Slicing strings

Learn the correct way to slice a substring.

Slicing strings

Slicing text with a byte range panics if the range falls in the middle of a character. That's why Rust doesn't allow single-index access like s[0] at all:

let s = "Çiçekçi";
// let c = s[0];        // error: str can't be indexed
// let x = &s[0..1];    // panic: Ç is two bytes
println!("{}", s.is_char_boundary(2));   // true
println!("{}", &s[0..2]);                // Ç

char_indices

If you want to slice at a character boundary without copying, char_indices gives the byte position of each character:

let s = "Çiçekçi";
for (i, c) in s.char_indices().take(4) {
    print!("{i}:{c} ");     // 0:Ç 2:i 3:ç 5:e
}
let (son, _) = s.char_indices().nth(3).unwrap();
println!("{}", &s[..son]);   // Çiç

The byte where the 4th character starts (5) is where the first three characters end.

Exercise Safe first three

For let s = "Çiçekçi";, find the byte position of the 4th character with char_indices().nth(3). Print the slice &s[..konum] and the position separated by a space.

Show example solution

fn main() {
    let s = "Çiçekçi";
    let (konum, _) = s.char_indices().nth(3).unwrap();
    println!("{} {}", &s[..konum], konum);
}

Test yourself

What is the simplest way to safely take the first 3 characters of a text?
- s[3]
- s.bytes().take(3)
- s.chars().take(3).collect::<String>()
- &s[0..3]
Show answer

s.chars().take(3).collect::<String>() — chars advances by character boundaries; &s[0..3] may panic with Turkish letters.

Byte positions

for (i, c) in s.____() {
    println!("{i}: {c}");
}

Show answer

for (i, c) in s.char_indices() {
    println!("{i}: {c}");
}

Expression and result on "Çiçekçi"

&s[0..2]

"Ç"
&s[0..1]

Panic
s.is_char_boundary(1)

false
s.char_indices().nth(3)

Some((5, 'e'))

Next lesson: contains and replace

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