How to get a substring of a &str based on character index?

Question

I am trying to write a program that takes a list of words and then, if the word has an even length, prints the two middle letters. If the word has an odd length, it prints the single middle letter.

I can find the index of the middle letter(s), but I do not know how to use that index to print the corresponding letters of the word.

fn middle(wds: &[&str)){
   for word in wds{
      let index = words.chars().count() /2;
      match words.chars().count() % 2{
          0 => println!("Even word found"),
          _ => println!("odd word found")
      }
   }

}

fn main(){

  let wordlist = ["Some","Words","to","test","testing","elephant","absolute"];
  middle(&wordlist);
}

Mikail Khan · Accepted Answer · 2021-12-03 22:45:16Z

2

You can use slices for this, specifically &str slices. Note the &.

These links might be helpful:

fn main() {
    let s = "elephant";
    let mid = s.len() / 2;

    let sliced = &s[mid - 1..mid + 1];
    println!("{}", sliced);
}

answered Dec 3, 2021 at 22:45

Mikail Khan

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1 Comment

user1003967 Over a year ago

Thanks, i was getting confused as i had been using .chars().nth(index).unwrap() to get one letter, then i tried slices but thought if i was specifying two values a start and end i had to use [start,end] syntax as opposed to [start..end] i have it working now

user1003967 · Accepted Answer · 2021-12-03 22:57:08Z

1

Hey after posting i found two different ways of doing it, the fact i had two seperate ways in my head was confusing me and stopping me finding the exact answer.

//i fixed printing the middle letter of the odd numbered string with
word.chars().nth(index).unwrap()

//to fix the even index problem i did
&word[index-1..index+1]

answered Dec 3, 2021 at 22:57

user1003967

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4 Comments

BurntSushi5 Over a year ago

Those aren't the same. The former yields the indexth codepoint. The latter yields the slice of bytes at the range given.

BurntSushi5 Over a year ago

The confusion here likely resides in the fact that your question is (unintentionally) underspecified. Namely, you use the word "letter" a few times without actually defining it. The word "letter" is an abstraction, and there are multiple different valid implementations of it depending on what you're doing.

user1003967 Over a year ago

I just meant it worked because it allowed me to display the correct Char to the screen, in the odd case the middle char, in the even case the middle 2 chars.

Joe_Jingyu Over a year ago

There is no problem with the second solution if all the characters are single-byte encoded, like ASCII. It will become much more complicated when dealing with characters of multi-byte encodings. For example, '中' in Chinese needs three bytes to represent in UTF-8. "中".chars().count() returns 1 while "中".len() returns 3. So, you need to make sure if encoding is a factor to consider for your case.

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