Chapter 12 - Options

Exercise 1

/ This function returns how much icecream there is left in the fridge.
// If it's before 10PM, there's 5 pieces left. At 10PM, someone eats them
// all, so there'll be no more left :(
fn maybe_icecream(time_of_day: u16) -> Option<u16> {
    // We use the 24-hour system here, so 10PM is a value of 22 and 12AM is a
    // value of 0 The Option output should gracefully handle cases where
    // time_of_day > 23.
    // TODO: Complete the function body - remember to return an Option!
    ???
}

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

    #[test]
    fn check_icecream() {
        assert_eq!(maybe_icecream(9), Some(5));
        assert_eq!(maybe_icecream(10), Some(5));
        assert_eq!(maybe_icecream(23), Some(0));
        assert_eq!(maybe_icecream(22), Some(0));
        assert_eq!(maybe_icecream(25), None);
    }

    #[test]
    fn raw_value() {
        // TODO: Fix this test. How do you get at the value contained in the
        // Option?
        let icecreams = maybe_icecream(12);
        assert_eq!(icecreams, 5);
    }
}

For the function maybe_icecream, we can use if or match to solve it:

fn maybe_icecream(time_of_day: u16) -> Option<u16> {
    // We use the 24-hour system here, so 10PM is a value of 22 and 12AM is a 
    // value of 0 The Option output should gracefully handle cases where
    // time_of_day > 23.
    // TODO: Complete the function body - remember to return an Option!
    if time_of_day < 22 {
        return Some(5);
    } else if time_of_day <= 23 {
        return Some(0);
    } else {
        return None;
    }
}

// OR

fn maybe_icecream(time_of_day: u16) -> Option<u16> {
    // We use the 24-hour system here, so 10PM is a value of 22 and 12AM is a
    // value of 0 The Option output should gracefully handle cases where
    // time_of_day > 23.
    // TODO: Complete the function body - remember to return an Option!
    match time_of_day  {
        0..=21 => return Some(5),
        22 | 23 => return Some(0),
        _ => return None
    }
}

Why we use 0..=21 instead of 0..21?
Because 0..21 is the inclusive range and the match keyword only supports inclusive ranges now.
Finally to get the value from an Option we can unwrap it: maybe_icecream(12).unwrap();

Exercise 2

#[cfg(test)]
mod tests {
    #[test]
    fn simple_option() {
        let target = "rustlings";
        let optional_target = Some(target);

        // TODO: Make this an if let statement whose value is "Some" type
        word = optional_target {
            assert_eq!(word, target);
        }
    }

    #[test]
    fn layered_option() {
        let range = 10;
        let mut optional_integers: Vec<Option<i8>> = vec![None];

        for i in 1..(range + 1) {
            optional_integers.push(Some(i));
        }

        let mut cursor = range;

        // TODO: make this a while let statement - remember that vector.pop also
        // adds another layer of Option<T>. You can stack `Option<T>`s into
        // while let and if let.
        integer = optional_integers.pop() {
            assert_eq!(integer, cursor);
            cursor -= 1;
        }

        assert_eq!(cursor, 0);
    }
}

We can using if let and while let to solve this exercise:

#[cfg(test)]
mod tests {
    #[test]
    fn simple_option() {
        let target = "rustlings";
        let optional_target = Some(target);

        // TODO: Make this an if let statement whose value is "Some" type
        if let Some(word) = optional_target {
            assert_eq!(word, target);
        }
    }

    #[test]
    fn layered_option() {
        let range = 10;
        let mut optional_integers: Vec<Option<i8>> = vec![None];

        for i in 1..(range + 1) {
            optional_integers.push(Some(i));
        }

        let mut cursor = range;

        // TODO: make this a while let statement - remember that vector.pop also
        // adds another layer of Option<T>. You can stack `Option<T>`s into
        // while let and if let.
        while let Some(Some(integer)) = optional_integers.pop() {
            assert_eq!(integer, cursor);
            cursor -= 1;
        }

        assert_eq!(cursor, 0);
    }
}

Exercise 3

struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let y: Option<Point> = Some(Point { x: 100, y: 200 });

    match y {
        Some(p) => println!("Co-ordinates are {},{} ", p.x, p.y),
        _ => panic!("no match!"),
    }
    y; // Fix without deleting this line.
}

Another way to handle the creation of a Point is to use let and if let:

fn main() {
    let y: Option<Point> = if let Some(p) = Some(Point { x: 100, y: 200 }) {
        println!("Co-ordinates are {},{} ", p.x, p.y);
        Some(p) // return Some(p) to y
    } else {
        panic!("no match!");
    };

    y; // Fix without deleting this line.
}

Conclusion

The 12th chapter of Rustlings - Options ends here.
TIL:

How to use Options, unwrap and match
How to handle Option with if let and while let

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