Mutable references in Rust - Mini Project: Build & Apply

Understanding Mutable References in Rust

📖 Scenario: Imagine you have a box with some toys. Sometimes, you want to look at the toys without changing them. Other times, you want to rearrange or add new toys inside the box. In Rust, mutable references let you change the contents of the box safely.

🎯 Goal: You will create a simple Rust program that uses mutable references to change the value inside a variable. This will help you understand how to borrow and modify data safely.

📋 What You'll Learn

Create a variable with an initial integer value

Create a mutable reference to that variable

Use the mutable reference to change the variable's value

Print the updated value to see the change

💡 Why This Matters

🌍 Real World

Mutable references are used in Rust to safely change data without copying it. This is useful in programs that need to update information efficiently, like games or data processing tools.

💼 Career

Understanding mutable references is essential for Rust developers, especially in systems programming, embedded development, and performance-critical applications where safe memory management is crucial.

1

Create a variable with an initial value

Create a variable called number and set it to 10.

Rust

# Create a variable called number and set it to 10
# Your code here

Hint

Remember to make the variable mutable by using mut so you can change it later.

2

Create a mutable reference to the variable

Create a mutable reference called number_ref to the variable number using &mut number.

Rust

fn main() {
    let mut number = 10;
    
    // Create a mutable reference to number
    # Your code here
}

Hint

Use &mut to borrow a mutable reference to the variable.

3

Change the variable's value using the mutable reference

Use the mutable reference number_ref to change the value of number to 20 by assigning *number_ref = 20.

Rust

fn main() {
    let mut number = 10;
    let number_ref = &mut number;
    
    // Change the value using the mutable reference
    # Your code here
}

Hint

Use the dereference operator * to change the value through the reference.

4

Print the updated value

Print the value of number using println!("{}", number) to see the updated value.

Rust

fn main() {
    let mut number = 10;
    let number_ref = &mut number;
    *number_ref = 20;
    
    // Print the updated value
    # Your code here
}

Hint

Use println!("{}", number) to display the value.

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