// [index](main.html) | previous | [next](part01.html)
-// Rust-101, Part 00
-// =================
+use std;
+
+// Rust-101, Part 00: Algebraic datatypes, expressions
+// ===================================================
// As a starter, we want to write a function that computes the minimum of a list.
-// First, we need to get a list. For now, let's just hard-code some sample list.
-// Later, we will ask the user to input a list.
-//
-// Observe how in Rust, the function type comes *after* the arguments (which, in
-// this case, the function does not have). `Vec<i32>` is the type of a growable list
-// of (signed, 32-bit) integers.
-
-pub fn read_vec() -> Vec<i32> {
- vec![0,1,2,3,4]
+// First, we need to write down the signature of the function: The types of its arguments and
+// of the return value. In the case of our minimum function,
+// we may be inclined to say that it returns a number. But then we would be in trouble: What's
+// the minimum of an empty list? The type of the function says we have to return *something*.
+// We could just choose 0, but that would be kind of arbitrary. What we need
+// is a type that is "a number, or nothing". Such a type (of multiple exclusive options)
+// is called an "algebraic datatype", and Rust lets us define such types with the keyword `enum`.
+// Coming from C(++), you can think of such a type as a `union`, together with a field that
+// stores the variant of the union that's currently used.
+
+enum NumberOrNothing {
+ Number(i32),
+ Nothing
+}
+
+// Notice that `i32` is the type of (signed, 32-bit) integers. To write down the type of
+// the minimum function, we need just one more ingredient: `Vec<i32>` is the type of
+// (growable) arrays of numbers, and we will use that as our list type.
+// Observe how in Rust, the function type comes *after* the arguments.
+
+fn vec_min_try1(vec: Vec<i32>) -> NumberOrNothing {
+ // First, we need some variable to store the minimum as computed so far.
+ // Since we start out with nothing computed, this will again be a
+ // "number or nothing". Notice that we do not have to write a type
+ // next to `min`, Rust can figure that out automatically (a bit like
+ // `auto` in C++11). Also notice the `mut`: In Rust, variables are
+ // immutable per default, and you need to tell Rust if you want
+ // to change a variable later.
+ 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 {
+ NumberOrNothing::Nothing => {
+ // `min` is currently nothing, so let's just make it the number `el`.
+ min = NumberOrNothing::Number(el);
+ },
+ NumberOrNothing::Number(n) => {
+ // `min` is currently the number `n`, so let's compute the new minimum and store it.
+ let new_min = std::cmp::min(n, el);
+ min = NumberOrNothing::Number(new_min);
+ }
+ }
+ }
+ // Finally, we return the result of the computation.
+ return min;
+}
+
+// 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:
+use self::NumberOrNothing::{Number,Nothing};
+// Try moving that above the function, and removing all the occurrences `NumberOrNothing::`.
+// Things should still compile, now being much less verbose!
+
+// However, the code is still not "idiomatic Rust code". To understand why, it is important to
+// understand that Rust is an "expression-based" language. This means that most of the
+// terms you write down are not just *statements* (executing code), but *expressions*
+// (returning a value). This applies even to the body of entire functions!
+
+// For example, consider `sqr`. Between the curly braces, we are giving the *expression*
+// that computes the return value. So we can just write `i * i`, the expression that
+// returns the square if `i`, and make that our return value! Note that this is
+// very close to how mathematicians write down functions (but with more types).
+fn sqr(i: i32) -> i32 { i * i }
+
+// Conditionals are also just expressions. You can compare this to the ternary `? :` operator
+// from languages like C.
+fn abs(i: i32) -> i32 { if i >= 0 { i } else { -i } }
+
+// And the same applies to case distinction with `match`: Every `arm` of the match
+// gives the expression that is returned in the respective case.
+fn number_or_default(n: NumberOrNothing, default: i32) -> i32 {
+ match n {
+ Nothing => default,
+ Number(n) => n,
+ }
}
+// With this fresh knowledge, let us now refactor `vec_min`.
+fn vec_min(v: Vec<i32>) -> NumberOrNothing {
+ let mut min = Nothing;
+ for e in v {
+ // First of all, notice that all we do here is compute a new value for `min`, and that we
+ // will always end up calling the `Number` constructor. In Rust, the structure of the code
+ // can express this uniformity as follows:
+ min = Number(match min {
+ Nothing => e,
+ Number(n) => std::cmp::min(n, e)
+ });
+ }
+ // The `return` keyword exists in Rust, but it is rarely used. Instead, we typically
+ // make use of the fact that the entire function body is an expression, so we can just
+ // write down the desired return value.
+ min
+}
+
+// Now that's already much shorter! Make sure you can go over the code above and actually understand
+// every step of what's going on.
+
+// 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:
+
+fn read_vec() -> Vec<i32> {
+ // `vec!` is a *macro* (as you can tell from the `!`) that constructs a constant `Vec` with the given
+ // elements.
+ 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. Of course Rust can print numbers,
+// but after calling `vec_min`, we have a `NumberOrNothing`. So let's write a small helper
+// function that can 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),
+ };
+}
+
+// So putting it all together - if you type `cargo run`, it will
+// run the following code:
pub fn part_main() {
-
+ let vec = read_vec();
+ let min = vec_min(vec);
+ print_number_or_nothing(min);
}
+
+// Yay, it said "1"! That's actually the right answer. Okay, we could have
+// computed that ourselves, but that's besides the point. More importantly:
+// You completed the first part of the course.
+
+// [index](main.html) | previous | [next](part01.html)
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