Rust by Example

25 Arrays and Slices

An array is a collection of objects of the same type T, stored in contiguous memory. Arrays are created using brackets [], and their size, which is known at compile time, is part of their type signature [T; size].

Slices are similar to arrays, but their size is not known at compile time. Instead, a slice is a two-word object, the first word is a pointer to the data, and the second word is the length of the slice. Slices can be used to borrow a section of an array, and have the type signature &[T].

use std::mem;
// This function borrows a slice
fn analyze_slice(slice: &[i32]) {
    println!("first element of the slice: {}", slice[0]);
    println!("the slice has {} elements", slice.len());
}
fn main() {
    // Fixed-size array (type signature is superfluous)
    let xs: [i32; 5] = [1, 2, 3, 4, 5];
    // All elements can be initialized to the same value
    let ys: [i32; 500] = [0; 500];
    // Indexing starts at 0
    println!("first element of the array: {}", xs[0]);
    println!("second element of the array: {}", xs[1]);
    // `len` returns the size of the array
    println!("array size: {}", xs.len());
    // Arrays are stack allocated
    println!("array occupies {} bytes", mem::size_of_val(&xs));
    // Arrays can be automatically borrowed as slices
    println!("borrow the whole array as a slice");
    analyze_slice(&xs);
    // Slices can point to a section of an array
    println!("borrow a section of the array as a slice");
    analyze_slice(&ys[1 .. 4]);
    // Out of bound indexing yields a panic
    println!("{}", xs[5]);
}
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