Ownership and Borrowing

Rust Slices: &str and &[T]

Borrow part of a string or array without copying a single byte.

Intermediate
2 min read
1 runnable example

What you'll learn

String slices
Array slices
Returning a slice

Run code →

&numbers[1..4]

100

201

302

403

504

The slice covers indexes 1, 2 and 3.

A view of part of a collection

A slice is a reference to a continuous part of a collection. &text[0..5] borrows the first five bytes of a string, and &numbers[1..4] borrows three items of an array.

first_word returns a slice of the text it was given, so nothing is copied. find returns the position of the first space, or None when there is none. A slice of an array works the same way and has methods such as iter and len.

Key points

&text[0..5] borrows the first five bytes.
A slice does not copy the data.
&numbers[1..4] has three items.

Runnable Rust example

Rust

Run code →

main.rs

fn first_word(text: &str) -> &str {
    match text.find(' ') {
        Some(index) => &text[..index],
        None => text,
    }
}

fn main() {
    let sentence = String::from("ownership makes Rust safe");
    println!("{}", first_word(&sentence));
    println!("{}", &sentence[10..15]);

    let numbers = [10, 20, 30, 40, 50];
    let middle = &numbers[1..4];
    println!("{:?} sum = {}", middle, middle.iter().sum::<i32>());
}

Get a Run button for this example Result of running the code

ownership
makes
[20, 30, 40] sum = 90

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

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