Array slices in Rust

Introduction

Array slices let you look at part of an array without copying it. They help you work with sections of data easily and safely.

When you want to process only a part of an array without making a new copy.

When you want to pass a section of an array to a function without giving full ownership.

When you want to read or modify a continuous range inside an array.

When you want to avoid copying large arrays but still access their parts.

When you want to share read-only access to a part of an array safely.

Syntax

Rust

fn main() {
    let array = [10, 20, 30, 40, 50];
    let slice: &[i32] = &array[1..4];
    // slice includes elements from index 1 up to but not including 4
}

Slices are references to a part of an array, so they do not own the data.

The syntax &array[start..end] creates a slice from index start up to but not including end.

Examples

This creates a slice of the whole array.

Rust

fn main() {
    let numbers = [1, 2, 3, 4, 5];
    let full_slice = &numbers[..];
    println!("Full slice: {:?}", full_slice);
}

This creates an empty slice because start and end are the same.

Rust

fn main() {
    let numbers = [1, 2, 3, 4, 5];
    let empty_slice = &numbers[2..2];
    println!("Empty slice: {:?}", empty_slice);
}

This slice contains only one element at index 3.

Rust

fn main() {
    let numbers = [1, 2, 3, 4, 5];
    let single_element_slice = &numbers[3..4];
    println!("Single element slice: {:?}", single_element_slice);
}

This slice goes from index 3 to the end of the array.

Rust

fn main() {
    let numbers = [1, 2, 3, 4, 5];
    let end_slice = &numbers[3..];
    println!("Slice from index 3 to end: {:?}", end_slice);
}

Sample Program

This program shows how to create slices from an array of fruit names. It prints the original array, a slice of part of the array, the full slice, and an empty slice.

Rust

fn main() {
    let fruits = ["apple", "banana", "cherry", "date", "elderberry"];
    println!("Original array: {:?}", fruits);

    // Slice from index 1 to 4 (banana, cherry, date)
    let fruit_slice = &fruits[1..4];
    println!("Slice of fruits from index 1 to 3: {:?}", fruit_slice);

    // Slice the whole array
    let full_slice = &fruits[..];
    println!("Full slice: {:?}", full_slice);

    // Empty slice example
    let empty_slice = &fruits[2..2];
    println!("Empty slice: {:?}", empty_slice);
}

Important Notes

Time complexity to create a slice is O(1) because it just creates a reference.

Space complexity is O(1) since no new data is copied.

Common mistake: Trying to slice with invalid indexes causes a panic at runtime.

Use slices when you want to borrow parts of arrays safely without copying.

Summary

Array slices let you borrow parts of arrays without copying.

Slices are references and do not own the data.

Use slices to work safely and efficiently with parts of arrays.

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>