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?
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.
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:
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!"),
}
You can try:
fn main() {
let stringthing = String::from("c");
match &*stringthing {
"a" => println!("0"),
"b" => println!("1"),
"c" => println!("2"),
_ => println!("else")
}
}
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 ie '\n' from the input. As in
match stringthing.as_str().trim() {...}
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");
}
}
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.