Loops in Rust

# CHAPTER 8

Loops in Rust #

1. Chapter Introduction #

2. Learning Objectives #

• Create infinite loops using loop.

• Create conditional loops using while.

• Iterate over collections and ranges using for.

• Control loop flow using break and continue.

• Return values from a loop.

3. The loop Keyword #

You stop a loop using the break keyword.

fn main() {
    let mut counter = 0;

    loop {
        counter += 1;
        println!("Counter is at: {}", counter);

        if counter == 3 {
            println!("Breaking the loop!");
            break; // Exits the loop completely
        }
    }
}

Returning Values from a Loop! Because everything in Rust is an expression, loop can actually return a value when you break!

let result = loop {
    counter += 1;
    if counter == 10 {
        break counter * 2; // Returns 20 to the 'result' variable!
    }
};

4. The while Loop #

fn main() {
    let mut fuel = 3;

    while fuel > 0 {
        println!("Engine running! Fuel remaining: {}", fuel);
        fuel -= 1; // Decrease fuel
    }

    println!("Out of fuel! System stopped.");
}

5. The for Loop (The Most Commonly Used) #

A. Looping through a Range We use the .. syntax to define a range. 1..5 means 1 up to (but not including) 5.

fn main() {
    // Prints 1, 2, 3, 4
    for number in 1..5 {
        println!("Number: {}", number);
    }
    
    // To include the final number, use ..=
    // Prints 1, 2, 3, 4, 5
    for number in 1..=5 {
        println!("Inclusive Number: {}", number);
    }
}

B. Looping through an Array Unlike while loops (which can accidentally cause "index out of bounds" panics if you calculate the array length wrong), for loops iterate safely.

fn main() {
    let servers = ["Alpha", "Beta", "Gamma"];

    // The safest way to loop through items
    for server in servers.iter() {
        println!("Pinging server: {}", server);
    }
}

6. The continue Keyword #

fn main() {
    for i in 1..=5 {
        if i == 3 {
            println!("Skipping 3!");
            continue; // Jumps directly to i = 4
        }
        println!("Processing: {}", i);
    }
}

7. Mini Project: Multiplication Table Generator #

fn main() {
    let table_number = 5;
    println!("--- Multiplication Table for {} ---", table_number);

    for i in 1..=10 {
        let result = table_number * i;
        println!("{} x {} = {}", table_number, i, result);
    }
}

8. Common Mistakes #

• Infinite Loops by Accident: Creating a loop or a while loop but forgetting to update the condition variable (e.g., forgetting fuel -= 1), causing the program to freeze in an infinite loop.

• Off-by-One Errors in Ranges: Thinking 1..10 includes 10. It only goes up to 9. You must use 1..=10 to include the 10.

9. Best Practices #

• Default to for loops: If you are iterating over an array or a known sequence of numbers, always use for. It prevents bugs and is highly optimized by the compiler. Only use loop when you truly need an infinite retry mechanism.

10. Exercises #

1. 1. Write a while loop that counts down from 10 to 1 and prints "Liftoff!".

1. 2. Write a for loop that iterates through an array of three names and prints "Hello, [name]".

11. MCQs with Answers #

12. Interview Questions #

• Q: Explain why you can return a value from a loop expression but not a while expression. (Hint: while might never run if the condition is false immediately, so it can't guarantee returning a value).

• Q: Contrast the use-cases for loop, while, and for.

13. Summary #

14. Next Chapter Recommendation #