Mutable references in Rust - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Rust code with mutable references?

Consider the following Rust code snippet. What will it print?

Rust

fn main() {
    let mut x = 10;
    let y = &mut x;
    *y += 5;
    println!("{}", x);
}

A 10

B 15

C Runtime panic

D Compilation error

💡 Hint

Remember that &mut allows you to change the value it points to.

The mutable reference y changes the value of x by adding 5. So x becomes 15.

🧠 Conceptual

intermediate

Which statement about mutable references in Rust is true?

Choose the correct statement about mutable references in Rust.

A You can have multiple mutable references to the same variable at the same time.

B Mutable references automatically clone the value they point to.

C Mutable references do not allow changing the value they point to.

D You can have only one mutable reference to a variable at a time.

💡 Hint

Think about Rust's borrowing rules to avoid data races.

Rust allows only one mutable reference at a time to ensure safe mutation without data races.

🔧 Debug

advanced

What error does this Rust code produce?

Analyze the code below. What error will the Rust compiler report?

Rust

fn main() {
    let mut val = 5;
    let r1 = &mut val;
    let r2 = &mut val;
    *r1 += 1;
    *r2 += 1;
    println!("{}", val);
}

A Cannot borrow `val` as mutable more than once at a time

B Use of moved value `val`

C Cannot assign twice to immutable variable `val`

D No error, prints 7

💡 Hint

Rust enforces exclusive access for mutable references.

The code tries to create two mutable references to val simultaneously, which Rust forbids.

📝 Syntax

advanced

Which option correctly declares a mutable reference in Rust?

Choose the correct syntax to create a mutable reference to a mutable variable num.

Rust

let mut num = 10;
// Which line correctly creates a mutable reference to num?

A let r = &mut num;

B let r = mut #

C let r = #

D let mut r = #

💡 Hint

Mutable reference syntax uses &mut.

To create a mutable reference, you write &mut variable. Option A is correct.

🚀 Application

expert

What is the value of `data` after this Rust code runs?

Consider this Rust code that uses mutable references and shadowing. What is the final value of data?

Rust

fn main() {
    let mut data = 3;
    {
        let r = &mut data;
        *r += 4;
    }
    let r2 = &mut data;
    *r2 *= 2;
    println!("{}", data);
}

A Compilation error due to multiple mutable borrows

B 7

C 14

D 10

💡 Hint

Mutable references in separate scopes do not overlap.

The first mutable reference adds 4 to 3, making 7. Then the second mutable reference doubles 7 to 14.

Practice

1. What does a mutable reference &mut allow you to do in Rust?

easy

A. Prevent any changes to the value it points to

B. Create multiple immutable references at the same time

C. Move ownership of the value to another variable

D. Change the value it points to without taking ownership

Solution

1. Step 1: Understand what &mut means

   The &mut symbol creates a mutable reference, which means you can change the value it points to without taking ownership.

2. Step 2: Compare with other options

   Options B and D describe immutable references or no changes allowed, and C talks about ownership transfer, which is different from mutable references.

3. Final Answer:

   Change the value it points to without taking ownership -> Option D

4. Quick Check:

   Mutable reference = change value without ownership [OK]

Hint: Mutable references let you change data without moving ownership [OK]

Common Mistakes:

• Confusing mutable references with ownership transfer
• Thinking multiple mutable references are allowed simultaneously
• Believing mutable references prevent changes

2. Which of the following is the correct syntax to create a mutable reference to a variable num in Rust?

easy

A. let ref = #

B. let ref = &mut num;

C. let ref = *num;

D. let ref = mut #

Solution

1. Step 1: Identify the syntax for mutable references

   In Rust, to create a mutable reference, you use &mut before the variable name.

2. Step 2: Check each option

   A creates an immutable reference, B dereferences the value, and D has incorrect syntax order.

3. Final Answer:

   let ref = &mut num; -> Option B

4. Quick Check:

   Mutable reference syntax = &mut variable [OK]

Hint: Use &mut before variable name for mutable reference [OK]

Common Mistakes:

• Forgetting the 'mut' keyword after &
• Placing 'mut' before & incorrectly
• Using * instead of &mut for references

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

fn main() {
    let mut x = 5;
    let r = &mut x;
    *r += 1;
    println!("{}", x);
}

medium

A. 6

B. Runtime panic

C. Compilation error

D. 5

Solution

1. Step 1: Understand the mutable reference usage

   The variable x is mutable. A mutable reference r is created to x. The value pointed by r is incremented by 1 using *r += 1;.

2. Step 2: Determine the final value of x

   Since r points to x, incrementing *r changes x from 5 to 6. The print statement outputs 6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Mutable reference modifies original value = 6 [OK]

Hint: Dereference mutable ref with * to change original value [OK]

Common Mistakes:

• Expecting original value unchanged
• Confusing reference with copy
• Thinking code causes compile error

4. What is wrong with this Rust code?

fn main() {
    let mut val = 10;
    let r1 = &mut val;
    let r2 = &mut val;
    *r1 += 1;
    *r2 += 1;
    println!("{}", val);
}

medium

A. Dereferencing mutable references is not allowed

B. Cannot mutate a variable declared as mutable

C. Cannot have two mutable references to the same variable at the same time

D. Missing semicolon after println!

Solution

1. Step 1: Identify the mutable references

   The code creates two mutable references r1 and r2 to the same variable val at the same time.

2. Step 2: Recall Rust's borrowing rules

   Rust allows only one mutable reference at a time to prevent data races. Having two mutable references simultaneously causes a compile-time error.

3. Final Answer:

   Cannot have two mutable references to the same variable at the same time -> Option C

4. Quick Check:

   Only one mutable reference allowed at once [OK]

Hint: Only one mutable reference allowed at a time [OK]

Common Mistakes:

• Trying to create multiple mutable references simultaneously
• Thinking mutable references can be copied freely
• Ignoring Rust's borrowing rules

5. Given the following code, what is the correct way to safely modify the value inside data using mutable references without causing a compile error?

fn main() {
    let mut data = 100;
    let r1 = &data;
    let r2 = &mut data;
    *r2 += 50;
    println!("{}", data);
}

hard

A. Remove the immutable reference r1 before creating r2

B. Change r2 to an immutable reference &data

C. Use two mutable references r1 and r2 simultaneously

D. Make data immutable by removing mut

Solution

1. Step 1: Understand Rust's borrowing rules

   Rust does not allow mutable and immutable references to the same variable at the same time. Here, r1 is immutable and r2 is mutable, causing a conflict.

2. Step 2: Fix the code by removing the immutable reference

   Removing r1 allows only the mutable reference r2 to exist, so modifying data through r2 is safe and allowed.

3. Final Answer:

   Remove the immutable reference r1 before creating r2 -> Option A

4. Quick Check:

   Mutable and immutable refs cannot coexist [OK]

Hint: Remove immutable refs before mutable refs to avoid errors [OK]

Common Mistakes:

• Trying to use mutable and immutable references simultaneously
• Changing mutable ref to immutable without reason
• Making variable immutable but still trying to mutate