For loop in Rust - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Rust for loop?

Consider the following Rust code. What will it print?

Rust

fn main() {
    let mut sum = 0;
    for i in 1..5 {
        sum += i;
    }
    println!("{}", sum);
}

A Error: cannot add i to sum

B 15

C 14

D 10

💡 Hint

Remember that 1..5 in Rust means 1 up to but not including 5.

The loop runs for i = 1, 2, 3, 4. Their sum is 1+2+3+4 = 10.

🧠 Conceptual

intermediate

How many times does this Rust for loop run?

How many iterations will this loop execute? for x in (0..10).step_by(3) { println!("{}", x); }

A 4

B Error: step_by requires usize

C 10

D 3

💡 Hint

Count numbers from 0 to 9 stepping by 3.

The loop runs for x = 0, 3, 6, 9, so 4 times.

❓ Predict Output

advanced

What is the output of this nested for loop?

Analyze the output of this Rust code snippet:

Rust

fn main() {
    for i in 1..3 {
        for j in 1..=i {
            print!("{}{} ", i, j);
        }
    }
}

A 11 21

B 11 12 21 22

C 11 21 22

D 12 22

💡 Hint

Remember that 1..3 means 1 and 2, and 1..=i includes i.

Outer loop i=1: inner j=1 prints '11 '. Outer loop i=2: inner j=1,2 prints '21 22 '.

🔧 Debug

advanced

What error does this Rust for loop code produce?

Identify the error when running this code:

Rust

fn main() {
    let mut numbers = vec![1, 2, 3];
    for num in numbers.iter() {
        numbers.push(*num + 1);
    }
}

A Compile error: cannot borrow `numbers` as mutable while iterating

B Syntax error: missing semicolon

C No error, prints numbers

D Compile error: cannot move out of `numbers`

💡 Hint

Think about modifying a vector while looping over it.

You cannot mutate a vector by pushing to it while iterating over it, causing a compile error.

🚀 Application

expert

What is the final value of `result` after this loop?

Given this Rust code, what is the value of `result` after the loop finishes?

Rust

fn main() {
    let mut result = 1;
    for i in (1..=4).rev() {
        if i % 2 == 0 {
            result *= i;
        }
    }
    println!("{}", result);
}

A 24

B 8

C 4

D 0

💡 Hint

Multiply only even numbers from 4 down to 1.

Even numbers in 4..=1 reversed are 4 and 2. Multiplying 1*4*2 = 8.

Practice

1. What does a for loop do in Rust?

easy

A. Declares a variable

B. Defines a new function

C. Repeats actions for each item in a collection

D. Stops the program immediately

Solution

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

   A for loop runs code repeatedly for each item in a list or range.

2. Step 2: Match the description to the options

   Only Repeats actions for each item in a collection describes repeating actions over items in a collection.

3. Final Answer:

   Repeats actions for each item in a collection -> Option C

4. Quick Check:

   For loop = repeat over items [OK]

Hint: For loops repeat actions over collections [OK]

Common Mistakes:

• Confusing for loop with function definition
• Thinking for loop declares variables
• Believing for loop stops program

2. Which of these is the correct syntax for a for loop iterating over numbers 1 to 3 in Rust?

easy

A. for i = 1 to 3 { println!("{}", i); }

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

C. for (i in 1..3) println!("{}", i);

D. for i in range(1,3) { println!("{}", i); }

Solution

1. Step 1: Recall Rust for loop syntax

   Rust uses for variable in range { } with ranges like 1..=3 to include 3.

2. Step 2: Check each option

   for i in 1..=3 { println!("{}", i); } uses correct Rust syntax with inclusive range. Others use invalid syntax or functions not in Rust.

3. Final Answer:

   for i in 1..=3 { println!("{}", i); } -> Option B

4. Quick Check:

   Rust for loop uses in and ranges [OK]

Hint: Use 'for var in start..=end' for inclusive ranges [OK]

Common Mistakes:

• Using 'to' instead of 'in' keyword
• Using parentheses around loop variable
• Using Python or other language range syntax

3. What is the output of this Rust code?

let mut sum = 0;
for i in 1..4 {
    sum += i;
}
println!("{}", sum);

medium

A. 6

B. 10

C. 9

D. 7

Solution

1. Step 1: Identify the loop range and iterations

   The loop runs for i = 1, 2, 3 because 1..4 excludes 4.

2. Step 2: Calculate the sum of iterations

   sum = 0 + 1 + 2 + 3 = 6

3. Final Answer:

   6 -> Option A

4. Quick Check:

   1+2+3 = 6 [OK]

Hint: Remember range excludes the end number [OK]

Common Mistakes:

• Including 4 in the sum
• Starting sum at 1 instead of 0
• Confusing inclusive and exclusive ranges

4. Find the error in this Rust for loop code:

for i in 0..5
    println!("{}", i);
}

medium

A. Variable i is not declared

B. Range syntax is incorrect

C. println! macro is used incorrectly

D. Missing curly braces around loop body

Solution

1. Step 1: Check loop syntax

   Rust requires curly braces { } around the loop body if it has more than one statement or to be explicit.

2. Step 2: Identify the missing brace

   The code has a closing brace '}' without an opening brace '{' before println!().

3. Final Answer:

   Missing curly braces around loop body -> Option D

4. Quick Check:

   For loop body needs braces [OK]

Hint: Always use braces {} for loop bodies [OK]

Common Mistakes:

• Forgetting opening brace
• Assuming range syntax is wrong
• Thinking variable needs declaration outside loop

5. How can you use a for loop in Rust to print only even numbers from 2 to 10 inclusive?

hard

A. for i in 2..=10 { if i % 2 == 0 { println!("{}", i); } }

B. for i in 2..10 { if i % 2 = 0 { println!("{}", i); } }

C. for i in 2..=10 { if i / 2 == 0 { println!("{}", i); } }

D. for i in 2..=10 { if i % 2 != 0 { println!("{}", i); } }

Solution

1. Step 1: Understand the range and condition

   We want numbers from 2 to 10 inclusive, so use 2..=10. To print even numbers, check if remainder when divided by 2 is zero.

2. Step 2: Check each option's condition

   for i in 2..=10 { if i % 2 == 0 { println!("{}", i); } } correctly uses i % 2 == 0. for i in 2..10 { if i % 2 = 0 { println!("{}", i); } } uses assignment (=) instead of comparison (==). for i in 2..=10 { if i / 2 == 0 { println!("{}", i); } } uses division instead of modulo. for i in 2..=10 { if i % 2 != 0 { println!("{}", i); } } prints odd numbers.

3. Final Answer:

   for i in 2..=10 { if i % 2 == 0 { println!("{}", i); } } -> Option A

4. Quick Check:

   Use modulo == 0 for even check [OK]

Hint: Use 'if i % 2 == 0' inside for loop [OK]

Common Mistakes:

• Using '=' instead of '==' in condition
• Using division instead of modulo
• Printing odd numbers by mistake