trying to trim and lowercase my String in rust

Question

I'm trying to trim and lowercase my String.

Currently I have

use dialoguer::Input;
    
let input: String = Input::new()
    .with_prompt("Guess a 5 letter word")
    .interact_text()
    .unwrap(); 
let guess: &str = input.as_str(); // trim and lowercase

I'm trying to transform String into a trimmed and lowercased &str but some functions are only on &str and others only on String so I'm having trouble coming up with an elegant solution.

TL;DR: End goal have guess be a trimmed and lowercase &str

String derefs to &str, so you can access str methods from String and if you want a &str as result, just do &input[..] — Rafaelplayerxd YT
– Rafaelplayerxd YT, Commented Feb 18, 2022 at 13:21

rodrigo · Accepted Answer · 2022-02-17 17:00:57Z

Rust stdlib is not about elegance, but about correctness and efficiency.

In your particular case, trim() is defined as str::trim(&self) -> &str because it always returns a substring of the original string, so it does not need to copy or allocate a new string, just compute the begin and end, and do the slice.

But to_lowercase() is defined as str::to_lowercase(&self) -> String because it changes each of its characters to the lowercase equivalent, so it must allocate and fill a new String.

You may thing that if you own the string you can mutate it to lowercase in-place. But that will not work in general because there is not a 1-to-1 map between lowercase and uppercase letters. Think of, for example ß <-> SS in German.

Naturally, you may know that your string only has ASCII characters... if so you can also use str::make_ascii_lowercase(&mut self) that does the change in-place, but only for ASCII characters that do have the 1-to-1 map.

So, summing up, the more ergonomic code would be, to trim input and copy to an owned lowercase:

let guess : String = input.trim().to_lowercase();

Or if you absolutely want to avoid allocating an extra string, but you are positive that only ASCII characters matter:

let mut input = input; //you could also add the mut above
input.make_ascii_lowercase();
let guess: &str = input.trim();

at54321 · Accepted Answer · 2022-02-17 16:52:22Z

1

Try this:

let s = "   aBcD ";
let s2 = s.trim().to_lowercase();
println!("[{s}], [{s2}]");

The above will work if s is &str (as in my example) or String and it will print:

[   aBcD ], [abcd]

So the last line in your code (if you insist on having guess as &str) should become:

let guess: &str = &input.trim().to_lowercase();

Otherwise if you write just:

let guess = input.trim().to_lowercase();

, guess will be of type String, as that's what to_lowercase() returns.

edited Feb 17, 2022 at 16:52

answered Feb 17, 2022 at 16:43

at54321

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2 Comments

Locke Over a year ago

I think it is also important to point out that since a &str is more or less just a reference to a String you can implicitly get a &str from a String when calling functions, but not the other way around. Since most string manipulation functions return a String, these functions can be easily chained together.

at54321 Over a year ago

@Locke Good point!

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