Array slices in Rust - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a slice of the first three elements from the array.

Rust

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

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

A[..3]

B[3..]

C[1..4]

D[..]

Attempts:

3 left

💡 Hint

Common Mistakes

Using [3..] which slices from index 3 to the end, not the first three elements.

Using [1..4] which starts from index 1, not from the beginning.

The syntax &arr[..3] creates a slice of the first three elements (indices 0, 1, 2) from the array.

2fill in blank

medium

Complete the code to create a slice of elements from index 2 up to (but not including) index 5.

Rust

let arr = [5, 10, 15, 20, 25, 30];
let slice = &arr[1];
println!("{:?}", slice);

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

A[2..5]

B[..5]

C[3..6]

D[1..4]

Attempts:

3 left

💡 Hint

Common Mistakes

Using [..5] which slices from start to index 4, but not starting at index 2.

Using [3..6] which starts at index 3, not 2.

The syntax &arr[2..5] creates a slice from index 2 up to (but not including) index 5.

3fill in blank

hard

Fix the error in the code to correctly create a slice of the last three elements.

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[5..]

B[3..]

C[..3]

D[2..5]

Attempts:

3 left

💡 Hint

Common Mistakes

Using [2..5] which slices from index 2 to 4, only last three elements but the question asks to fix the error (original code presumably incorrect).

Using [..3] which slices the first three elements, not the last.

The last three elements are at indices 2, 3, and 4, so &arr[3..] correctly slices them.

4fill in blank

hard

Complete the code to create a slice of elements from index 1 up to (but not including) index 4.

Rust

let arr = 7, 8, 9, 10, 11];
let slice = &arr[[1];
println!("{:?}", slice);

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

A[

B]

C1..4]

D2..5

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to add square brackets around the range.

Using the wrong range like 2..5 which slices different elements.

.4].

5fill in blank

hard

Fill both blanks to create a slice of the middle three elements and print its length.

Rust

let arr = 100, 200, 300, 400, 500];
let slice = &arr[[1];
println!("Length: {}", slice[2]());

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

A[

B2..5]

C.len

D]

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting the closing bracket for the slice.

Using incorrect range values.

Trying to get length without calling the method (missing parentheses).

.5] gets elements at indices 2, 3, 4. Using .len() prints the length of the slice.

Practice

1. What is an array slice in Rust?

easy

A. A new array created by copying elements

B. A macro to print arrays

C. A function that modifies an array

D. A reference to a part of an array without copying data

Solution

1. Step 1: Understand what a slice is

   A slice is a reference to a contiguous part of an array, not a new array.

2. Step 2: Compare options

   Only A reference to a part of an array without copying data correctly describes a slice as a reference without copying.

3. Final Answer:

   A reference to a part of an array without copying data -> Option D

4. Quick Check:

   Slice = reference to part of array [OK]

Hint: Remember: slices borrow, they don't copy [OK]

Common Mistakes:

• Thinking slices create new arrays
• Confusing slices with functions
• Assuming slices own data

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

easy

A. let slice = &arr[1..4];

B. let slice = arr.slice(1, 4);

C. let slice = arr[1:4];

D. let slice = &arr(1..4);

Solution

1. Step 1: Recall Rust slice syntax

   Rust uses &array[start..end] to create a slice referencing elements from start up to but not including end.

2. Step 2: Check each option

   let slice = &arr[1..4]; uses correct Rust syntax. Options A, B, and C use invalid syntax.

3. Final Answer:

   let slice = &arr[1..4]; -> Option A

4. Quick Check:

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

Hint: Use &array[start..end] for slices [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using colons instead of two dots
• Calling slice as a method

3. What is the output of this Rust code?

let arr = [10, 20, 30, 40, 50];
let slice = &arr[1..4];
println!("{:?}", slice);

medium

A. [30, 40, 50]

B. [10, 20, 30]

C. [20, 30, 40]

D. [1, 4]

Solution

1. Step 1: Understand the slice range

   The slice &arr[1..4] includes elements at indices 1, 2, and 3 (20, 30, 40).

2. Step 2: Check the printed output

   Printing the slice shows the elements in the slice as an array-like list.

3. Final Answer:

   [20, 30, 40] -> Option C

4. Quick Check:

   Slice arr[1..4] = [20, 30, 40] [OK]

Hint: Remember slice excludes the end index element [OK]

Common Mistakes:

• Including the element at index 4
• Starting slice at index 0
• Printing the slice as indices

4. Identify the error in this Rust code snippet:

let arr = [1, 2, 3, 4, 5];
let slice = &arr[2..6];
println!("{:?}", slice);

medium

A. The slice range goes out of bounds

B. The slice syntax is incorrect

C. The array is not mutable

D. The println! macro is used incorrectly

Solution

1. Step 1: Check the slice range

   The array has length 5, valid indices are 0 to 4. The slice uses 2..6, which tries to access index 5 (out of bounds).

2. Step 2: Identify the error type

   Rust will panic at runtime due to out-of-bounds slicing.

3. Final Answer:

   The slice range goes out of bounds -> Option A

4. Quick Check:

   Slice range must be within array length [OK]

Hint: Slice end index must be ≤ array length [OK]

Common Mistakes:

• Using an end index beyond array length
• Confusing syntax errors with runtime errors
• Assuming arrays auto-expand

5. Given a function that takes a slice of integers and returns the sum of its elements, which of the following implementations is correct?

fn sum_slice(slice: &[i32]) -> i32 {
    // fill in
}

hard

A. slice.to_vec().sum()

B. slice.iter().sum()

C. slice[0] + slice[1]

D. slice.sum()

Solution

1. Step 1: Understand slice methods

   A slice has an iterator via .iter(), and iterators have a .sum() method to add elements.

2. Step 2: Evaluate each option

   slice.iter().sum() correctly uses slice.iter().sum(). slice.to_vec().sum() is valid because Vec<i32> has sum() method. slice[0] + slice[1] only sums two elements, not all. slice.sum() is invalid because slices don't have sum() directly.

3. Final Answer:

   slice.iter().sum() -> Option B

4. Quick Check:

   Use iter().sum() to sum slice elements [OK]

Hint: Use slice.iter().sum() to sum elements [OK]

Common Mistakes:

• Trying to call sum() directly on slice
• Summing only first two elements
• Calling sum() on Vec<i32>