While loop in Rust - Mini Project: Build & Apply

Counting Down with a While Loop in Rust

📖 Scenario: Imagine you have a countdown timer for a game or event. You want to count down from a starting number to zero, showing each number as it goes down.

🎯 Goal: You will create a Rust program that uses a while loop to count down from a number to zero and print each number.

📋 What You'll Learn

Create a variable called count with the starting number 5

Create a variable called step with the value 1

Use a while loop that runs while count is greater than or equal to 0

Inside the loop, print the current value of count

Inside the loop, decrease count by step

Print the final message "Countdown finished!" after the loop ends

💡 Why This Matters

🌍 Real World

Countdown timers are used in games, cooking timers, and event reminders to show time left.

💼 Career

Understanding loops and conditions is essential for programming tasks like automation, game development, and system monitoring.

1

Create the countdown starting number

Create a variable called count and set it to the number 5.

Rust

# Create a variable called count and set it to 5
# Your code here

Hint

Use let mut count = 5; to create a variable that can change.

2

Add the step variable

Add a variable called step and set it to 1 inside the main function, below count.

Rust

fn main() {
    let mut count = 5;
    // Create a variable called step and set it to 1
    // Your code here
}

Hint

Use let step = 1; to create a fixed step size.

3

Write the while loop to count down

Write a while loop that runs while count is greater than or equal to 0. Inside the loop, print the value of count and then subtract step from count.

Rust

fn main() {
    let mut count = 5;
    let step = 1;

    // Write a while loop that runs while count >= 0
    // Inside the loop, print count and subtract step from count
    // Your code here
}

Hint

Use while count >= 0 { ... } and inside print with println!("{}", count); then subtract with count -= step;.

4

Print the final message after the loop

After the while loop, write a println! statement to print "Countdown finished!".

Rust

fn main() {
    let mut count = 5;
    let step = 1;

    while count >= 0 {
        println!("{}", count);
        count -= step;
    }

    // Print "Countdown finished!" here
    // Your code here
}

Hint

Use println!("Countdown finished!"); after the loop.

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