// 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)
-// 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.
+//@ ## Getting started
+//@ 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)
+//@ 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.
// An `enum` for "a number or nothing" could look as follows:
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.
+//@ 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 return type comes *after* the arguments.
fn vec_min(vec: Vec<i32>) -> NumberOrNothing {
- // In the function, we first 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":
+ //@ In the function, we first 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":
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.
+ //@ 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.
// Now we want to *iterate* over the list. Rust has some nice syntax for
// iterators:
match min {
// In this case (*arm*) of the `match`, `min` is currently nothing, so let's just make it the number `el`.
NumberOrNothing::Nothing => {
- min = NumberOrNothing::Number(el);
+ min = NumberOrNothing::Number(el); /*@*/
},
- // In this arm, `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. We will write
+ // the function `min_i32` just after we completed this one.
NumberOrNothing::Number(n) => {
- let new_min = std::cmp::min(n, el);
- min = NumberOrNothing::Number(new_min);
+ let new_min = min_i32(n, el); /*@*/
+ min = NumberOrNothing::Number(new_min); /*@*/
}
}
}
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 {
+ return a; /*@*/
+ } else {
+ return b; /*@*/
+ }
+}
+
// Phew. We wrote our first Rust function! But all this `NumberOrNothing::` is getting kind of
// ugly. Can't we do that nicer?
// 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.
-// `println!` is again a macro, where the first argument is a *format string*. For
-// now, you just need to know that `{}` is the placeholder for a value, and that Rust
-// will check at compile-time that you supplied the right number of arguments.
+//@ `println!` is again a macro, where the first argument is a *format string*. For
+//@ now, you just need to know that `{}` is the placeholder for a value, and that Rust
+//@ will check at compile-time that you supplied the right number of arguments.
fn print_number_or_nothing(n: NumberOrNothing) {
match n {
Nothing => println!("The number is: <nothing>"),
// 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.
+//@ 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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