X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/800c78a128bd22bbe700164de5019ff8270fd746..d7b96e63505812685191c3dad091453284892943:/src/part00.rs diff --git a/src/part00.rs b/src/part00.rs index 308c242..1c7d3ce 100644 --- a/src/part00.rs +++ b/src/part00.rs @@ -1,14 +1,14 @@ -// [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) @@ -24,17 +24,17 @@ enum NumberOrNothing { // 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 function type comes *after* the arguments. +// Observe how in Rust, the return type comes *after* the arguments. fn vec_min_try1(vec: Vec) -> 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: @@ -43,11 +43,11 @@ fn vec_min_try1(vec: Vec) -> NumberOrNothing { // 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); } @@ -64,16 +64,16 @@ 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 +// 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. @@ -92,8 +92,8 @@ fn number_or_default(n: NumberOrNothing, default: i32) -> i32 { fn vec_min(v: Vec) -> 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, @@ -109,19 +109,19 @@ fn vec_min(v: Vec) -> NumberOrNothing { // 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 { + 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 { @@ -130,8 +130,7 @@ fn print_number_or_nothing(n: NumberOrNothing) { }; } -// 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(); @@ -139,6 +138,8 @@ pub fn part_main() { 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.