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.
as_str did not exist yet. I changed the accepted answer but thank all people who answered this question!
UPDATE: Use .as_str() like this to convert the String to an &str:
match stringthing.as_str() {
"a" => println!("0"),
"b" => println!("1"),
"c" => println!("2"),
_ => println!("something else!"),
}
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.
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!"),
}
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
.as_str() and that is way shorter.
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!"),
}
.as_ref() or .as_str(), both did not take ownership.
You can try:
fn main() {
let stringthing = String::from("c");
match &*stringthing {
"a" => println!("0"),
"b" => println!("1"),
"c" => println!("2"),
_ => println!("else")
}
}
&*stringthing means and does.
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"),
}
}
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 i.e '\n' from the input. As in
match stringthing.as_str().trim() {...}
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trim()function. Just usingas_ref()doesn't match the string.trim()removes. This is nice for deferencing to match against user input.read_linereads a line. Lines end in\n..trim_end()(or, more precisely,trim_end_matches('\n')) will remove that for you.