How do I get the first character out of a string?

UTF-8 does not define what "character" is so it depends on what you want. In this case, chars are Unicode scalar values, and so the first char of a &str is going to be between one and four bytes.

If you want just the first char, then don't collect into a Vec<char>, just use the iterator:

let text = "hello world!";
let ch = text.chars().next().unwrap();

Alternatively, you can use the iterator's nth method:

let ch = text.chars().nth(0).unwrap();

Bear in mind that elements preceding the index passed to nth will be consumed from the iterator.

I wrote a function that returns the head of a &str and the rest:

fn car_cdr(s: &str) -> (&str, &str) {
    for i in 1..5 {
        let r = s.get(0..i);
        match r {
            Some(x) => return (x, &s[i..]),
            None => (),
        }
    }

    (&s[0..0], s)
}

Use it like this:

let (first_char, remainder) = car_cdr("test");
println!("first char: {}\nremainder: {}", first_char, remainder);

The output looks like:

first char: t
remainder: est

It works fine with chars that are more than 1 byte.

It would be nice to have something similar to Haskell's head function and tail function for such cases.

I wrote this function to act like head and tail together (doesn't match exact implementation)

pub fn head_tail<T: Iterator, O: FromIterator<<T>::Item>>(iter: &mut T) -> (Option<<T>::Item>, O) {
    (iter.next(), iter.collect::<O>())
}

Usage:

// works with Vec<i32>
let mut val = vec![1, 2, 3].into_iter(); 
println!("{:?}", head_tail::<_, Vec<i32>>(&mut val));
// works with chars in two ways
let mut val = "thanks! bedroom builds YT".chars();
println!("{:?}", head_tail::<_, String>(&mut val));
// calling the function with Vec<char>
let mut val = "thanks! bedroom builds YT".chars();
println!("{:?}", head_tail::<_, Vec<char>>(&mut val));

NOTE: The head_tail function doesn't panic! if the iterator is empty. If this matched Haskell's head/tail output, this would have thrown an exception if the iterator was empty. It might also be good to use iterable trait to be more compatible to other types.

How do I get the first character out of a string?

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