We want to find a first word from a string joined by space.(sentence)

Bad case

1fn first_word(s: &String) -> usize {
2    let bytes = s.as_bytes();
3
4    for (i, &item) in bytes.iter().enumerate() {
5        if item == b' ' {
6            return i;
7        }
8    }
9
10    s.len()
11}

This code looks for an index for a first space in the string.

But, this is bad code.

1fn main() {
2    let mut s = String::from("hello world");
3
4    let word = first_word(&s); // word will get the value 5
5
6    s.clear(); // this empties the String, making it equal to ""
7}

As shown above, word is still alive even after the string is cleared.

Someone may use the index (word variable) to find a first word.

It no longer exists.

Good case

The article says

Having to worry about the index in word getting out of sync with the data in s is tedious and error prone!.

syncing data is good behavior in Rust.