Slice lifetime basics in Rust - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a string slice with a lifetime.

Rust

fn print_slice[1](s: &str) {
    println!("{}", s);
}

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

A<'static>

B<'a>

C"static"

D<'b>

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to use angle brackets for lifetimes.

Using quotes around lifetime names incorrectly.

The correct syntax to declare a static lifetime parameter is <'static>.

2fill in blank

medium

Complete the function signature to accept a string slice with a generic lifetime.

Rust

fn first_char[1](s: &str) -> char {
    s.chars().next().unwrap()
}

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

A"static"

B<'a>

C<'b>

D<'static>

Attempts:

3 left

💡 Hint

Common Mistakes

Using a fixed lifetime like 'static' when a generic one is needed.

Omitting the angle brackets.

Using a generic lifetime parameter <'a> allows the function to accept slices with any lifetime.

3fill in blank

hard

Fix the error in the function signature by adding the correct lifetime annotation.

Rust

fn longest<'a>(x: &[1]str, y: &'a str) -> &'a str {
    if x.len() > y.len() { x } else { y }
}

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

A, 'a

B<'a>

C<'b>

D'a

Attempts:

3 left

💡 Hint

Common Mistakes

Annotating only one parameter with a lifetime.

Not annotating the return type.

Both parameters and the return type must share the same lifetime 'a, so &'a str is needed.

4fill in blank

hard

Complete the code to correctly define a function returning a slice with lifetime.

Rust

fn first_word<'a>(s: &'a str) -> &'a str {
    let bytes = s.as_bytes();
    let mut i = 0;
    while i < bytes.len() && bytes[i] != b' ' {
        i += 1;
    }
    s[1]i
}

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

A[..

B[i..]

C]

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect slice syntax like [i..] for the first word.

Forgetting the closing bracket.

To return the first word slice, use s[.i] with proper slice syntax.

5fill in blank

hard

Fill all three blanks to create a function that returns the longer of two slices with lifetime.

Rust

fn longer<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() [1] y.len() {
        [2]
    } else {
        [3]
    }
}

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

A>

Bx

Cy

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of > for comparison.

Returning the wrong variable.

The function compares lengths with > and returns x if longer, else y.

Practice

1. What does a slice lifetime in Rust indicate?
fn example<'a>(slice: &'a [i32]) {}

easy

A. How long the slice reference is valid in the program

B. The size of the slice in memory

C. The number of elements in the slice

D. The type of elements inside the slice

Solution

1. Step 1: Understand what a lifetime means in Rust

   Lifetimes tell Rust how long a reference is valid to prevent using invalid data.

2. Step 2: Apply this to a slice reference

   The lifetime on a slice reference means how long the slice can be safely used.

3. Final Answer:

   How long the slice reference is valid in the program -> Option A

4. Quick Check:

   Slice lifetime = reference validity [OK]

Hint: Lifetimes track reference validity duration [OK]

Common Mistakes:

• Confusing lifetime with size or length
• Thinking lifetime is about element type
• Ignoring lifetime annotations on references

2. Which of the following is the correct syntax to declare a function that takes a slice with a lifetime parameter?

fn process_slice(____) {}

easy

A. &'a [i32]

B. &'a [i32];

C. &[i32]

D. &[i32]'a

Solution

1. Step 1: Recall lifetime syntax for references

   Lifetimes are written as &'a before the type to indicate the lifetime of the reference.

2. Step 2: Check each option

   &'a [i32] correctly places &'a before the slice type [i32]. &[i32] misses lifetime, &'a [i32]; has an extra semicolon, &[i32]'a places lifetime incorrectly.

3. Final Answer:

   &'a [i32] -> Option A

4. Quick Check:

   Lifetime syntax = &'a [type] [OK]

Hint: Lifetimes go right after & before type [OK]

Common Mistakes:

• Putting lifetime after the type
• Adding extra semicolons inside parameters
• Omitting lifetime when needed

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

fn get_first<'a>(slice: &'a [i32]) -> &'a i32 {
    &slice[0]
}

fn main() {
    let numbers = vec![10, 20, 30];
    let first = get_first(&numbers);
    println!("{}", first);
}

medium

A. 20

B. 10

C. Compilation error due to lifetime mismatch

D. Runtime panic

Solution

1. Step 1: Understand the function behavior

   The function returns a reference to the first element of the slice, which is 10.

2. Step 2: Check lifetime correctness and usage

   The lifetime 'a ensures the returned reference is valid as long as the input slice is valid. The vector lives long enough, so no error.

3. Final Answer:

   10 -> Option B

4. Quick Check:

   Returned first element = 10 [OK]

Hint: Returned slice element matches first index [OK]

Common Mistakes:

• Thinking lifetime causes compile error here
• Confusing element values
• Expecting runtime panic without reason

4. Identify the error in this Rust code snippet:

fn invalid<'a>() -> &'a [i32] {
    let data = vec![1, 2, 3];
    &data[..]
}

medium

A. Missing lifetime annotation on data

B. Syntax error in slice syntax

C. The function returns a reference to data that will be dropped

D. No error, code is correct

Solution

1. Step 1: Analyze the function return and data lifetime

   The function returns a slice reference with lifetime 'a, but data is created inside the function and dropped when it ends.

2. Step 2: Understand why this causes an error

   Returning a reference to local data is invalid because the data won't live long enough, causing a dangling reference.

3. Final Answer:

   The function returns a reference to data that will be dropped -> Option C

4. Quick Check:

   Returning local reference = error [OK]

Hint: Never return reference to local variable [OK]

Common Mistakes:

• Assuming lifetime annotation fixes local data drop
• Ignoring ownership rules
• Thinking slice syntax is wrong

5. Given this function that returns a slice from two input slices, which lifetime annotation is correct to ensure safety?

fn longest_slice<'a>(x: &'a [i32], y: &'a [i32]) -> &____ [i32] {
    if x.len() > y.len() { x } else { y }
}

hard

A. 'a + 'b

B. 'b

C. 'static

D. 'a

Solution

1. Step 1: Understand the function's return lifetime

   The function returns a slice that could come from either input slice, both sharing lifetime 'a.

2. Step 2: Use shared lifetime for safety

   The correct lifetime for the return is 'a, matching the lifetime of both input slices.

3. Final Answer:

   'a -> Option D

4. Quick Check:

   Return lifetime = 'a of inputs [OK]

Hint: Return lifetime must cover both inputs [OK]

Common Mistakes:

• Choosing only one input lifetime
• Using 'static without reason
• Ignoring lifetime intersection rules