Why can't I collect strings from a Vec after calling `push` on them?

Question

I am trying to collect vector of string to string with separator $.

let v = [String::from("bump"), String::from("sage"),String::from("lol"), String::from("   kek   ")]; 
let s: String = v.into_iter().map(|x| x.push_str("$")).collect();
println!("{:?}",s );

The code above does not work, but this:

let v = [String::from("hello"), String::from("world"),String::from("shit"), String::from("   +15   ")]; 
let s: String = v.into_iter().collect();
println!("{:?}",s );

is working. How do I solve this problem?

Please post any errors or evidence on why the code isn't working. — possum, Oct 09 '22 at 16:22
You can also simply use `v.join("$")`. See my updated answer. — Peterrabbit, Oct 10 '22 at 07:14
[How do I concatenate strings?](https://stackoverflow.com/a/41688369/155423) — Shepmaster, Oct 10 '22 at 15:21

Finomnis · Accepted Answer · 2022-10-09T21:32:14.170

Your code isn't working because push_str() does not return the string.

So your map() function maps from String to (), because you don't return x from it.

Further, x is not mutable, so you cannot call push_str() on it. You have to declare it mut.

This is your code, minimally modified so that it works:

fn main(){
    let v = [String::from("bump"), String::from("sage"),String::from("lol"), String::from("   kek   ")]; 
    let s: String = v.into_iter().map(|mut x| {x.push_str("$"); x}).collect();
    println!("{:?}",s );
}

"bump$sage$lol$   kek   $"

Further, if you only push a single character, do push('$') instead.

You will notice, however, that there is a $ at the end of the string. Your usecase is perfect for reduce(), so I'd use @Aleksander's answer instead.

Peterrabbit · Answer 2 · 2022-10-10T08:08:38.060

You must return strings from your map.

Note that push_str doesn't return anything.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=50534d454a46093299ce38682733c86a

fn main() {
    let v = [
        String::from("bump"),
        String::from("sage"),
        String::from("lol"),
        String::from(" kek "),
    ];
    let s: String = v
        .iter()
        .map(|x| {
            let mut x = x.to_owned();
            x.push_str("$");
            x
        })
        .collect();
    println!("{}", s);
}

EDIT

If your real use case is more complex and you must use a iterator and a map, you should prefer above answers which are better done (no need to own the returned strings from the map because you collect them into a new string anyway).

But that said if the only purpose is to join your Vec with a separator you should simply do

fn main() {
    let v = [
        String::from("bump"),
        String::from("sage"),
        String::from("lol"),
        String::from(" kek "),
    ];
    
    println!("{}", v.join("$"));
}

Aleksander Krauze · Answer 3 · 2022-10-09T21:26:25.927

You can use Iterator::reduce. Note that it will put separators only between items (and not at the end of string like in Petterrabit's answer) and it will re-use the allocation of fist string (which results in slightly better memory efficiency).

fn main() {
    let v = [
        String::from("bump"),
        String::from("sage"),
        String::from("lol"),
        String::from(" kek "),
    ];
    let s: String = v
        .into_iter()
        .reduce(|mut acc, x| {
            acc.push('$');
            acc.push_str(&x);
            acc
        })
        .unwrap_or_default();
    println!("{}", s);
}

@Finomnis good call. Updated answer. – Aleksander Krauze Oct 09 '22 at 21:26 — Aleksander Krauze, Oct 09 '22 at 21:26

Why can't I collect strings from a Vec after calling `push` on them?

3 Answers3