Slice syntax in Rust - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a slice of the first 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[1..4]

B[3..]

C[..3]

D[..]

Attempts:

3 left

💡 Hint

Common Mistakes

Using [3..] which slices from index 3 to the end.

Using [1..4] which slices from index 1 to 3, not the first three elements.

The syntax [..3] creates a slice from the start up to, but not including, index 3.

2fill in blank

medium

Complete the code to create a slice from index 2 to the end.

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[..2]

B[2..]

C[..]

D[1..3]

Attempts:

3 left

💡 Hint

Common Mistakes

Using [..2] which slices from start up to index 2, not from index 2 to end.

Using [1..3] which slices from index 1 to 2 only.

The syntax [2..] creates a slice starting from index 2 to the end.

3fill in blank

hard

Fix the error in the slice syntax to get elements from index 1 to 3.

Rust

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

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

A[1..3]

B[1:3]

C[..3]

D[3..1]

Attempts:

3 left

💡 Hint

Common Mistakes

Using [1:3] which is invalid syntax in Rust.

Using [3..1] which is an invalid range.

Rust slice syntax uses start..end inside square brackets, so [1..3] is correct.

4fill in blank

hard

Complete the code to create a slice from index 1 to 4 (exclusive).

Rust

let arr = [2, 4, 6, 8, 10];
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 include brackets around the range.

Using an incorrect range like 2..5.

The slice syntax requires brackets around the range: [1.4].

5fill in blank

hard

Complete the code to create a slice from the start to index 3 (exclusive) and print its length.

Rust

let arr = [7, 14, 21, 28, 35];
let slice = &arr[[1]];
println!("Length: {}", slice.len());

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

A[

B..3

C]

D..

Attempts:

3 left

💡 Hint

Common Mistakes

Using .. without specifying the end index.

Forgetting to close the brackets.

The slice syntax is [.3] with brackets around the range.

Practice

1. What does the slice syntax &array[2..5] mean in Rust?

easy

A. It creates a view of elements from index 2 up to but not including 5.

B. It copies elements from index 2 to 5 inclusive into a new array.

C. It creates a slice starting at index 5 and ending at index 2.

D. It returns the element at index 2 only.

Solution

1. Step 1: Understand slice range syntax

   The syntax start..end means from start index up to but not including end index.

2. Step 2: Apply to given slice

   So &array[2..5] means elements at indices 2, 3, and 4 are included in the slice.

3. Final Answer:

   It creates a view of elements from index 2 up to but not including 5. -> Option A

4. Quick Check:

   Slice range excludes end index = A [OK]

Hint: Remember slice excludes the end index, like half-open intervals [OK]

Common Mistakes:

• Thinking the slice copies elements instead of borrowing
• Including the end index element in the slice
• Confusing slice with single element access

2. Which of the following is the correct way to get a slice of the first 4 elements of an array named nums?

easy

A. &nums[1..4]

B. &nums[0..4]

C. &nums[0..5]

D. &nums[4..0]

Solution

1. Step 1: Identify slice start and end for first 4 elements

   The first 4 elements are at indices 0, 1, 2, and 3, so slice should start at 0 and end at 4 (exclusive).

2. Step 2: Check options for correct syntax

   &nums[0..4] uses &nums[0..4], which correctly slices elements 0 to 3.

3. Final Answer:

   &nums[0..4] -> Option B

4. Quick Check:

   Slice first 4 elements = &nums[0..4] [OK]

Hint: Start at 0, end at count for first N elements [OK]

Common Mistakes:

• Starting slice at 1 instead of 0
• Including one extra element by using end index too high
• Using reversed range which is invalid

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. [20, 30, 40, 50]

B. [10, 20, 30, 40]

C. [20, 30, 40]

D. [30, 40, 50]

Solution

1. Step 1: Identify slice indices and elements

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

2. Step 2: Understand print output

   Printing the slice with {:?} shows the elements in array-like format.

3. Final Answer:

   [20, 30, 40] -> Option C

4. Quick Check:

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

Hint: Slice excludes end index, so count elements carefully [OK]

Common Mistakes:

• Including element at index 4 mistakenly
• Printing the whole array instead of slice
• Confusing slice with copying elements

4. What is wrong with this Rust code?

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

medium

A. The slice start index is greater than the end index, causing a panic.

B. The slice syntax is correct and will return an empty slice.

C. The code will compile but slice will contain elements 3 and 2.

D. The slice must use parentheses instead of brackets.

Solution

1. Step 1: Analyze slice indices order

   The slice &arr[3..2] has start index 3 and end index 2, which is invalid because start must be less than or equal to end.

2. Step 2: Understand Rust slice behavior

   Rust panics at runtime if slice start is greater than end; it does not return empty slice or reorder indices.

3. Final Answer:

   The slice start index is greater than the end index, causing a panic. -> Option A

4. Quick Check:

   Slice start > end causes panic [OK]

Hint: Slice start must be ≤ end; else panic occurs [OK]

Common Mistakes:

• Assuming reversed slice returns empty slice
• Thinking slice silently swaps indices
• Using parentheses instead of brackets for slices

5. Given a string let text = "hello world";, which slice expression extracts the word "world" correctly?

hard

A. &text[..5]

B. &text[5..10]

C. &text[6..10]

D. &text[6..]

Solution

1. Step 1: Identify indices of "world" in the string

   "hello world" has "world" starting at index 6 and goes to the end (index 10 inclusive).

2. Step 2: Check slice options

   &text[6..] uses &text[6..] which means from index 6 to the end, correctly extracting "world".

3. Final Answer:

   &text[6..] -> Option D

4. Quick Check:

   Slice from start index to end = &text[6..] [OK]

Hint: Use start.. to slice from start index to string end [OK]

Common Mistakes:

• Using wrong end index excluding last character
• Confusing byte indices with character indices (not in this example)
• Using slice that extracts wrong substring