Chapter 9 - Strings

Exercise 1

// Make me compile without changing the function signature!

fn main() {
    let answer = current_favorite_color();
    println!("My current favorite color is {}", answer);
}

fn current_favorite_color() -> String {
    "blue"
}

To create a new String, we have 2 ways:

Use to_string() macro: "blue".to_string()
Use String::from: String::from("blue")

// Make me compile without changing the function signature!

fn main() {
    let answer = current_favorite_color();
    println!("My current favorite color is {}", answer);
}

fn current_favorite_color() -> String {
    // "blue".to_string()
    String::from("blue")
}

Exercise 2

// Make me compile without changing the function signature!

fn main() {
    let word = String::from("green"); // Try not changing this line :)
    if is_a_color_word(word) {
        println!("That is a color word I know!");
    } else {
        println!("That is not a color word I know.");
    }
}

fn is_a_color_word(attempt: &str) -> bool {
    attempt == "green" || attempt == "blue" || attempt == "red"
}

The is_a_color_word function's parameter expected &str, so we need to pass a string reference to the function.

fn main() {
    let word = String::from("green"); // Try not changing this line :)
    if is_a_color_word(&word) {
        println!("That is a color word I know!");
    } else {
        println!("That is not a color word I know.");
    }
}

fn is_a_color_word(attempt: &str) -> bool {
    attempt == "green" || attempt == "blue" || attempt == "red"
}

The interesting thing here is "green" is &str. You can do st like this:

fn main() {
    let word = String::from("green"); // Try not changing this line :)
    if is_a_color_word("green") {
        println!("That is a color word I know!");
    } else {
        println!("That is not a color word I know.");
    }
}

fn is_a_color_word(attempt: &str) -> bool {
    attempt == "green" || attempt == "blue" || attempt == "red"
}

Exercise 3

fn trim_me(input: &str) -> String {
    // TODO: Remove whitespace from both ends of a string!
    ???
}

fn compose_me(input: &str) -> String {
    // TODO: Add " world!" to the string! There's multiple ways to do this!
    ???
}

fn replace_me(input: &str) -> String {
    // TODO: Replace "cars" in the string with "balloons"!
    ???
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn trim_a_string() {
        assert_eq!(trim_me("Hello!     "), "Hello!");
        assert_eq!(trim_me("  What's up!"), "What's up!");
        assert_eq!(trim_me("   Hola!  "), "Hola!");
    }

    #[test]
    fn compose_a_string() {
        assert_eq!(compose_me("Hello"), "Hello world!");
        assert_eq!(compose_me("Goodbye"), "Goodbye world!");
    }

    #[test]
    fn replace_a_string() {
        assert_eq!(replace_me("I think cars are cool"), "I think balloons are cool");
        assert_eq!(replace_me("I love to look at cars"), "I love to look at balloons");
    }
}

We have tons of String methods here. Just uses them to solve this exercise.
For trim_me function, we can simply use the trim() method input.trim()
For compose_me me function, we have several ways to do it:

Using format: format!("{} {}", input, "world!")
Using string concentrate - Create a String on the left and use + to add "&str": input.to_string() + " world!"

For replace_me function, we can use replace() method: input.replace("cars", "balloons")

fn trim_me(input: &str) -> String {
    input.trim().to_string()
}

fn compose_me(input: &str) -> String {
    input.to_string() + " world!"
}

fn replace_me(input: &str) -> String {
    input.replace("cars", "balloons")
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn trim_a_string() {
        assert_eq!(trim_me("Hello!     "), "Hello!");
        assert_eq!(trim_me("  What's up!"), "What's up!");
        assert_eq!(trim_me("   Hola!  "), "Hola!");
    }

    #[test]
    fn compose_a_string() {
        assert_eq!(compose_me("Hello"), "Hello world!");
        assert_eq!(compose_me("Goodbye"), "Goodbye world!");
    }

    #[test]
    fn replace_a_string() {
        assert_eq!(replace_me("I think cars are cool"), "I think balloons are cool");
        assert_eq!(replace_me("I love to look at cars"), "I love to look at balloons");
    }
}

Exercise 4

// Ok, here are a bunch of values-- some are `String`s, some are `&str`s. Your
// task is to call one of these two functions on each value depending on what
// you think each value is. That is, add either `string_slice` or `string`
// before the parentheses on each line. If you're right, it will compile!

fn string_slice(arg: &str) {
    println!("{}", arg);
}
fn string(arg: String) {
    println!("{}", arg);
}

fn main() {
    ???("blue");
    ???("red".to_string());
    ???(String::from("hi"));
    ???("rust is fun!".to_owned());
    ???("nice weather".into());
    ???(format!("Interpolation {}", "Station"));
    ???(&String::from("abc")[0..1]);
    ???("  hello there ".trim());
    ???("Happy Monday!".to_string().replace("Mon", "Tues"));
    ???("mY sHiFt KeY iS sTiCkY".to_lowercase());
}

Just check the doc for the return type of each method!

fn string_slice(arg: &str) {
    println!("{}", arg);
}
fn string(arg: String) {
    println!("{}", arg);
}

fn main() {
    string_slice("blue");
    string("red".to_string());
    string(String::from("hi"));
    string("rust is fun!".to_owned());
    string("nice weather".into());
    string(format!("Interpolation {}", "Station"));
    string_slice(&String::from("abc")[0..1]);
    string_slice("  hello there ".trim());
    string("Happy Monday!".to_string().replace("Mon", "Tues"));
    string("mY sHiFt KeY iS sTiCkY".to_lowercase());
}

Conclusion

The 9th chapter of Rustlings - Strings ends here.
TIL:

Distinguish String and $str
Methods of String
Doc link: https://doc.rust-lang.org/std/string/struct.String.html

Thanks for reading and please add comments below if you have any questions