Why slices are used in Rust - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a slice from the array.

Rust

let arr = [1, 2, 3, 4, 5];
let slice = &arr[1];
println!("{:?}", slice);

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

A[1..3]

B[..3]

C[3..]

D[..]

Attempts:

3 left

💡 Hint

Common Mistakes

Using a range that is out of bounds.

Forgetting the & to borrow the slice.

The slice &arr[..] creates a slice of the whole array.

2fill in blank

medium

Complete the code to get a slice of the first three elements.

Rust

let arr = [10, 20, 30, 40, 50];
let first_three = &arr[1];
println!("{:?}", first_three);

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

A[..3]

B[..]

C[1..4]

D[3..]

Attempts:

3 left

💡 Hint

Common Mistakes

Using a range starting at 3 instead of ending at 3.

Using incorrect indices.

The slice &arr[..3] gets elements from start up to index 3 (exclusive).

3fill in blank

hard

Fix the error in the slice syntax to borrow a slice of the last two elements.

Rust

let arr = [5, 6, 7, 8, 9];
let last_two = &arr[1];
println!("{:?}", last_two);

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

A[3..5]

B[..2]

C[2..]

D[4..]

Attempts:

3 left

💡 Hint

Common Mistakes

Using a range that starts too late or too early.

Using a range that is out of bounds.

The slice &arr[3..5] gets elements at indices 3 and 4 (last two elements).

4fill in blank

hard

Complete the code to create a slice of elements from index 1 to 3.

Rust

let arr = 100, 200, 300, 400, 500];
let mid_slice = &arr[[1];
println!("{:?}", mid_slice);

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

A[

B1

C1..3]

D..

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting brackets.

Using incorrect range syntax.

.3] to get elements at indices 1 and 2.

5fill in blank

hard

Fill both blanks to create a slice of the middle three elements.

Rust

let arr = 7, 14, 21, 28, 35, 42];
let middle = &arr[[1][2];
println!("{:?}", middle);

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

A[

B1

C..4]

D..

Attempts:

3 left

💡 Hint

Common Mistakes

Missing brackets.

Using wrong range endpoints.

.4] gets elements at indices 1, 2, and 3.

Practice

1. Why are slices used in Rust?
let arr = [1, 2, 3, 4]; let slice = &arr[1..3];

easy

A. To access a part of a collection without copying the data

B. To create a new independent array from the original

C. To permanently delete elements from the array

D. To convert an array into a string

Solution

1. Step 1: Understand what a slice does

   A slice is a reference to a part of a collection, like a section of an array, without copying the data.

2. Step 2: Analyze the example code

   The code &arr[1..3] creates a slice referencing elements at index 1 and 2, not a new array or deletion.

3. Final Answer:

   To access a part of a collection without copying the data -> Option A

4. Quick Check:

   Slices = references to parts of collections [OK]

Hint: Slices reference parts without copying data [OK]

Common Mistakes:

• Thinking slices copy data
• Confusing slices with new arrays
• Assuming slices delete elements

2. Which of the following is the correct syntax to create a slice of the first three elements of an array arr in Rust?

easy

A. let slice = arr[0..3];

B. let slice = arr[0, 3];

C. let slice = arr.slice(0, 3);

D. let slice = &arr[0..3];

Solution

1. Step 1: Recall slice syntax in Rust

   Rust slices are references, so you must use & before the range to create a slice.

2. Step 2: Check each option

   let slice = &arr[0..3]; uses &arr[0..3], which is the correct syntax. let slice = arr[0..3]; misses the reference, B uses invalid syntax, and C uses a non-existent method.

3. Final Answer:

   let slice = &arr[0..3]; -> Option D

4. Quick Check:

   Slice syntax = &array[start..end] [OK]

Hint: Slices need & before range to reference data [OK]

Common Mistakes:

• Omitting & before the slice range
• Using invalid methods like slice()
• Using comma instead of range syntax

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

let arr = [10, 20, 30, 40, 50];
let slice = &arr[2..4];
println!("{:?}", slice);

medium

A. [30, 40]

B. [20, 30]

C. [30, 40, 50]

D. [10, 20]

Solution

1. Step 1: Understand the slice range

   The slice &arr[2..4] includes elements at index 2 and 3 (end index 4 is exclusive).

2. Step 2: Identify elements at those indices

   Index 2 is 30, index 3 is 40, so the slice contains [30, 40].

3. Final Answer:

   [30, 40] -> Option A

4. Quick Check:

   Slice range excludes end index [OK]

Hint: Slice end index is exclusive, so stop before it [OK]

Common Mistakes:

• Including the end index element
• Confusing indices with values
• Printing the whole array instead of slice

4. Find the error in this Rust code that tries to create a slice:

let arr = [1, 2, 3, 4];
let slice = arr[1..5];

medium

A. Array declaration syntax is wrong

B. Slice range is out of bounds and missing reference operator

C. Slice syntax is correct, no error

D. Slice must use parentheses instead of brackets

Solution

1. Step 1: Check the slice range validity

   The array has length 4, so index 5 is out of bounds causing an error.

2. Step 2: Check slice syntax

   The code misses the & reference operator needed to create a slice.

3. Final Answer:

   Slice range is out of bounds and missing reference operator -> Option B

4. Quick Check:

   Slice must be &array[start..end] within bounds [OK]

Hint: Slice must be referenced and within array bounds [OK]

Common Mistakes:

• Ignoring out of bounds index
• Forgetting & before slice
• Thinking brackets are wrong syntax

5. You want to write a function that takes a string slice and prints its first 3 characters safely. Which function signature and usage is correct in Rust?

hard

A. fn print_first_three(s: String) { println!("{}", &s[0..3]); }

B. fn print_first_three(s: &String) { println!("{}", s[0..3]); }

C. fn print_first_three(s: &str) { println!("{}", &s[0..3]); }

D. fn print_first_three(s: &str) { println!("{}", s[0..3]); }

Solution

1. Step 1: Understand string slices and indexing

   String slices &str are references to string data and support slicing with range syntax and & to get a slice.

2. Step 2: Analyze each function signature and usage

   fn print_first_three(s: &str) { println!("{}", &s[0..3]); } correctly takes &str and slices with &s[0..3]. fn print_first_three(s: String) { println!("{}", &s[0..3]); } takes ownership (String) but slices correctly. fn print_first_three(s: &String) { println!("{}", s[0..3]); } misses & before slice. fn print_first_three(s: &str) { println!("{}", s[0..3]); } slices without &, causing error.

3. Final Answer:

   fn print_first_three(s: &str) { println!("{}", &s[0..3]); } -> Option C

4. Quick Check:

   Use &str and &s[start..end] for safe string slices [OK]

Hint: Use &str and &s[start..end] to slice strings safely [OK]

Common Mistakes:

• Using String instead of &str for slicing
• Omitting & before slice range
• Trying to slice without reference