While loop in Rust

Introduction

A while loop helps you repeat a set of actions as long as a condition is true. It saves you from writing the same code many times.

When you want to keep asking a user for input until they give a valid answer.

When you want to count up or down until a certain number is reached.

When you want to keep checking if a task is done before moving on.

When you want to repeat a game turn until the player wins or loses.

Syntax

Rust

while condition {
    // code to repeat
}

The condition is checked before each loop. If it is true, the code inside runs.

If the condition is false at the start, the code inside does not run at all.

Examples

This loop prints numbers 0, 1, and 2. It stops when count reaches 3.

Rust

let mut count = 0;
while count < 3 {
    println!("Count is {}", count);
    count += 1;
}

This loop keeps asking for user input until they type "exit".

Rust

let mut input = String::new();
while input.trim() != "exit" {
    input.clear();
    std::io::stdin().read_line(&mut input).expect("Failed to read line");
    println!("You typed: {}", input.trim());
}

Sample Program

This program counts down from 5 to 1, printing each number. After the loop ends, it prints "Blast off!".

Rust

fn main() {
    let mut number = 5;
    while number > 0 {
        println!("Countdown: {}", number);
        number -= 1;
    }
    println!("Blast off!");
}

Important Notes

Make sure the condition will eventually become false, or the loop will run forever.

You can change the variable inside the loop to control when it stops.

Summary

A while loop repeats code while a condition is true.

The condition is checked before each repetition.

Use it when you don't know how many times you need to repeat in advance.

Practice

1.

What does a while loop do in Rust?

easy

A. Stops the program immediately

B. Runs code only once

C. Repeats code a fixed number of times

D. Repeats code as long as a condition is true

Solution

1. Step 1: Understand the purpose of a while loop

   A while loop runs repeatedly while its condition is true.

2. Step 2: Compare options with this behavior

   Only Repeats code as long as a condition is true describes repeating code while a condition is true.

3. Final Answer:

   Repeats code as long as a condition is true -> Option D

4. Quick Check:

   while loop = repeat while true [OK]

Hint: While loops run while condition is true [OK]

Common Mistakes:

• Confusing while loop with for loop
• Thinking while loop runs fixed times
• Believing while loop runs once only

2.

Which of the following is the correct syntax for a while loop in Rust?

??? x < 5 {
    println!("{}", x);
    x += 1;
}

easy

A. while

B. loop

C. for

D. if

Solution

1. Step 1: Identify the keyword for a while loop

   The keyword to start a while loop is while.

2. Step 2: Check other keywords

   loop is an infinite loop, for is for fixed iteration, if is conditional, not looping.

3. Final Answer:

   while -> Option A

4. Quick Check:

   while keyword starts while loop [OK]

Hint: While loops start with 'while' keyword [OK]

Common Mistakes:

• Using 'loop' instead of 'while'
• Using 'for' for while loops
• Using 'if' instead of loop keyword

3.

What will be the output of this Rust code?

let mut count = 1;
while count <= 3 {
    println!("{}", count);
    count += 1;
}

medium

A. No output

B. 1 2 3 4

C. 1 2 3

D. 0 1 2

Solution

1. Step 1: Analyze the loop condition and variable

   The loop runs while count is less than or equal to 3, starting at 1.

2. Step 2: Trace the output values

   It prints 1, then 2, then 3, then stops because count becomes 4.

3. Final Answer:

   1 2 3 -> Option C

4. Quick Check:

   Prints 1 to 3 inclusive [OK]

Hint: Count from 1 to 3 prints 1 2 3 [OK]

Common Mistakes:

• Thinking it prints 4
• Starting count at 0
• Assuming no output

4.

Find the error in this Rust code using a while loop:

let mut i = 0;
while i < 5 {
    println!("{}", i);
}

medium

A. Wrong comparison operator in while condition

B. Missing increment of 'i' inside the loop

C. 'i' should be immutable, not mutable

D. println! macro syntax is incorrect

Solution

1. Step 1: Check loop variable update

   The variable 'i' is never increased inside the loop, so the condition never becomes false.

2. Step 2: Understand effect of missing increment

   Without increment, the loop runs forever (infinite loop).

3. Final Answer:

   Missing increment of 'i' inside the loop -> Option B

4. Quick Check:

   Increment needed to avoid infinite loop [OK]

Hint: Always update loop variable inside while loop [OK]

Common Mistakes:

• Forgetting to increment loop variable
• Using wrong operator but code still compiles
• Misunderstanding mutability requirement

5.

Write a while loop in Rust that prints all even numbers from 2 to 10 inclusive.

Which code snippet correctly does this?

???

hard

A. let mut n = 2;
while n <= 10 {
println!("{}", n);
n += 2;
}

B. let mut n = 1;
while n < 10 {
if n % 2 == 0 {
println!("{}", n);
}
n += 1;
}

C. let mut n = 2;
while n < 10 {
println!("{}", n);
n += 2;
}

D. let mut n = 0;
while n <= 10 {
println!("{}", n);
n += 2;
}

Solution

1. Step 1: Check the range and values printed

   We want even numbers from 2 to 10 inclusive, so start at 2 and go up to 10.

2. Step 2: Analyze each option

   let mut n = 2; while n <= 10 { println!("{}", n); n += 2; } starts at 2, increments by 2, and stops at 10 inclusive. let mut n = 1; while n < 10 { if n % 2 == 0 { println!("{}", n); } n += 1; } starts at 1 and checks evenness each time but stops before 10, missing 10. let mut n = 2; while n < 10 { println!("{}", n); n += 2; } stops before 10, missing 10. let mut n = 0; while n <= 10 { println!("{}", n); n += 2; } starts at 0, which is even but outside the requested range.

3. Final Answer:

   let mut n = 2; while n <= 10 { println!("{}", n); n += 2; } -> Option A

4. Quick Check:

   Start 2, step 2, end 10 inclusive [OK]

Hint: Start at 2, add 2 each loop until 10 [OK]

Common Mistakes:

• Starting at 0 instead of 2
• Stopping before 10
• Not incrementing by 2