Comparing string in Rust

Question

I want to compare a string input from stdin to a static string with no luck.

Here is what I have tried so far:

fn main() -> () {
    let mut line = "".to_string();
    let exit = "exit".to_string(); 

    while line.as_slice() != exit.as_slice() {
        let input = std::io::stdin().read_line().ok();

        line.push_str( input.unwrap().to_string().as_slice() );

        assert_eq!(line, exit);
    }   
}

However during assertion it failed. How should I compare a string input to a static string in Rust?

Your help is very much appreciated.

Answer 1

First of all, the line contains the line terminator. You probably want to use trim (or one of its variants) to ignore that.

Secondly, you're doing a lot of unnecessary conversions and allocations. Try to avoid those.

Third, to_string is (or at least, was the last time I checked) inefficient due to over-allocation. You want into_string.

Fourth, the quickest way to go from a String to a &str is to "cross-borrow" it; given a String s, &*s will re-borrow it as a &str. This is because a String implements Deref<&str>; in other words, String acts kind of like a smart pointer to a borrowed string, allowing it to decay into a simpler form.

Fifth, unless you're doing something unusual, you can rewrite this as a for loop using the lines iterator method.

Sixth, be aware that stdin() actually allocates a new buffered reader every time you call it. Not only that, but characters read into the buffer do not get "pushed back" into STDIN when a new buffer is created; that data is simply lost. So you really don't want to be calling it in a loop. If you need to, call it once and keep the result in a variable.

So, I end up with this:

fn main() {
    for line in std::io::stdin().lines() {
        // Extract the line, or handle the error.
        let line = match line {
            Ok(line) => line,
            Err(err) => panic!("failed to read line: {}", err)
        };

        assert_eq!(line.trim(), "exit");
    }
}