X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/e2374eed1c3ae8d0063138ea011e86bbd42473ab..229b86d07e94cd3ec175051a44b3f3cb45b40b65:/src/part00.rs?ds=sidebyside diff --git a/src/part00.rs b/src/part00.rs index 4deb829..2bc40d9 100644 --- a/src/part00.rs +++ b/src/part00.rs @@ -4,39 +4,36 @@ // 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; - -// ## 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. +//@ ## 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` 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` 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) -> 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: @@ -46,12 +43,13 @@ fn vec_min(vec: Vec) -> NumberOrNothing { 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); /*@*/ } } } @@ -59,6 +57,15 @@ fn vec_min(vec: Vec) -> NumberOrNothing { 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? @@ -81,9 +88,9 @@ fn read_vec() -> Vec { // 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: "), @@ -100,8 +107,8 @@ pub fn main() { // 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. // [index](main.html) | previous | [next](part01.html)