While loop in Rust - Step-by-Step Execution

Concept Flow - While loop

Initialize variable

↓

Check condition

|Yes↓

Execute loop body

↓

Update variable

Back to Check↓

Exit loop

↩Back to Check

The while loop starts by checking a condition. If true, it runs the loop body, updates variables, then checks again. It stops when the condition is false.

Execution Sample

Rust

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

This code prints the count from 0 to 2 using a while loop that runs while count is less than 3.

Execution Table

Step	count	Condition (count < 3)	Action	Output
1	0	true	Print and increment count to 1	Count is 0
2	1	true	Print and increment count to 2	Count is 1
3	2	true	Print and increment count to 3	Count is 2
4	3	false	Exit loop

💡 count reaches 3, condition 3 < 3 is false, loop stops

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
count	0	1	2	3	3

Key Moments - 2 Insights

Why does the loop stop when count is 3 even though the code increments count inside the loop?

The condition is checked before each loop iteration (see execution_table step 4). When count is 3, the condition count < 3 is false, so the loop exits before running the body again.

What happens if the condition is false at the start?

The loop body does not run at all. The condition is checked first (execution_table step 4 style), so if false initially, the loop exits immediately.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of count at step 2?

A 1

B 0

C 2

D 3

💡 Hint

Check the 'count' column in execution_table row 2.

At which step does the condition become false and the loop stops?

A Step 3

B Step 4

C Step 2

D Step 1

💡 Hint

Look at the 'Condition' column in execution_table where it changes to false.

If we change the condition to count < 2, how many times will the loop run?

A 1 time

B 3 times

C 2 times

D 0 times

💡 Hint

Refer to variable_tracker and execution_table to see how condition affects loop count.

Concept Snapshot

while condition {
    // code to run
}

- Checks condition before each loop
- Runs body only if condition true
- Stops when condition false
- Useful for repeating until a condition changes

Full Transcript

This visual execution shows how a while loop works in Rust. We start with a variable count set to 0. The loop checks if count is less than 3. If yes, it prints the count and adds 1 to it. This repeats until count reaches 3. Then the condition is false and the loop stops. The variable tracker shows count changing from 0 to 3 step by step. Key moments explain why the loop stops when count is 3 and what happens if the condition is false initially. The quiz questions help check understanding of the loop steps and variable changes.

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