Summary - Slices

Combine string and array slices to write a first-word function.

Slices summary

A slice is a borrowed view of contiguous data: &str for text, &[T] for arrays and vectors. Ranges take the forms a..b (b excluded), a..=b (included), ..b, a.. and ... String indexes are byte positions; slicing off a char boundary panics, and the safe option is get. Taking slices in function parameters is the most flexible choice.

What to choose when?

If you only read the data, take a slice; if you return a part of the input, return a slice. If you produced new data (joining, uppercasing), return an owned type: String or Vec<T>. Since a slice can't outlive its source, returning a slice of a local variable is a compile error.

Exercise Last n elements

Write fn son_n(v: &[i32], n: usize) -> &[i32]; it returns &v[v.len() - n..]. Print the result for [1, 2, 3, 4, 5] and n = 2 with {:?}.

Show example solution

fn son_n(v: &[i32], n: usize) -> &[i32] {
    &v[v.len() - n..]
}

fn main() {
    let a = [1, 2, 3, 4, 5];
    println!("{:?}", son_n(&a, 2));
}

Test yourself

For <code>let v = [1, 2, 3, 4]; let s = &v[1..3];</code>, what is <code>s.len()</code>?
- 1
- 2
- 3
- 4
Show answer

2 — The range 1..3 covers indexes 1 and 2: [2, 3].

Safe slice

let s = "abc";
let x = s.____(1..5);   // None

Show answer

let s = "abc";
let x = s.get(1..5);   // None

Type and meaning

&str

Text slice
&[T]

Element slice
&mut [T]

Mutable slice
RangeInclusive

a..=b

Next lesson: Game skeleton

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