Step 6 of 6

Comparing strings

== and != compare the text of two strings, character by character. They work between a String and a &str in either order:

fn main() {
    let answer = String::from("yes");
    println!("{}", answer == "yes");
    println!("{}", "Yes" == "yes");
    println!("{}", answer.to_lowercase() == "YES".to_lowercase());
    println!("{}", "apple" < "banana");
    println!("{}", "Zebra" < "apple");
}

true
false
true
true
true

• Comparison is case-sensitive: "Yes" and "yes" are different. To ignore case, lowercase both sides. (For plain ASCII text there's also a.eq_ignore_ascii_case(b), which doesn't build new strings.)
• < and > compare in dictionary-like order, character by character, using each character's Unicode number. All uppercase letters come before all lowercase ones, which is why "Zebra" < "apple". For a human-friendly order, compare lowercased copies.

Choosing what to do based on text

An if / else if chain handles commands. Clean the input first, then compare:

fn reply(cmd: &str) -> String {
    let cmd = cmd.trim().to_lowercase();
    if cmd == "start" {
        "Starting engine".to_string()
    } else if cmd == "stop" {
        "Stopping engine".to_string()
    } else {
        format!("Unknown command: {cmd}")
    }
}

fn main() {
    println!("{}", reply("START "));
    println!("{}", reply("stop"));
    println!("{}", reply("jump"));
}

Starting engine
Stopping engine
Unknown command: jump

let cmd = cmd.trim().to_lowercase(); shadows the &str parameter with a cleaned-up String, a common idiom. Module 14 introduces match, which expresses this kind of choice even more neatly.

Common mistakes

• Comparing raw input. Text typed by a user usually ends with a newline: "yes\n" == "yes" is false. Trim first.
• Comparing a string with a char:

fn main() {
    let answer = String::from("y");
    if answer == 'y' {
        println!("ok");
    }
}

error[E0277]: can't compare `String` with `char`

'y' is a char, "y" is a string. Compare with "y".

Your task

Write bot_reply(msg) for a support chat bot. Ignore spaces around the message and ignore case, then:

• exactly hi or hello (the whole message) → Hello! How can I help?
• a message containing refund anywhere → Refunds take 5 days.
• an empty message → Say something!
• anything else → Sorry, I don't understand: followed by the message, trimmed but with its original case.

Challenge 2

Write same_user(a, b), which returns true when two usernames are the same once you ignore spaces around them and upper/lower case. For example same_user(" Ferris", "ferris ") is true.

Challenge 3

Write check_password(p), which returns the first problem it finds, checking in this order:

1. fewer than 8 characters → too short
2. the password, lowercased, is password or qwertyuiop → too common
3. no digit ('0' to '9') anywhere → needs a digit
4. otherwise → ok

Previous: Changing case, searching, replacing and trimming