Why doesn't read_line mutate its argument?

Question

fn main() {
    let mut input = String::new();

    loop {
        std::io::stdin()
            .read_line(&mut input)
            .expect("error: unable to read user input");

        println!("input is: {}", input);
    }
}

When I first enter hello it will show input is: hello, then no matter what I enter later it will always show input is: hello, so read_line doesn't mutate the input variable. Why does this happen?

It appends line inputs to the buffer you provide, are you sure it just prints only `input is: hello`? — Ömer Erden, Jan 29 '21 at 15:01

score 3 · Accepted Answer · answered Jan 29 '21 at 14:59

read_line does mutate the input variable, but it does so by appending to it, not overwriting it. If you'd like to overwrite input you should first clear it before passing it to read_line which will give you the behavior you're looking for:

fn main() {
    let mut input = String::new();

    loop {
        input.clear(); // truncate string contents
        std::io::stdin()
            .read_line(&mut input)
            .expect("error: unable to read user input");

        println!("input is: {}", input);
    }
}

Why doesn't read_line mutate its argument?

1 Answers1