我如何追加期权 <char> 到一个字符串？

Question

I am using a match statement with .chars().next() and want to append a character to a string if it matches a certain character. I am trying to do so like this

keyword.push(line.chars().next()) 

but get an error:

expected type 'char' found type Option<<char>>

How would I go about appending this onto my string?

Answer 1

Well, thats the thing: because next() returns an Option<char> , its possible that it returns None . You need to account for that scenario... otherwise you'll likely cause a panic and your application will exit.

So, the blind and error-prone way is to unwrap it:

keyword.push(line.chars().next().unwrap());

That will likely crash at some point. What you want is to destructure it and make sure there's something there:

match line.chars().next() {
    Some(c) => {
        if c == 'H' || c == 'W' {
             keyword.push(c);
        }
    },
    None => ()
}

As Shepmaster points out in the comments, the particular scenario above (where we only care about a single arm of the match ) can be condensed to an if let binding:

if let Some(c) = line.chars().next() {
    if c == 'H' || c == 'W' {
       keyword.push(c);
    }
}

That said - you get this all for free by iterating via a for loop:

for c in line.chars() {
    if c == 'H' || c == 'W' {
        keyword.push(c);
    }
}

Playground example