如何将字符串与字符串文字匹配？

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 String s in Rust?

Answer 1

UPDATE Use .as_str()

Reason .as_str() is more concise and enforces stricter type checking. The trait as_ref is implemented for multiple types and its behaviour could be changed for type String , leading to unexpected results. Similarly, if the input argument changes type, the compiler will not signal a problem when that type implements the trait as_ref .

The docs suggest to use as_str as well https://doc.rust-lang.org/std/string/struct.String.html , https://doc.rust-lang.org/std/primitive.str.html

Old answer:

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.

Answer 2

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!"),
}

Answer 3

You could also do

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

See:

• std::string::String
• std::ops::Deref
• Deref coercions

Answer 4

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!"),
}

Answer 5

You can try:

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

Answer 6

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"),
    }
}

Answer 7

Use as_str() on Strings to get string slice

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

if your taking input from the console and want to perform match on it be sure to call trim() after as_str() to remove escape character ie '\n' from the input. As in

match stringthing.as_str().trim() {...}

Answer 8

You can convert the String to a &str using the as_str() method, and then match on the &str value like so:

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

Or you can bind the String value to a variable, and then match on the variable like so:

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

Or you can use the == operator to compare the String value with a string literal like so:

fn main() {
    let stringthing = String::from("c");
    if stringthing == "a" {
        println!("0");
    } else if stringthing == "b" {
        println!("1");
    } else if stringthing == "c" {
        println!("2");
    } else {
        println!("other");
    }
}