If statements

pub fn reply(msg: &str) -> &str {
    let message = msg.trim_end();
    if message.is_empty() {
        return "Fine. Be that way!";
    }

    let is_questioning = message.ends_with('?');
    let is_yelling =
        message.chars().any(|ch| ch.is_alphabetic()) && message == message.to_uppercase();

    if is_yelling {
        return if is_questioning {
            "Calm down, I know what I'm doing!"
        } else {
            "Whoa, chill out!"
        };
    }
    if is_questioning {
        return "Sure.";
    }
    "Whatever."
}

In this approach you have a series of if statements to evaluate the conditions. As soon as the right condition is found, the correct response is returned.

• First, the str method trim_end is used to remove whitespace from the end of the input.
• If the trimmed input is_empty, then the response for nothing said is returned.
• The ends_with method is used to determine if the input ends with a question mark.
• The first half of the yell condition

message.chars().any(|ch| ch.is_alphabetic())

calls the str method chars to create an Iterator for the characters in the trimmed input.

• The iterated characters are passed into the Iterator method any.

• any passes each character to a closure using the char method is_alphabetic to ensure there is at least one letter character in the str. This is because the second half of the condition tests that the uppercased input is the same as the input. If the input were only "123" it would equal itself uppercased, but without letters it would not be a yell. The uppercasing is done by using the str method to_uppercase.

• If the input is a yell, then the function returns the response for if the input is a yelled question os just a yell.

• If the input is a question, then the function returns the response for that.

• Finally, if the function has not returned by the end, the response for neither a yell nor a question is returned.