Example 1: Day of the Week

cpp

#include <iostream>
using namespace std;

int main() {
    int day;
    cout << "Enter day number (1-7): ";
    cin >> day;
    
    switch (day) {
        case 1:
            cout << "Monday";
            break;
        case 2:
            cout << "Tuesday";
            break;
        case 3:
            cout << "Wednesday";
            break;
        case 4:
            cout << "Thursday";
            break;
        case 5:
            cout << "Friday";
            break;
        case 6:
            cout << "Saturday";
            break;
        case 7:
            cout << "Sunday";
            break;
        default:
            cout << "Invalid day number!";
    }
    
    return 0;
}

Input:

4

Output:

Thursday

Input:

9

Output:

Invalid day number!

Example 2: Simple Calculator

cpp

#include <iostream>
using namespace std;

int main() {
    double num1, num2;
    char operation;
    
    cout << "Enter first number: ";
    cin >> num1;
    cout << "Enter operation (+, -, *, /): ";
    cin >> operation;
    cout << "Enter second number: ";
    cin >> num2;
    
    switch (operation) {
        case '+':
            cout << num1 << " + " << num2 << " = " << num1 + num2;
            break;
        case '-':
            cout << num1 << " - " << num2 << " = " << num1 - num2;
            break;
        case '*':
            cout << num1 << " * " << num2 << " = " << num1 * num2;
            break;
        case '/':
            if (num2 != 0)
                cout << num1 << " / " << num2 << " = " << num1 / num2;
            else
                cout << "Division by zero is not allowed!";
            break;
        default:
            cout << "Invalid operation!";
    }
    
    return 0;
}

Input:

10
*
5

Output:

10 * 5 = 50

Important Details About Switch Case

1. Fall-through Behavior

When you omit the break statement, C++ will continue executing the code for all subsequent cases until it encounters a break or reaches the end of the switch statement. This is called "fall-through" behavior.

cpp

#include <iostream>
using namespace std;

int main() {
    int x = 2;
    
    switch (x) {
        case 1:
            cout << "One" << endl;
            // No break, so it falls through to the next case
        case 2:
            cout << "Two" << endl;
            // No break, so it falls through to the next case
        case 3:
            cout << "Three" << endl;
            break;
        case 4:
            cout << "Four" << endl;
            break;
        default:
            cout << "Other number" << endl;
    }
    
    return 0;
}

Output:

Two
Three

Fall-through can be useful in certain scenarios, but it can also lead to bugs if used accidentally. Always include break statements unless you explicitly want fall-through behavior.

2. Valid Types for Switch Expression

The switch expression and case values must be of integral types (int, char, enum) or of a class type with a single conversion function to an integral type. You cannot use:

• Floating-point values (float, double)
• Strings (string)
• Booleans (bool) (though they work since they convert to integers)

3. Case Values Must Be Constant Expressions

Case values must be compile-time constants:

cpp

int main() {
    const int x = 5; // This is a compile-time constant
    int y = 10;      // This is not a compile-time constant
    int value = 5;
    
    switch (value) {
        case 1 + 2:  // Valid - compile-time constant expression
            cout << "Three";
            break;
        case x:      // Valid - x is a const
            cout << "Five";
            break;
        // case y:   // INVALID - y is not a compile-time constant
        //     cout << "Ten";
        //     break;
        default:
            cout << "Other";
    }
    
    return 0;
}

Practical Applications

Menu-Based Systems

cpp

#include <iostream>
using namespace std;

int main() {
    int choice;
    
    do {
        // Display menu
        cout << "\n----- STUDENT DATABASE -----\n";
        cout << "1. Add new student\n";
        cout << "2. View all students\n";
        cout << "3. Search student\n";
        cout << "4. Update student information\n";
        cout << "5. Delete student\n";
        cout << "0. Exit\n";
        cout << "Enter your choice: ";
        cin >> choice;
        
        switch (choice) {
            case 1:
                cout << "Adding new student...\n";
                // Code to add student
                break;
            case 2:
                cout << "Viewing all students...\n";
                // Code to view students
                break;
            case 3:
                cout << "Searching for a student...\n";
                // Code to search student
                break;
            case 4:
                cout << "Updating student information...\n";
                // Code to update student
                break;
            case 5:
                cout << "Deleting student...\n";
                // Code to delete student
                break;
            case 0:
                cout << "Exiting program. Goodbye!\n";
                break;
            default:
                cout << "Invalid choice! Please try again.\n";
        }
    } while (choice != 0);
    
    return 0;
}

State Machine Implementation

cpp

#include <iostream>
using namespace std;

enum TrafficLightState {
    RED,
    YELLOW,
    GREEN
};

int main() {
    TrafficLightState currentState = RED;
    char input;
    
    cout << "Traffic Light Simulator\n";
    cout << "Press 'n' to change to next state, 'q' to quit\n";
    
    while (true) {
        // Display current state
        switch (currentState) {
            case RED:
                cout << "Current state: RED - Stop!\n";
                break;
            case YELLOW:
                cout << "Current state: YELLOW - Prepare to stop/go!\n";
                break;
            case GREEN:
                cout << "Current state: GREEN - Go!\n";
                break;
        }
        
        cout << "Command: ";
        cin >> input;
        
        if (input == 'q') {
            break;
        } else if (input == 'n') {
            // Transition to next state
            switch (currentState) {
                case RED:
                    currentState = GREEN;
                    break;
                case YELLOW:
                    currentState = RED;
                    break;
                case GREEN:
                    currentState = YELLOW;
                    break;
            }
        } else {
            cout << "Invalid command!\n";
        }
    }
    
    cout << "Simulator ended.\n";
    return 0;
}

Switch vs. If-Else

While both switch and if-else statements can be used for decision making, each has its advantages:

When to use Switch:

• For selecting from multiple options based on a single value
• When comparing a variable against constant values
• When readability and performance matter for many conditions

When to use If-Else:

• For logical conditions (not just equality)
• When conditions involve ranges of values
• When using non-integral types (strings, floats)
• For complex Boolean expressions

Best Practices

1. Always include a default case to handle unexpected values
2. Don't forget break statements unless fall-through is specifically needed
3. Add comments when using fall-through to indicate it's intentional
4. Keep case blocks short for better readability
5. Consider using enums with switch statements to improve code clarity
6. Be consistent with your code formatting and style

Common Pitfalls

1. Missing break statements causing unintended fall-through
2. Duplicate case values which will cause compilation errors
3. Attempting to use non-constant expressions in case labels
4. Trying to use switch with non-integral types like strings or floats
5. Defining variables inside a case without using curly braces

Summary

The switch statement in C++ is a powerful control flow mechanism for handling multi-way decisions based on a single expression. It provides a cleaner and potentially more efficient alternative to multiple if-else statements when comparing a variable against multiple constant values.

Key points to remember:

• The switch expression must be of integral type
• Case values must be constant expressions
• Break statements prevent fall-through behavior
• Include a default case to handle unexpected values

By mastering switch statements, you can write more readable and maintainable code, especially when dealing with multiple conditions based on a single value.

Exercises

1. Write a program that takes a month number (1-12) and prints the number of days in that month.
2. Create a menu-driven program that converts temperatures between Celsius, Fahrenheit, and Kelvin.
3. Implement a simple text-based game where the player can choose different actions using a switch statement.
4. Write a program that takes a character and determines if it's a vowel, consonant, digit, or special character.
5. Create a calculator that can perform basic arithmetic as well as modulus, exponent, and square root operations using a switch statement.

Additional Resources

• C++ Documentation on Switch Statements
• C++ Control Flow
• Book: "The C++ Programming Language" by Bjarne Stroustrup
• C++ Switch Statement on cplusplus.com