Update Exercises Directory Names to Reflect Order

exercises/12_options/README.md

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+# Options
+
+Type Option represents an optional value: every Option is either Some and contains a value, or None, and does not.
+Option types are very common in Rust code, as they have a number of uses:
+
+- Initial values
+- Return values for functions that are not defined over their entire input range (partial functions)
+- Return value for otherwise reporting simple errors, where None is returned on error
+- Optional struct fields
+- Struct fields that can be loaned or "taken"
+- Optional function arguments
+- Nullable pointers
+- Swapping things out of difficult situations
+
+## Further Information
+
+- [Option Enum Format](https://doc.rust-lang.org/stable/book/ch10-01-syntax.html#in-enum-definitions)
+- [Option Module Documentation](https://doc.rust-lang.org/std/option/)
+- [Option Enum Documentation](https://doc.rust-lang.org/std/option/enum.Option.html)
+- [if let](https://doc.rust-lang.org/rust-by-example/flow_control/if_let.html)
+- [while let](https://doc.rust-lang.org/rust-by-example/flow_control/while_let.html)

exercises/12_options/options1.rs

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+// options1.rs
+//
+// Execute `rustlings hint options1` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+// 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);
+    }
+}

exercises/12_options/options2.rs

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+// options2.rs
+//
+// Execute `rustlings hint options2` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+#[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);
+    }
+}

exercises/12_options/options3.rs

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+// options3.rs
+//
+// Execute `rustlings hint options3` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+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.
+}