Loop execution flow in Rust - Step-by-Step Execution

Concept Flow - Loop execution flow

Initialize counter i=0

↓

Check condition i < 3?

No→EXIT LOOP

Yes↓

Execute loop body

↓

Update counter i = i + 1

↩Back to condition check

The loop starts by setting a counter, checks if it meets the condition, runs the loop body if yes, updates the counter, and repeats until the condition is false.

Execution Sample

Rust

fn main() {
    let mut i = 0;
    while i < 3 {
        println!("i = {}", i);
        i += 1;
    }
}

This Rust code runs a loop printing the value of i from 0 to 2.

Execution Table

Step	i value	Condition i < 3?	Action	Output
1	0	true	Print i, i = 0; i = i + 1	i = 0
2	1	true	Print i, i = 1; i = i + 1	i = 1
3	2	true	Print i, i = 2; i = i + 1	i = 2
4	3	false	Exit loop

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

Variable Tracker

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

Key Moments - 2 Insights

Why does the loop stop when i equals 3?

Because the condition i < 3 becomes false at step 4 in the execution_table, so the loop exits.

When is the value of i increased during the loop?

After printing i in each iteration, i is increased by 1 as shown in the Action column of steps 1 to 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of i at step 2?

A 2

B 0

C 1

D 3

💡 Hint

Check the 'i value' column at step 2 in the execution_table.

At which step does the loop condition become false?

A Step 4

B Step 3

C Step 1

D Step 2

💡 Hint

Look at the 'Condition i < 3?' column in the execution_table.

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

A 3 times

B 2 times

C 1 time

D 0 times

💡 Hint

Refer to the variable_tracker and how the condition controls loop iterations.

Concept Snapshot

Rust loop execution flow:
- Initialize counter before loop
- Check condition before each iteration
- Run loop body if condition true
- Update counter inside loop
- Repeat until condition false
- Loop exits cleanly

Full Transcript

This example shows a Rust while loop starting with i = 0. Each step checks if i is less than 3. If yes, it prints i and increases i by 1. When i reaches 3, the condition fails and the loop stops. The variable i changes from 0 to 3 over the loop. Key points are when the condition is checked and when i is updated. The loop runs exactly 3 times, printing i values 0, 1, and 2.

Practice

1. What does the break statement do inside a Rust loop?

easy

A. It restarts the loop from the beginning.

B. It skips the current iteration and continues with the next one.

C. It pauses the loop until a condition is met.

D. It stops the loop immediately and exits it.

Solution

1. Step 1: Understand the role of break in loops

   The break statement immediately stops the loop and exits it.

2. Step 2: Compare with other loop controls

   continue skips to the next iteration, but break ends the loop entirely.

3. Final Answer:

   It stops the loop immediately and exits it. -> Option D

4. Quick Check:

   break stops loop = C [OK]

Hint: Remember: break = stop loop, continue = skip iteration [OK]

Common Mistakes:

• Confusing break with continue
• Thinking break pauses instead of stops
• Assuming break restarts the loop

2. Which of the following is the correct syntax to skip the current iteration in a Rust for loop?

easy

A. skip;

B. continue;

C. break;

D. next;

Solution

1. Step 1: Identify the keyword to skip iteration

   In Rust, continue; skips the rest of the current loop iteration and moves to the next.

2. Step 2: Check other options for correctness

   break; stops the loop, skip; and next; are not valid Rust keywords for this purpose.

3. Final Answer:

   continue; -> Option B

4. Quick Check:

   Skip iteration = continue [OK]

Hint: Use continue to skip current loop iteration [OK]

Common Mistakes:

• Using break instead of continue
• Using non-existent keywords like skip or next
• Omitting the semicolon

3. What will be the output of this Rust code?

let mut count = 0;
loop {
    count += 1;
    if count == 3 {
        break;
    }
    println!("{}", count);
}

medium

A. 1 2 3

B. 0 1 2

C. 1 2

D. 3

Solution

1. Step 1: Trace the loop iterations and print statements

   Loop increments count from 0 to 1, prints 1; then 2, prints 2; when count is 3, break stops loop before printing.

2. Step 2: Determine printed values

   Only 1 and 2 are printed before break stops the loop.

3. Final Answer:

   1 2 -> Option C

4. Quick Check:

   Prints before break = 1 2 [OK]

Hint: Break stops before printing count 3 [OK]

Common Mistakes:

• Assuming count 3 is printed
• Starting count from 1 instead of 0
• Confusing break and continue effects

4. Identify the error in this Rust loop code:

for i in 1..5 {
    if i == 3 {
        continue
    }
    println!("{}", i);
}

medium

A. Missing semicolon after continue

B. Incorrect range syntax

C. println! macro is used incorrectly

D. continue cannot be used inside for loops

Solution

1. Step 1: Check syntax of continue statement

   In Rust, statements like continue require a semicolon to end the statement.

2. Step 2: Verify other parts of the code

   The range 1..5 and println! usage are correct; continue is valid inside for loops.

3. Final Answer:

   Missing semicolon after continue -> Option A

4. Quick Check:

   Statements need semicolon = A [OK]

Hint: Add semicolon after continue to fix syntax [OK]

Common Mistakes:

• Forgetting semicolon after continue
• Thinking continue is invalid in for loops
• Misreading range syntax

5. Given this Rust code, what will be the final value of sum?

let mut sum = 0;
for i in 1..6 {
    if i % 2 == 0 {
        continue;
    }
    if i == 5 {
        break;
    }
    sum += i;
}

hard

A. 4

B. 5

C. 6

D. 9

Solution

1. Step 1: Analyze loop iterations and conditions

   Loop runs i from 1 to 5 (exclusive 6). For even i, continue skips adding. For i == 5, break stops loop before adding.

2. Step 2: Calculate sum for valid i values

   i=1 (odd, add 1), i=2 (even, skip), i=3 (odd, add 3), i=4 (even, skip), i=5 (break before add). sum = 1 + 3 = 4.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   Sum odd before 5 = 4 [OK]

Hint: Add only odd numbers before 5, skip evens [OK]

Common Mistakes:

• Adding even numbers mistakenly
• Including 5 in sum despite break
• Confusing continue and break effects