X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/f21475ab3d58a70db564410f2b81572a4dbb492c..a3a64118702b4f75691de78d42256c306f286014:/src/part01.rs diff --git a/src/part01.rs b/src/part01.rs index 927a617..fa06a80 100644 --- a/src/part01.rs +++ b/src/part01.rs @@ -1,27 +1,28 @@ // Rust-101, Part 01: Expressions, Inherent methods // ================================================ -use std; +// For Rust to compile this file, make sure to enable the corresponding line +// in `main.rs` before going on. -// Even though our code from the first part works, we can still learn a -// lot by making it prettier. 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! +//@ Even though our code from the first part works, we can still learn a +//@ lot by making it prettier. That's because Rust is an "expression-based" language, which +//@ 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`: +// ## Expression-based programming +//@ 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). +//@ 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 +// Conditionals are also just expressions. This is comparable 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. -// (We repeat the definition from the previous part here.) +//@ And the same applies to case distinction with `match`: Every `arm` of the match +//@ gives the expression that is returned in the respective case. +//@ (We repeat the definition from the previous part here.) enum NumberOrNothing { Number(i32), Nothing @@ -34,37 +35,45 @@ fn number_or_default(n: NumberOrNothing, default: i32) -> i32 { } } -// With this fresh knowledge, let us now refactor `vec_min`. First of all, we are doing a small change -// to the type: `&Vec` denotes a *reference* to a `Vec`. You can think of this as a pointer -// (in C terms): Arguments in Rust are passed *by value*, so we need to employ explicit references if -// that's not what we want. References are per default immutable (like variables), a mutable reference -// would be denoted `&mut Vec`. -fn vec_min(v: &Vec) -> NumberOrNothing { +// It is even the case that blocks are expressions, evaluating to the last expression they contain. +fn compute_stuff(x: i32) -> i32 { + let y = { let z = x*x; z + 14 }; + y*y +} + +// Let us now refactor `vec_min`. +fn vec_min(v: Vec) -> NumberOrNothing { + //@ Remember that helper function `min_i32`? Rust allows us to define such helper functions *inside* other + //@ functions. This is just a matter of namespacing, the inner function has no access to the data of the outer + //@ one. Still, being able to nicely group functions can significantly increase readability. + fn min_i32(a: i32, b: i32) -> i32 { + if a < b { a } else { b } /*@*/ + } + let mut min = Nothing; for e in v { - // Now that `v` is just a reference, the same goes for `e`, so we have to dereference the pointer. - let e = *e; - // Notice that all we do here is compute a new value for `min`, and that it will always end - // up being a `Number` rather than `Nothing`. In Rust, the structure of the code - // can express this uniformity. - min = Number(match min { - Nothing => e, - Number(n) => std::cmp::min(n, e) - }); + //@ Notice that all we do here is compute a new value for `min`, and that it will always end + //@ up being a `Number` rather than `Nothing`. In Rust, the structure of the code + //@ can express this uniformity. + min = Number(match min { /*@*/ + Nothing => e, /*@*/ + Number(n) => min_i32(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. + //@ 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. -// So much for `vec_min`. Let us now reconsider `print_number_or_nothing`. That function -// really belongs pretty close to the type `NumberOrNothing`. In C++ or Java, you would -// probably make it a method of the type. In Rust, we can achieve something very similar -// by providing an *inherent implementation*. +// ## Inherent implementations +//@ So much for `vec_min`. Let us now reconsider `print_number_or_nothing`. That function +//@ really belongs pretty close to the type `NumberOrNothing`. In C++ or Java, you would +//@ probably make it a method of the type. In Rust, we can achieve something very similar +//@ by providing an *inherent implementation*. impl NumberOrNothing { fn print(self) { match self { @@ -73,28 +82,28 @@ impl NumberOrNothing { }; } } -// So, what just happened? Rust separates code from data, so the definition of the -// methods on an `enum` (and also on `struct`, which we will learn about later) -// is independent of the definition of the type. `self` is like `this` in other -// languages, and its type is always implicit. So `print` is now a method that -// takes as first argument a `NumberOrNothing`, just like `print_number_or_nothing`. -// -// Try making `number_or_default` from above an inherent method as well! +//@ So, what just happened? Rust separates code from data, so the definition of the +//@ methods on an `enum` (and also on `struct`, which we will learn about later) +//@ is independent of the definition of the type. `self` is like `this` in other +//@ languages, and its type is always implicit. So `print` is now a method that +//@ takes as first argument a `NumberOrNothing`, just like `print_number_or_nothing`. +//@ +//@ Try making `number_or_default` from above an inherent method as well! // With our refactored functions and methods, `main` now looks as follows: fn read_vec() -> Vec { vec![18,5,7,2,9,27] } -pub fn part_main() { +pub fn main() { let vec = read_vec(); - let min = vec_min(&vec); - min.print(); + let min = vec_min(vec); + min.print(); /*@*/ } // You will have to replace `part00` by `part01` in the `main` function in // `main.rs` to run this code. -// **Exercise**: Write a funtion `vec_avg` that computes the average value of a `Vec`. -// -// *Hint*: `vec.len()` returns the length of a vector `vec`. +// **Exercise 01.1**: Write a function `vec_sum` that computes the sum of all values of a `Vec`. + +// **Exercise 01.2**: Write a function `vec_print` that takes a vector and prints all its elements. -// [index](main.html) | [previous](part00.html) | [next](part02.html) +//@ [index](main.html) | [previous](part00.html) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part01.rs) | [next](part02.html)