Mutable references in Rust - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a mutable reference to the variable num.

Rust

let mut num = 5;
let num_ref = &[1] num;

Drag options to blanks, or click blank then click option'

Amut

Bref

Cconst

Dlet

Attempts:

3 left

💡 Hint

Common Mistakes

Using &ref instead of &mut

Forgetting the ampersand &

Using const instead of mut

To create a mutable reference in Rust, you use &mut before the variable name.

2fill in blank

medium

Complete the code to change the value through the mutable reference num_ref.

Rust

let mut num = 10;
let num_ref = &mut num;
*[1] = 20;

Drag options to blanks, or click blank then click option'

Anum

Bnum_ref

Cmut

Dref

Attempts:

3 left

💡 Hint

Common Mistakes

Dereferencing the original variable instead of the reference

Forgetting the dereference operator *

Using the wrong variable name

To change the value through a mutable reference, you dereference it with * and assign a new value.

3fill in blank

hard

Fix the error in the code by completing the blank to allow mutable borrowing.

Rust

let mut value = 15;
let ref2 = &[1] value;
*ref2 += 1;
println!("Value: {}", *ref2);

Drag options to blanks, or click blank then click option'

Aconst

Blet

Cref

Dmut

Attempts:

3 left

💡 Hint

Common Mistakes

Using &const instead of &mut

Forgetting to mark the original variable as mutable

To borrow a variable mutably, you must use &mut. This allows changing the value through the reference.

4fill in blank

hard

Fill both blanks to create a mutable reference and modify the value through it.

Rust

let mut count = 0;
let count_ref = &[1] count;
*count_ref [2] 5;

Drag options to blanks, or click blank then click option'

Amut

B+=

C=

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using = instead of += for addition

Forgetting mut in the reference

Using - instead of +=

The first blank needs mut to create a mutable reference. The second blank uses += to add 5 to the value.

5fill in blank

hard

Fill all three blanks to create a mutable reference, modify the value, and print it.

Rust

let mut score = 10;
let score_ref = &[1] score;
*score_ref [2] 15;
println!("Score: {}", [3]);

Drag options to blanks, or click blank then click option'

Amut

B+=

C*score_ref

Dscore

Attempts:

3 left

💡 Hint

Common Mistakes

Printing the original variable instead of the dereferenced reference

Using = instead of +=

Forgetting mut in the reference

Use mut to create a mutable reference. Use += to add 15 to the value. Print the updated value by dereferencing the mutable reference with *score_ref.

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