Immutable references in Rust - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an immutable reference to the variable num.

Rust

let num = 10;
let r = [1]num;
println!("{}", r);

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

A&

Bref

Cmut

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using * which dereferences a pointer instead of creating a reference.

Using mut which is for mutable references.

Using ref which is used in patterns, not for creating references.

The ampersand & creates an immutable reference to the variable.

2fill in blank

medium

Complete the code to print the value through an immutable reference r.

Rust

let num = 20;
let r = #
println!("{}", [1]r);

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

Amut

B&

Cref

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using & which creates a reference instead of dereferencing.

Using ref which is not used here.

Using mut which is for mutability.

The asterisk * dereferences the immutable reference to access the value.

3fill in blank

hard

Fix the error by completing the code to borrow text immutably.

Rust

let text = String::from("hello");
let r = [1]text;
println!("{}", r);

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

Aref

B*

C&

D&mut

Attempts:

3 left

💡 Hint

Common Mistakes

Using &mut which tries to borrow mutably.

Using * which dereferences instead of borrowing.

Using ref which is not correct here.

Using & borrows text immutably, fixing the error.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps words to their lengths only if the length is greater than 3.

Rust

let words = vec!["apple", "cat", "banana"];
let lengths = words.iter().filter(|&&word| word.len() [1] 3).map(|&word| (word, word.len())).collect::<std::collections::HashMap<_, _>>();

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

A==

B>

C<

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using < which filters words shorter than 3.

Using == which filters words exactly length 3.

Using != which filters words not length 3.

The operator > filters words with length greater than 3.

5fill in blank

hard

Fill all three blanks to create a HashMap from words to their lengths, filtering words with length greater than 3.

Rust

let words = vec!["apple", "cat", "banana"];
let lengths = words.iter().filter(|&&word| word.len() [1] 3).map(|&word| ([2], [3])).collect::<std::collections::HashMap<_, _>>();

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

A>

Bword

Cword.len()

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of > for filtering.

Swapping key and value in the tuple.

Using incorrect variable names.

Use > to filter, then map each word to a tuple of (word, word.len()).

Practice

1. What does an immutable reference in Rust allow you to do?

easy

A. Prevent any access to the data

B. Read data without changing it

C. Create multiple mutable references

D. Change data while reading it

Solution

1. Step 1: Understand the purpose of immutable references

   Immutable references allow you to look at data without changing it.

2. Step 2: Compare options with this understanding

   Only Read data without changing it correctly states that immutable references let you read data without changing it.

3. Final Answer:

   Read data without changing it -> Option B

4. Quick Check:

   Immutable reference = read-only access [OK]

Hint: Immutable references only allow reading data [OK]

Common Mistakes:

• Thinking immutable references allow data modification
• Confusing mutable and immutable references
• Believing immutable references block all access

2. Which of the following is the correct syntax to create an immutable reference to a variable x in Rust?

easy

A. let ref = x&;

B. let ref = *x;

C. let ref = &mut x;

D. let ref = &x;

Solution

1. Step 1: Recall syntax for immutable references

   In Rust, an immutable reference is created using the ampersand before the variable: &x.

2. Step 2: Check each option

   let ref = &x; uses &x correctly. let ref = *x; uses dereference operator incorrectly. let ref = &mut x; creates a mutable reference, not immutable. let ref = x&; is invalid syntax.

3. Final Answer:

   let ref = &x; -> Option D

4. Quick Check:

   Immutable reference syntax = &variable [OK]

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

Common Mistakes:

• Using *x instead of &x
• Confusing mutable (&mut) with immutable (&)
• Writing invalid syntax like x&

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

let x = 10;
let r1 = &x;
let r2 = &x;
println!("{} and {}", r1, r2);

medium

A. Compilation error due to multiple references

B. Runtime error

C. 10 and 10

D. 10 and 0

Solution

1. Step 1: Understand multiple immutable references

   Rust allows multiple immutable references to the same data at the same time without error.

2. Step 2: Check the print statement

   Both r1 and r2 point to x which is 10, so printing them shows "10 and 10".

3. Final Answer:

   10 and 10 -> Option C

4. Quick Check:

   Multiple immutable refs print same value [OK]

Hint: Multiple immutable references can coexist safely [OK]

Common Mistakes:

• Thinking multiple immutable references cause errors
• Expecting runtime errors from references
• Confusing mutable and immutable references rules

4. Identify the error in this Rust code snippet:

let mut x = 5;
let r1 = &x;
let r2 = &mut x;
println!("{} and {}", r1, r2);

medium

A. Cannot have mutable and immutable references at the same time

B. Missing semicolon after let r1

C. Cannot print references directly

D. Variable x should not be mutable

Solution

1. Step 1: Check reference rules in Rust

   Rust forbids having a mutable reference while immutable references exist to the same variable.

2. Step 2: Analyze the code

   Here, r1 is an immutable reference and r2 is a mutable reference to x at the same time, causing a compile error.

3. Final Answer:

   Cannot have mutable and immutable references at the same time -> Option A

4. Quick Check:

   Mutable + immutable refs together = error [OK]

Hint: Mutable and immutable refs can't coexist simultaneously [OK]

Common Mistakes:

• Ignoring Rust's borrowing rules
• Thinking printing references causes error
• Assuming mutability of variable is the problem

5. Given this Rust function:

fn print_value(val: &i32) {
    println!("Value is {}", val);
}

fn main() {
    let mut num = 20;
    let r1 = #
    print_value(r1);
    let r2 = &mut num;
    *r2 += 5;
    print_value(&num);
}

What is the output when this program runs?

hard

A. Value is 20\nValue is 25

B. Compilation error due to mutable and immutable references

C. Value is 25\nValue is 25

D. Runtime error

Solution

1. Step 1: Analyze references in main function

   First, an immutable reference r1 to num is created and passed to print_value. Then a mutable reference r2 is created after r1 is no longer used.

2. Step 2: Check Rust borrowing rules

   Rust allows mutable references after immutable references are no longer used. Here, r1 is used only in print_value, so r2 creation is valid.

3. Step 3: Determine output

   First print_value prints 20, then after incrementing via r2, second print_value prints 25.

4. Final Answer:

   Value is 20 Value is 25 -> Option A

5. Quick Check:

   Immutable ref used before mutable ref creation [OK]

Hint: Mutable ref can be created after immutable ref is no longer used [OK]

Common Mistakes:

• Assuming code runs into compile error
• Ignoring Rust's borrowing rules about scopes
• Thinking mutable ref can coexist with immutable ref simultaneously