Introduction to Control Flow

Control flow structures allow you to control the execution order of statements in a C++ program. They enable conditional execution (executing code based on certain conditions) and iteration (repeating code multiple times). Mastering these structures is crucial for writing efficient and effective C++ code.

Key Concepts

• Conditional Statements: Execute certain blocks of code based on whether a condition is true or false.
• Loops: Repeat blocks of code multiple times until a condition is met.
• Branching: Directs the flow of control to different parts of the program based on conditions.

If-Else Statements

The if-else statements are used for decision-making in C++. They allow you to execute certain code blocks based on whether a condition evaluates to true or false.

Basic Syntax

if (condition) {
    // Code to execute if condition is true
} else {
    // Code to execute if condition is false
}

Example

int age = 18;

if (age >= 18) {
    std::cout << "You are an adult." << std::endl;
} else {
    std::cout << "You are a minor." << std::endl;
}

Nested If-Else

You can nest if-else statements to handle multiple conditions.

Example:

int score = 85;

if (score >= 90) {
    std::cout << "Grade: A" << std::endl;
} else if (score >= 80) {
    std::cout << "Grade: B" << std::endl;
} else if (score >= 70) {
    std::cout << "Grade: C" << std::endl;
} else {
    std::cout << "Grade: D" << std::endl;
}

If-Else If-Else Ladder

An if-else if-else ladder is used when you have multiple conditions to check sequentially.

Example:

int temperature = 25;

if (temperature < 0) {
    std::cout << "Freezing cold" << std::endl;
} else if (temperature < 15) {
    std::cout << "Cold" << std::endl;
} else if (temperature < 30) {
    std::cout << "Warm" << std::endl;
} else {
    std::cout << "Hot" << std::endl;
}

Switch Statements

The switch statement provides a way to execute different parts of code based on the value of a variable. It is often used as a more readable alternative to multiple if-else statements when checking a single variable against different values.

Basic Syntax

switch (expression) {
    case value1:
        // Code to execute if expression equals value1
        break;
    case value2:
        // Code to execute if expression equals value2
        break;
    default:
        // Code to execute if expression does not match any case
        break;
}

Example

int day = 3;

switch (day) {
    case 1:
        std::cout << "Monday" << std::endl;
        break;
    case 2:
        std::cout << "Tuesday" << std::endl;
        break;
    case 3:
        std::cout << "Wednesday" << std::endl;
        break;
    case 4:
        std::cout << "Thursday" << std::endl;
        break;
    case 5:
        std::cout << "Friday" << std::endl;
        break;
    case 6:
        std::cout << "Saturday" << std::endl;
        break;
    case 7:
        std::cout << "Sunday" << std::endl;
        break;
    default:
        std::cout << "Invalid day" << std::endl;
        break;
}

Key Points

• Break Statement: Exits the switch block after executing the code associated with a case. Without a break, execution continues into the next case.
• Default Case: Handles any values not explicitly covered by other cases.

Loops: For, While, and Do-While

Loops are used to execute a block of code repeatedly based on a condition. C++ provides three main types of loops: for, while, and do-while.

1. For Loop

The for loop is used when you know in advance how many times you need to execute a statement or block of statements.

Basic Syntax

for (initialization; condition; update) {
    // Code to execute repeatedly
}

Example

for (int i = 0; i < 5; ++i) {
    std::cout << i << std::endl;
}

Explanation:

• Initialization: int i = 0 initializes the loop control variable.
• Condition: i < 5 is checked before each iteration. The loop continues while this condition is true.
• Update: ++i increments the loop control variable after each iteration.

2. While Loop

The while loop is used when you want to repeat a block of code an unknown number of times, as long as a condition remains true.

Basic Syntax

while (condition) {
    // Code to execute repeatedly
}

Example

int i = 0;
while (i < 5) {
    std::cout << i << std::endl;
    ++i;
}

Explanation:

• Condition: i < 5 is checked before each iteration. The loop continues as long as this condition is true.
• Update: ++i increments the loop control variable within the loop.

3. Do-While Loop

The do-while loop is similar to the while loop, but it guarantees that the loop body is executed at least once before checking the condition.

Basic Syntax

do {
    // Code to execute repeatedly
} while (condition);

Example

int i = 0;
do {
    std::cout << i << std::endl;
    ++i;
} while (i < 5);

Explanation:

• Loop Body: The code inside the do block is executed first.
• Condition: i < 5 is checked after each iteration. The loop continues as long as this condition is true.

Practical Examples

Example 1: Counting Down

Use a for loop to count down from 10 to 1.

for (int i = 10; i > 0; --i) {
    std::cout << i << std::endl;
}

Example 2: Checking for Prime Numbers

Use a while loop to determine if a number is prime.

int number = 29;
bool isPrime = true;
int i = 2;

while (i <= number / 2) {
    if (number % i == 0) {
        isPrime = false;
        break;
    }
    ++i;
}

if (isPrime) {
    std::cout << number << " is a prime number." << std::endl;
} else {
    std::cout << number << " is not a prime number." << std::endl;
}

Example 3: User Input Validation

Use a do-while loop to repeatedly prompt the user until they enter a valid input.

int userInput;

do {
    std::cout << "Enter a positive number: ";
    std::cin >> userInput;
} while (userInput <= 0);

std::cout << "You entered: " << userInput << std::endl;

Example 4: Factorial Calculation

Calculate the factorial of a number using a for loop.

int n = 5;
int factorial = 1;

for (int i = 1; i <= n; ++i) {
    factorial *= i;
}

std::cout << "Factorial of " << n << " is " << factorial << std::endl;

Conclusion

Control flow mechanisms are vital for directing the execution of your C++ programs. Understanding if-else statements, switch statements, and loops enables you to handle various programming scenarios effectively. By mastering these constructs, you can write more efficient and readable code, manage program execution flow, and handle complex logic with ease. Continue practicing with different examples to strengthen your understanding and improve your problem-solving skills in C++.