Why does a truncated string Rust print as an empty pair of parenthesis?

Question

I have

use std::io;

fn main() {
    println!("CHAR COUNT");
    let mut guess = String::new();
    io::stdin().read_line(&mut guess).expect(
        "Failed to read line",
    );

    let string_length = guess.len() - 2;
    let correct_string_length = guess.truncate(string_length);

    println!("Your text: {}", guess);
    println!("Your texts wrong length is: {}", string_length);
    println!("Your texts correct length: {}", correct_string_length);
}

The last line gives me

error[E0277]: the trait bound `(): std::fmt::Display` is not satisfied
  --> src/main.rs:15:47
   |
15 |     println!("Your texts correct length: {}", correct_string_length);
   |                                               ^^^^^^^^^^^^^^^^^^^^^ `()` cannot be formatted with the default formatter; try using `:?` instead if you are using a format string
   |
   = help: the trait `std::fmt::Display` is not implemented for `()`
   = note: required by `std::fmt::Display::fmt`

What am I doing wrong? If I use {:?} then I get () instead of a formatted string.

You should read the documentation a bit more closely :)

Kroltan
– Kroltan

2017-08-23 14:16:02 +00:00
Commented Aug 23, 2017 at 14:16 — Kroltan
– Kroltan, Commented Aug 23, 2017 at 14:16
Possible duplicate of How do I sort an array?

Boiethios
– Boiethios

2017-08-23 14:27:54 +00:00
Commented Aug 23, 2017 at 14:27 — Boiethios
– Boiethios, Commented Aug 23, 2017 at 14:27

Joe Clay · Accepted Answer · 2017-08-23 10:39:56Z

9

When in doubt, go to the docs - here's the function signature of String::truncate:

fn truncate(&mut self, new_len: usize)

Note two things:

It takes self as &mut.
It has no return value.

From that, the problem becomes pretty clear - truncate does not return a new truncated string, it truncates the existing string in place.

This might seem a little unintuitive at first, but Rust APIs tend not to allocate new objects in memory unless you specifically ask them to - if you're never going to use guess again, then it'd be ineffecient to create a whole new String. If you wanted to make a truncated copy, then you'd need to be explicit:

let truncated = guess.clone();
truncated.truncate(string_length);

Or if you just wanted to reference part of the existing string, you could do what Ryan's answer suggests.

edited Aug 23, 2017 at 10:39

answered Aug 23, 2017 at 10:30

Joe Clay

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Comments

Simon Whitehead · Accepted Answer · 2017-08-23 11:38:32Z

Just to compliment the other answers here..

Attempting to truncate a string in Rust that is not on a character boundary will cause a runtime panic.

So while this works now:

let correct_string_length = &guess[..string_length];

If you're trying to truncate a string with wider characters, your code will panic at runtime. This is especially true if you're truncating user input.. who knows what it could be. For example:

fn main() {
    let s = "Hello, 世界";

    println!("{}", &s[..8]); // <--- panic
}

You can use the str::is_char_boundary(usize) method to make sure you're not about to break up a wide character accidentally:

fn print_safely(s: &str, mut idx: usize) {
    loop {
        if s.is_char_boundary(idx) || idx >= s.len() - 1 {
            break;
        }
        idx += 1;
    }

    println!("{}", &s[..idx]);
}

User input could be anything so this is just something to consider.

Playground link: http://play.integer32.com/?gist=632ff6c81c56f9ba52e0837ff25939bc&version=stable

Ry- · Accepted Answer · 2017-08-23 10:32:18Z

4

truncate operates in place, which is why it returns (). Looks like you’re just looking for a regular non-mutating substring:

let correct_string_length = &guess[..string_length];

answered Aug 23, 2017 at 10:32

Ry-♦

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