How do I concatenate strings

Question

How do I concatenate the following combinations of types    str and str String and str String and String

User · Answer

To concatenate multiple strings into a single string, separated by another character, there are a couple of ways.

The nicest I have seen is using the join method on an array:

fn main() {
    let a = "Hello";
    let b = "world";
    let result = [a, b].join("\n");

    print!("{}", result);
}

Depending on your use case you might also prefer more control:

fn main() {
    let a = "Hello";
    let b = "world";
    let result = format!("{}\n{}", a, b);

    print!("{}", result);
}

There are some more manual ways I have seen, some avoiding one or two allocations here and there. For readability purposes I find the above two to be sufficient.

User · Answer

2020 Update  Concatenation by String Interpolation RFC 2795 issued 2019-10-27  Suggests support for implicit arguments to do what many people would know as  quot string interpolation quot  -- a way of embedding arguments within a string to concatenate them  RFC  https   rust-lang github io rfcs 2795-format-args-implicit-identifiers html Latest issue status can be found here  https   github com rust-lang rust issues 67984 At the time of this writing  2020-9-24   I believe this feature should be available in the Rust Nightly build  This will allow you to concatenate via the following shorthand  format args   quot hello  person  quot    It is equivalent to this  format args   quot hello  person  quot   person person   There is also the  quot ifmt quot  crate  which provides its own kind of string interpolation  https   crates io crates ifmt

User · Answer

When you concatenate strings  you need to allocate memory to store the result  The easiest to start with is String and  amp str  fn main         let mut owned string  String    quot hello  quot  to owned        let borrowed string   amp str    quot world quot            owned string push str borrowed string       println   quot    quot   owned string      Here  we have an owned string that we can mutate  This is efficient as it potentially allows us to reuse the memory allocation  There s a similar case for String and String  as  amp String can be dereferenced as  amp str  fn main         let mut owned string  String    quot hello  quot  to owned        let another owned string  String    quot world quot  to owned             owned string push str  amp another owned string       println   quot    quot   owned string      After this  another owned string is untouched  note no mut qualifier   There s another variant that consumes the String but doesn t require it to be mutable  This is an implementation of the Add trait that takes a String as the left-hand side and a  amp str as the right-hand side  fn main         let owned string  String    quot hello  quot  to owned        let borrowed string   amp str    quot world quot            let new owned string   owned string   borrowed string      println   quot    quot   new owned string      Note that owned string is no longer accessible after the call to    What if we wanted to produce a new string  leaving both untouched  The simplest way is to use format   fn main         let borrowed string   amp str    quot hello  quot       let another borrowed string   amp str    quot world quot            let together   format   quot      quot   borrowed string  another borrowed string           After https   rust-lang github io rfcs 2795-format-args-implicit-identifiers html        let together   format   quot  borrowed string  another borrowed string  quot         println   quot    quot   together      Note that both input variables are immutable  so we know that they aren t touched  If we wanted to do the same thing for any combination of String  we can use the fact that String also can be formatted  fn main         let owned string  String    quot hello  quot  to owned        let another owned string  String    quot world quot  to owned             let together   format   quot      quot   owned string  another owned string           After https   rust-lang github io rfcs 2795-format-args-implicit-identifiers html        let together   format   quot  owned string  another owned string  quot        println   quot    quot   together      You don t have to use format  though  You can clone one string and append the other string to the new string  fn main         let owned string  String    quot hello  quot  to owned        let borrowed string   amp str    quot world quot            let together   owned string clone     borrowed string      println   quot    quot   together      Note - all of the type specification I did is redundant - the compiler can infer all the types in play here  I added them simply to be clear to people new to Rust  as I expect this question to be popular with that group

User · Answer

I think that concat method and   should be mentioned here as well   assert eq       My  to owned            string        My         string   concat        and there is also concat  macro but only for literals   let s   concat   test   10   b   true   assert eq  s   test10btrue

User · Answer

Simple ways to concatenate strings in Rust There are various methods available in Rust to concatenate strings First method  Using concat       fn main         println   quot    quot   concat   quot a quot    quot b quot       The output of the above code is    ab   Second method  using push str   and   operator   fn main         let mut  a    quot a quot  to string        let  b    quot b quot  to string        let  c    quot c quot  to string          a push str  amp  b        println   quot    quot    a        println   quot    quot    a    amp  c      The output of the above code is   ab abc   Third method  Using format      fn main         let mut  a    quot a quot  to string        let  b    quot b quot  to string        let  c   format   quot      quot    a   b        println   quot    quot    c      The output of the above code is    ab  Check it out and experiment with Rust playground

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