🦀 Rust – While Loops: Conditional Repetition with Safe Control

🧲 Introduction – Why Learn while Loops in Rust?

In Rust, the while loop lets you repeat a block of code as long as a condition is true. It’s ideal when you don’t know the exact number of iterations in advance. Rust’s strict type system ensures your loop logic is safe, explicit, and error-resistant.

🎯 In this guide, you’ll learn:

• How to use while loops effectively
• Compare while vs. loop and for
• Use break, continue, and nested loops
• Examples with output and line-by-line explanation

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🔁 Basic while Loop

🔹 Code Example:

fn main() {
    let mut count = 3;

    while count > 0 {
        println!("Counting down: {}", count);
        count -= 1;
    }

    println!("Liftoff!");
}

🧠 Explanation:

• let mut count = 3; → A mutable counter starts at 3.
• while count > 0 → Condition is checked before each iteration.
• count -= 1; → Reduces count by 1 on every loop.

📤 Output:

Counting down: 3
Counting down: 2
Counting down: 1
Liftoff!

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🧪 Looping Until a Condition is Met

🔹 Code Example:

fn main() {
    let mut num = 1;

    while num * num < 100 {
        println!("num: {}, square: {}", num, num * num);
        num += 1;
    }
}

🧠 Explanation:

• Loops until num * num >= 100.
• Each iteration increases num.

📤 Output (first few lines):

num: 1, square: 1
num: 2, square: 4
num: 3, square: 9
...
num: 9, square: 81

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🔄 Using continue to Skip Iterations

🔹 Code Example:

fn main() {
    let mut i = 0;

    while i < 5 {
        i += 1;

        if i % 2 == 0 {
            continue; // skip even numbers
        }

        println!("Odd number: {}", i);
    }
}

🧠 Explanation:

• i += 1; is placed before the condition to avoid infinite loop.
• continue skips printing when i is even.

📤 Output:

Odd number: 1
Odd number: 3
Odd number: 5

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🛑 Using break to Exit Early

🔹 Code Example:

fn main() {
    let mut attempts = 0;

    while attempts < 10 {
        println!("Attempt: {}", attempts);
        if attempts == 3 {
            break;
        }
        attempts += 1;
    }
}

🧠 Explanation:

• Loop exits when attempts == 3.

📤 Output:

Attempt: 0
Attempt: 1
Attempt: 2
Attempt: 3

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🔁 Nested while Loops

🔹 Code Example:

fn main() {
    let mut outer = 1;

    while outer <= 2 {
        let mut inner = 1;

        while inner <= 3 {
            print!("({},{}) ", outer, inner);
            inner += 1;
        }

        println!();
        outer += 1;
    }
}

🧠 Explanation:

• A while loop inside another—prints coordinate pairs.

📤 Output:

(1,1) (1,2) (1,3) 
(2,1) (2,2) (2,3) 

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⚠️ Common Mistakes with while Loops

Mistake	Problem	Fix
Forgetting mut for variable	“cannot assign twice to immutable”	Use let mut variable = value
Infinite loop	Missing update to loop variable	Ensure loop condition eventually fails
Misplaced continue or break	May skip increment or exit too early	Structure control statements properly

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📌 Summary – Recap & Next Steps

Rust’s while loop is ideal for conditional repetition. It gives you precise control while avoiding common mistakes like off-by-one errors and infinite loops—thanks to Rust’s compiler checks.

🔍 Key Takeaways:

• Use while when loop count is not predetermined
• Loop condition is checked before each iteration
• Use continue to skip and break to exit
• Use nested loops for grid-like control

⚙️ Next up: Dive into Rust – For Loops for range-based and iterator-based looping.

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❓FAQs

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❓ When should I use a while loop in Rust?
✅ Use while when the number of iterations is unknown and based on a condition, such as reading from a file or waiting for input.

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❓ What happens if the loop condition never becomes false?
✅ You’ll create an infinite loop. Rust won’t stop it unless you break manually or the condition eventually fails.

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❓ Can I return a value from a while loop?
❌ No. Unlike loop, a while loop cannot return a value directly via break value;.

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❓ Does Rust support do…while loops?
❌ No. Rust does not have a built-in do…while. You can simulate it using loop with manual condition checks.

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