CODEWITHSUNDEEP
rust
Muhammad Lukman Low
Muhammad Lukman Low

Reputation: 8533

Reversing a string in Rust

What is wrong with this:

fn main() {
    let word: &str = "lowks";
    assert_eq!(word.chars().rev(), "skwol");
}

I get an error like this:

error[E0369]: binary operation `==` cannot be applied to type `std::iter::Rev<std::str::Chars<'_>>`
 --> src/main.rs:4:5
  |
4 |     assert_eq!(word.chars().rev(), "skwol");
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: an implementation of `std::cmp::PartialEq` might be missing for `std::iter::Rev<std::str::Chars<'_>>`
  = note: this error originates in a macro outside of the current crate

What is the correct way to do this?

Upvotes: 80

Views: 77678

Answers (3)

Kaplan
Kaplan

Reputation: 3738

If you work with ASCII characters only, you can make the reversal in place (w/o memory allocation) with the reverse function for slices .

fn main() {
    let mut slice = *b"lowks";
    slice.reverse();
    assert_eq!(std::str::from_utf8(&slice).unwrap(), "skwol");
}

Playground

Upvotes: 2

DK.
DK.

Reputation: 59065

The first, and most fundamental, problem is that this isn't how you reverse a Unicode string. You are reversing the order of the code points, where you want to reverse the order of graphemes. There may be other issues with this that I'm not aware of. Text is hard.

The second issue is pointed out by the compiler: you are trying to compare a string literal to a char iterator. chars and rev don't produce new strings, they produce lazy sequences, as with iterators in general. The following works:

/*!
Add the following to your `Cargo.toml`:

```cargo
[dependencies]
unicode-segmentation = "0.1.2"
```
*/
extern crate unicode_segmentation;
use unicode_segmentation::UnicodeSegmentation;

fn main() {
    let word: &str = "loẅks";
    let drow: String = word
        // Split the string into an Iterator of &strs, where each element is an
        // extended grapheme cluster.
        .graphemes(true)
        // Reverse the order of the grapheme iterator.
        .rev()
        // Collect all the chars into a new owned String.
        .collect();

    assert_eq!(drow, "skẅol");

    // Print it out to be sure.
    println!("drow = `{}`", drow);
}

Note that graphemes used to be in the standard library as an unstable method, so the above will break with sufficiently old versions of Rust. In that case, you need to use UnicodeSegmentation::graphemes(s, true) instead.

Upvotes: 75

bright-star
bright-star

Reputation: 6447

Since, as @DK. suggested, .graphemes() isn't available on &str in stable, you might as well just do what @huon suggested in the comments:

fn main() {
    let foo = "palimpsest";
    println!("{}", foo.chars().rev().collect::<String>());
}

Upvotes: 112

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