Slices and functions

Pass slices to functions to borrow data.

Slices and functions

Taking &str instead of &String, and &[T] instead of &Vec<T>, lets a function accept Strings, literals and sub-slices alike. The returned slice is a part of the input:

fn ilk_kelime(s: &str) -> &str {
    for (i, b) in s.bytes().enumerate() {
        if b == b' ' {
            return &s[..i];
        }
    }
    &s[..]
}

As long as the returned &str borrows s, the source can't be changed.

Built-in helpers

The standard library offers methods that produce slices without hand-written loops:

let s = "ad soyad yaş";
let ilk = s.split(' ').next().unwrap_or("");   // "ad"
let (sol, sag) = s.split_at(2);               // ("ad", " soyad yaş")
let temiz = "  rust  ".trim();                // "rust"

split_whitespace, lines and trim all return parts of the source as &str without copying.

Exercise First word

Write fn ilk_kelime(s: &str) -> &str: find the first space with bytes().enumerate() and return &s[..i]; if there is no space, &s[..]. Print the results for "merhaba dünya" and "tek" separated by a space.

Show example solution

fn ilk_kelime(s: &str) -> &str {
    for (i, b) in s.bytes().enumerate() {
        if b == b' ' {
            return &s[..i];
        }
    }
    &s[..]
}

fn main() {
    println!("{} {}", ilk_kelime("merhaba dünya"), ilk_kelime("tek"));
}

Test yourself

How is <code>ilk_kelime</code> called with a <code>String</code>?
- ilk_kelime(metin)
- ilk_kelime(*metin)
- It can't be called
- ilk_kelime(&metin)
Show answer

ilk_kelime(&metin) — &String is converted to &str automatically (deref coercion).

Slice parameter

fn en_buyuk(v: ____) -> i32 {
    *v.iter().max().unwrap()
}

Show answer

fn en_buyuk(v: &[i32]) -> i32 {
    *v.iter().max().unwrap()
}

Method and result on " ad soyad "

trim()

"ad soyad"
split_whitespace().count()

2
len()

11
trim().split_at(2).0

"ad"

Next lesson: Range types

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