Array slices in Rust - Mini Project: Build & Apply

Array slices

📖 Scenario: You are working with a list of daily temperatures recorded over a week. You want to look at a part of this list to analyze the middle days.

🎯 Goal: Learn how to create and use array slices in Rust to access parts of an array without copying data.

📋 What You'll Learn

Create an array with exact temperature values

Create a slice from the array using a range

Print the slice to see the selected temperatures

💡 Why This Matters

🌍 Real World

Array slices let you look at parts of data efficiently, useful in many programs like analyzing sensor data or processing text.

💼 Career

Understanding slices is important for Rust programming jobs, especially when working with performance-sensitive applications.

1

Create the temperature array

Create an array called temperatures with these exact values: [20, 22, 19, 24, 25, 23, 21].

Rust

# Create the array 'temperatures' with the given values
# Your code here

Hint

Use square brackets to create an array and assign it to temperatures.

2

Create a slice of the middle days

Create a slice called mid_week that includes the temperatures from index 2 up to (but not including) index 5 from the temperatures array.

Rust

fn main() {
    let temperatures = [20, 22, 19, 24, 25, 23, 21];
    // Create a slice 'mid_week' from index 2 to 5
    // Your code here
}

Hint

Use &array[start..end] syntax to create a slice.

3

Print the slice

Use println! to print the mid_week slice so you can see the temperatures it contains.

Rust

fn main() {
    let temperatures = [20, 22, 19, 24, 25, 23, 21];
    let mid_week = &temperatures[2..5];
    // Print the slice 'mid_week'
    // Your code here
}

Hint

Use println!("{:?}", variable) to print arrays or slices.

4

Run and see the output

Run the program and observe the output. It should show the temperatures from the middle of the week.

Rust

fn main() {
    let temperatures = [20, 22, 19, 24, 25, 23, 21];
    let mid_week = &temperatures[2..5];
    println!("{:?}", mid_week);
    // Observe the output
}

Hint

The output should be the slice showing the temperatures 19, 24, and 25.

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>