How to Concatenate Strings in Rust

Maxwell DeMers — Tue, 15 Sep 2020 04:44:36 +0000

If you are new to Rust, you many have to re-learn how to do certain tasks that you almost did automatically in other languages.

When I was working on swcli, I needed to create a URL dynamically, based on user input. I ended up finding 5 approaches to create the URL. Now, I am going to share those 5 approaches, and highlight which ones may be the best going forward.

Good ol' + Operator

let url = String::from("http://swapi.dev/api/") + &args.attributes + "/" + &args.id;

I just tried this as a carry over from the pre-template literal days. This is one of the ways you could concatenate strings in Javascript.

I should point out a limitation to using the + operator that Chris Biscardi called out in his post. You can only use the + operator on owned values which means you can't use + with string slices.

Format Macro

let url = format!("http://swapi.dev/api/{}/{}", args.attributes, args.id);

Straight from the rust docs:

A common use for format! is concatenation and interpolation of strings.

Use `push_str` on a `String` struct

let mut url = String::from("http://swapi.dev/api/");
url.push_str(&args.attributes);
url.push_str("/");
url.push_str(&args.id);

This pattern is great, but does require a mutable reference. Everything in Rust is immutable by default. If you don't want a mutable reference to your string, consider one of the other options.

Array.concat()

let url = ["http://swapi.dev/api/", &args.attributes, "/", &args.id].concat();

This pattern leverages an array to concatenate the items together. This was recommended by scottmcm as the cleanest and fully efficient way of concatenating strings.

The Actual `url` Crate

let paths = [&args.attributes, "/", &args.id].concat();
let url = Url::parse("http://swapi.dev/api/")?;
let url = url.join(&paths)?;

If you recall, I needed to create a URL dynamically. What I should have started with, and what you should too if you need to create a URL, is just use the url crate.

The .join() method only allows you to append one string to the end of the url and path. In this case, I had to use the .concat() approach mentioned above to concatenate my cli arguments to then join them to the base url.

If you need to concatenate strings for a more general purpose, follow the .concat() recommendations above.

Edit Stackoverflow Response

There is a stackoverflow question that has a lot of great responses as well. Give it a look for some additional details.

Happy coding! 🤓

DEV Community: Maxwell DeMers