The Slice Type

Another data type that does not have ownership is the slice.

• String 속 첫 단어 출력하기

fn first_word(s: $string) -> usize {
  let bytes = s.as_bytes(); // convert `String` to an array of bytes.
  
  for (i, &item) in bytes.iter().enumerate() {
    // `iter` is a method that returns each element in a collection
    // `enumerate` wraps the result of `iter` and returns each element as part of a tuple instead : (index, reference) .
    if item == b' ' {
      return i;
    }
  }
  s.len()
}
fn main() {
  let mut s = String::from("hello world");
  let word = first_word(&s);
  s.clear(); // empties the String -> making it to ""
  // but word is still 5 -> it whould be a bug.
}

Function first_word returns the index of the end of the first word in the string.

But, it's only a meaningful number in the context of the &String.

Solution : String Slices

let s = String::from("hello world");

let hello = &s[0..5]; // &s[..5];
let workd = &s[6..11]; // &s[6..];
// &s[..]; is possible

• make first_word function to return slice

// let s = "Hello, world!";
// type of s : &str, &str is an immutable reference.
fn first_word(s: &String) -> &str {
  let bytes = s.as_bytes();
  
  for (i, &item) in bytes.iter().enumerate() {
    if item ==b' ' {
      return &s[0..i];
    }
  }
  &s[..]
}
fn main() {
  let mut s = String::from("hello world");
  let word = first_word(&s);
  s.clear(); // error : s is borrowed!
}

• better code:

fn first_word(s: &str) -> &str {
  let bytes=s.as_bytes();
  for (i, &item) in bytes.iter().enumerate() {
    if item==b' '{
      return &s[..i];
    }
  }
  &s[..]
}
fn main() {
  let my_string = String::from("hello world");
  let word = first_word(&my_string[..]);
  // String type -> make it to string slice.
  // more general code. 
}

Other Slices

let a = [1,2,3,4,5];
let slice = &a[1..3]; // type of slice : &[i32]