How to match a String against string literals?

Question

I'm trying to figure out how to match a String in Rust.

I initially tried matching like this, but I figured out Rust cannot implicitly cast from std::string::String to &str.

fn main() {
    let stringthing = String::from("c");
    match stringthing {
        "a" => println!("0"),
        "b" => println!("1"),
        "c" => println!("2"),
    }
}

This has the error:

error[E0308]: mismatched types
 --> src/main.rs:4:9
  |
4 |         "a" => println!("0"),
  |         ^^^ expected struct `std::string::String`, found reference
  |
  = note: expected type `std::string::String`
             found type `&'static str`

I then tried to construct new String objects, as I could not find a function to cast a String to a &str.

fn main() {
    let stringthing = String::from("c");
    match stringthing {
        String::from("a") => println!("0"),
        String::from("b") => println!("1"),
        String::from("c") => println!("2"),
    }
}

This gave me the following error 3 times:

error[E0164]: `String::from` does not name a tuple variant or a tuple struct
 --> src/main.rs:4:9
  |
4 |         String::from("a") => return 0,
  |         ^^^^^^^^^^^^^^^^^ not a tuple variant or struct

How to actually match Strings in Rust?

`stringthing.as_str()` is probably the most straightforward of all the answers; I don't like `as_ref` because it's unnecessarily general, which can lead to bugs, and not as explicit, it isn't completely clear that `as_ref()` is going to be a `&str`, `as_str` is simple and clear. — Zorf, Jan 10 '20 at 21:00
@Zorf You are right. The answer was accepted when `as_str` did not exist yet. I changed the accepted answer but thank all people who answered this question! — Jeroen, Jan 10 '20 at 21:04

score 219 · Answer 1 · edited Nov 11 '16 at 17:21

219

as_slice is deprecated, you should now use the trait std::convert::AsRef instead:

match stringthing.as_ref() {
    "a" => println!("0"),
    "b" => println!("1"),
    "c" => println!("2"),
    _ => println!("something else!"),
}

Note that you also have to explicitly handle the catch-all case.

edited Nov 11 '16 at 17:21

Shepmaster

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answered Mar 25 '15 at 22:58

Tijs Maas

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3

using rust 1.4.0 one can use the `trim()` function. Just using `as_ref()` doesn't match the string. – futtetennista Dec 01 '15 at 20:17
1

I think the match fails because of whitespace that `trim()` removes. This is nice for deferencing to match against user input. – Gerard Sexton Sep 20 '16 at 13:25
1

It doesn't work. It can only match _ if I get String from read_line. – Masked Man Nov 26 '16 at 10:20
Don't know much about how rust manages different types of strings, but [it seems to work on a basic example](https://play.rust-lang.org/?gist=d3a1379484fd4a01aa1c16e6cb32d187&version=nightly). – tforgione Jan 24 '18 at 09:40

score 152 · Accepted Answer · edited Jun 12 '20 at 19:31

152

You can do something like this:

match &stringthing[..] {
    "a" => println!("0"),
    "b" => println!("1"),
    "c" => println!("2"),
    _ => println!("something else!"),
}

There's also an as_str method as of Rust 1.7.0:

match stringthing.as_str() {
    "a" => println!("0"),
    "b" => println!("1"),
    "c" => println!("2"),
    _ => println!("something else!"),
}

edited Jun 12 '20 at 19:31

Shepmaster

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answered Sep 25 '15 at 20:56

Anonymous Coward

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score 20 · Answer 3 · edited Nov 11 '16 at 17:26

20

You could also do

match &stringthing as &str {
    "a" => println!("0"),
    "b" => println!("1"),
    "c" => println!("2"),
    _ => println!("something else!"),
}

See:

edited Nov 11 '16 at 17:26

Shepmaster

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answered Jan 22 '16 at 22:28

Marco Scannadinari

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24

This is effectively the same as `.as_str()` and that is way shorter. – amyiris Mar 08 '21 at 16:44
1

".as_str()" is literally 1 more character than "as &str" though? am I missing something? @amyiris – Luis Octavio Lomeli Navarrete Mar 25 '21 at 06:26

score 9 · Answer 4 · edited Jan 12 '18 at 16:34

9

Editor's note: This answer pertains to an version of Rust before 1.0 and does not work in Rust 1.0

You can match on a string slice.

match stringthing.as_slice() {
    "a" => println!("0"),
    "b" => println!("1"),
    "c" => println!("2"),
    _ => println!("something else!"),
}

edited Jan 12 '18 at 16:34

Shepmaster

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answered Aug 19 '14 at 12:44

A.B.

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better to use `.as_ref()` or `.as_str()`, both did not take ownership. – Abrar Khan Dec 08 '19 at 16:00

score 1 · Answer 5 · answered May 03 '20 at 07:33

1

You can try:

fn main() {
    let stringthing = String::from("c");
    match &*stringthing {
        "a" => println!("0"),
        "b" => println!("1"),
        "c" => println!("2"),
        _ => println!("else")
    }
}

answered May 03 '20 at 07:33

omrihhh

49
4

8

It might improve the usefulness of your answer if you explain what `&*stringthing` means and does. – Seth Difley May 03 '20 at 15:53

score 1 · Answer 6 · answered Jan 20 '21 at 18:03

1

You can convert the String into &str by doing this:

fn main() {
let stringthing = String::from("c");
match &stringthing[..] {
    "a" => println!("0"),
    "b" => println!("1"),
    "c" => println!("2"),
}

}

answered Jan 20 '21 at 18:03

crimsondamask

21
1

How to match a String against string literals?

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