Return a Formatted String in Rust

Question

I'd like to create a function that takes an x and y coordinate values and returns a string of the format (x,y) :

pub struct Coord {
    x: i32,
    y: i32,
}

fn main() {
    let my_coord = Coord {
        x: 10,
        y: 12
    };

    let my_string = coords(my_coord.x, my_coord.y);

    fn coords(x: i32, y: i32) -> &str{
        let l = vec!["(", x.to_string(), ",", y.to_string(), ")"];
        let j = l.join("");
        println!("{}", j);
        return &j
    }
}

This gives me the error:

   |
14 |     fn coords(x: i32, y: i32) -> &str {
   |                                  ^ expected named lifetime parameter
   |
   = help: this function's return type contains a borrowed value with an elided lifetime, but the lifetime cannot be derived from the arguments
help: consider using the `'static` lifetime
   |

Adding the 'static lifetime seems to cause a host of other problems with this function? How can I fix this?

Answer 1

The more idiomatic approach would be to implement the Display trait for your type Coord which would allow you to call to_string() on it directly, and also would allow you to use it in the println! macro directly. Example:

use std::fmt::{Display, Formatter, Result};

pub struct Coord {
    x: i32,
    y: i32,
}

impl Display for Coord {
    fn fmt(&self, f: &mut Formatter) -> Result {
        write!(f, "({}, {})", self.x, self.y)
    }
}

fn main() {
    let my_coord = Coord { x: 10, y: 12 };
    
    // create string by calling to_string()
    let my_string = my_coord.to_string();
    println!("{}", my_string); // prints "(10, 12)"
    
    // can now also directly pass to println! macro
    println!("{}", my_coord); // prints "(10, 12)"
}

playground

Answer 2

What you are trying to do is not possible. The String you are creating is local to the function and you are trying to return a reference to it.

j will be dropped at the end of the function, so you can't return a reference to it.

You will have to return a String :

fn coords(x: i32, y: i32) -> String {
    let l = vec![
        "(".into(),
        x.to_string(),
        ",".into(),
        y.to_string(),
        ")".into(),
    ];
    let j = l.join("");
    println!("{}", j);
    return j;
}

Playground

---

A better way to do the same:

fn coords(x: i32, y: i32) -> String {
    let x = format!("({},{})", x, y);
    println!("{}", x);
    return x;
}

Playground

Answer 3

this ended up working for me:

pub struct Coord{
    x: i32,
    y: i32,
}

fn main(){
    let my_coord = Coord{
        x: 10,
        y: 12
    };

    let my_string = coords(my_coord.x, my_coord.y);

    fn coords(x: i32, y: i32) -> String{
        let myx = x.to_string(); 
        let myy = y.to_string(); 
        let l = vec!["(", &myx, ",", &myy, ")"];
        let j = l.join("");
        return j; 
    }
}