-// [index](main.html) | previous | [next](part01.html)
-
-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 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
+// As our first piece of Rust code, we want to write a function that computes the
+// minimum of a list. We are going to make use of the standard library, so let's import that:
+
+use std;
+
+// Let us start by thinking about the *type* of our function. Rust forces us to give the types of
+// all arguments, and the return type, before we even start writing the body. 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)
// 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.
+// Observe how in Rust, the return 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
+ // "number or nothing":
+ let mut min = NumberOrNothing::Nothing;
+ // 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:
// 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`.
+ // In this case (*arm*) of the `match`, `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.
+ // In this arm, `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);
}
// 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
+// There is more prettification we can do. To understand how, 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).
+// For example, consider `sqr`:
fn sqr(i: i32) -> i32 { i * i }
+// 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`!
+// This is very close to how mathematicians write down functions (but with more types).
// Conditionals are also just expressions. You can compare this to the ternary `? :` operator
// from languages like C.
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
+ // First of all, notice that all we do here is compute a new value for `min`, and that it
+ // will always end up being `Number` rather than `Nothing`. In Rust, the structure of the code
// can express this uniformity as follows:
min = Number(match min {
Nothing => e,
// 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
+// 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![18,5,7,1,9,27]
// `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.
+// 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 {
};
}
-// So putting it all together - if you type `cargo run`, it will
-// run the following code:
+// Putting it all together:
pub fn part_main() {
let vec = read_vec();
print_number_or_nothing(min);
}
+// Now try `cargo run` on the console to run above code.
+
// 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.
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