X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/3192415c398766f2dba869c6a3c6dad69f300cd7..229b86d07e94cd3ec175051a44b3f3cb45b40b65:/workspace/src/part00.rs

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+// ***Remember to enable/add this part in `main.rs`!***
+
+// Rust-101, Part 00: Algebraic datatypes
+// ======================================
+
+// As our first piece of Rust code, we want to write a function that computes the
+// minimum of a list.
+
+
+// An `enum` for "a number or nothing" could look as follows:
+enum NumberOrNothing {
+    Number(i32),
+    Nothing
+}
+
+// Observe how in Rust, the return type comes *after* the arguments.
+fn vec_min(vec: Vec<i32>) -> NumberOrNothing {
+    let mut min = NumberOrNothing::Nothing;
+
+    // Now we want to *iterate* over the list. Rust has some nice syntax for
+    // iterators:
+    for el in vec {
+        // So `el` is al element of the list. We need to update `min` accordingly, but how do we get the current
+        // number in there? This is what pattern matching can do:
+        match min {
+            // In this case (*arm*) of the `match`, `min` is currently nothing, so let's just make it the number `el`.
+            NumberOrNothing::Nothing => {
+                unimplemented!()
+            },
+            // In this arm, `min` is currently the number `n`, so let's compute the new minimum and store it. We will write
+            // the function `min_i32` just after we completed this one.
+            NumberOrNothing::Number(n) => {
+                unimplemented!()
+            }
+        }
+    }
+    // Finally, we return the result of the computation.
+    return min;
+}
+
+// Now that we reduced the problem to computing the minimum of two integers, let's do that.
+fn min_i32(a: i32, b: i32) -> i32 {
+    if a < b {
+        unimplemented!()
+    } else {
+        unimplemented!()
+    }
+}
+
+// Phew. We wrote our first Rust function! But all this `NumberOrNothing::` is getting kind of
+// ugly. Can't we do that nicer?
+
+// Indeed, we can: The following line tells Rust to take
+// the constructors of `NumberOrNothing` into the local namespace.
+// Try moving that above the function, and removing all the occurrences `NumberOrNothing::`.
+use self::NumberOrNothing::{Number,Nothing};
+
+// To call this function, we now just need a list. Of course, ultimately we want to ask the user for
+// a list of numbers, but for now, let's just hard-code something.
+
+// `vec!` is a *macro* (as you can tell from the `!`) that constructs a constant `Vec<_>` with the given
+// elements.
+fn read_vec() -> Vec<i32> {
+    vec![18,5,7,1,9,27]
+}
+
+// Finally, let's call our functions and run the code!
+// But, wait, we would like to actually see something, so we need to print the result.
+// Of course Rust can print numbers, but after calling `vec_min`, we have a `NumberOrNothing`.
+// So let's write a small helper function that prints such values.
+
+fn print_number_or_nothing(n: NumberOrNothing) {
+    match n {
+        Nothing => println!("The number is: <nothing>"),
+        Number(n) => println!("The number is: {}", n),
+    };
+}
+
+// Putting it all together:
+pub fn main() {
+    let vec = read_vec();
+    let min = vec_min(vec);
+    print_number_or_nothing(min);
+}
+
+// Now try `cargo run` on the console to run above code.
+
+
+// [index](main.html) | previous | [next](part01.html)