String joins strings in Vec in reverse without `collect`

Question

I am trying to join the lines in a vector in one line, in the reverse order of my order in the vector. The following works:

let v = vec!["a".to_string(), "b".to_string(), "c".to_string()];
v.iter().rev().map(|s| s.clone()).collect::<Vec<String>>().connect(".")

However, this leads to the creation of a temporary vector that I really don't need. Is it possible to do this without collect? I see that connectthis is a method StrVector. Is there nothing for raw iterators?

Answer 1

Here is the iterator extension that I whipped, just for you!

pub trait InterleaveExt: Iterator + Sized {
    fn interleave(self, value: Self::Item) -> Interleave<Self> {
        Interleave {
            iter: self.peekable(),
            value: value,
            me_next: false,
        }
    }
}

impl<I: Iterator> InterleaveExt for I {}

pub struct Interleave<I>
where
    I: Iterator,
{
    iter: std::iter::Peekable<I>,
    value: I::Item,
    me_next: bool,
}

impl<I> Iterator for Interleave<I>
where
    I: Iterator,
    I::Item: Clone,
{
    type Item = I::Item;

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        // Don't return a value if there no next item
        if let None = self.iter.peek() {
            return None;
        }

        let next = if self.me_next {
            Some(self.value.clone())
        } else {
            self.iter.next()
        };

        self.me_next = !self.me_next;
        next
    }
}

It can be called like this:

fn main() {
    let a = &["a", "b", "c"];
    let s: String = a.iter().cloned().rev().interleave(".").collect();
    println!("{}", s);

    let v = vec!["a".to_string(), "b".to_string(), "c".to_string()];
    let s: String = v.iter().map(|s| s.as_str()).rev().interleave(".").collect();
    println!("{}", s);
}

Since then, I found out that this iterator adapter already exists in itertools under the name intersperse- use it instead !.

Cheating answer

, , join...

let mut v = vec!["a".to_string(), "b".to_string(), "c".to_string()];
v.reverse();
println!("{}", v.join("."))

Answer 2

, , :

fn main() {
    let v = vec!["a".to_string(), "b".to_string(), "c".to_string()];
    let mut r = v.iter()
        .rev()
        .fold(String::new(), |r, c| r + c.as_str() + ".");
    r.pop();
    println!("{}", r);
}

String , - . clone() .

, , concat()/connect() . .

Answer 3

, , itertools , - join.

:

use itertools::Itertools;
let v = ["a", "b", "c"];
let connected = v.iter().rev().join(".");