Loop execution flow in Rust - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested loops with break

What is the output of this Rust code snippet?

Rust

fn main() {
    let mut count = 0;
    for i in 1..=3 {
        for j in 1..=3 {
            if i * j > 4 {
                break;
            }
            count += 1;
        }
    }
    println!("{}", count);
}

A 9

B 5

C 7

D 6

💡 Hint

Count how many times the inner loop runs before break triggers.

The inner loop breaks when i*j > 4. Counting iterations: i=1 (j=1,2,3) all <=4, count=3; i=2 (j=1,2) 2*3=6>4 break at j=3, count=5; i=3 (j=1) 3*2=6>4 break at j=2, count=6 total.

❓ Predict Output

intermediate

Loop with continue and variable update

What will be printed by this Rust program?

Rust

fn main() {
    let mut sum = 0;
    for x in 1..=5 {
        if x % 2 == 0 {
            continue;
        }
        sum += x;
    }
    println!("{}", sum);
}

A 6

B 9

C 15

D 10

💡 Hint

Sum only odd numbers from 1 to 5.

The loop skips even numbers (2,4). Sum is 1 + 3 + 5 = 9.

🔧 Debug

advanced

Identify the error in this loop

What error will this Rust code produce when compiled?

Rust

fn main() {
    let mut i = 0;
    loop {
        if i == 5 {
            break;
        }
        i += 1;
    }
    println!("{}", i);
}

A No error, prints 5

B RuntimeError: infinite loop

C SyntaxError: missing semicolon after i += 1

D SyntaxError: missing curly brace

💡 Hint

Check if all statements end properly.

The original code was missing a semicolon after 'i += 1'. Adding the semicolon fixes the syntax error, so the code compiles and prints 5.

❓ Predict Output

advanced

Loop with labeled break

What will this Rust program print?

Rust

fn main() {
    let mut count = 0;
    'outer: for i in 1..=3 {
        for j in 1..=3 {
            if i + j == 4 {
                break 'outer;
            }
            count += 1;
        }
    }
    println!("{}", count);
}

A 2

B 6

C 9

D 3

💡 Hint

The labeled break exits both loops when i+j == 4.

The labeled break exits the outer loop when i+j == 4. For i=1: j=1 (2!=4) count=1; j=2 (3!=4) count=2; j=3 (4==4) break 'outer (no increment). No further iterations. Total count=2.

❓ Predict Output

expert

Complex loop with multiple control flow statements

What is the final value of `result` after running this Rust code?

Rust

fn main() {
    let mut result = 0;
    let mut i = 0;
    while i < 5 {
        i += 1;
        if i == 3 {
            continue;
        }
        if i == 4 {
            break;
        }
        result += i;
    }
    println!("{}", result);
}

A 6

B 5

C 3

D 4

💡 Hint

Trace each iteration and note when continue and break happen.

i=1 result=1; i=2 result=3; i=3 continue (skip adding); i=4 break loop; final result=3.

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