X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/229b86d07e94cd3ec175051a44b3f3cb45b40b65..ab7f9b241429bd675b437d2437799de75d2f409b:/workspace/src/part01.rs?ds=sidebyside

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-// ***Remember to enable/add this part in `main.rs`!***
-
-// Rust-101, Part 01: Expressions, Inherent methods
-// ================================================
-
-// 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!
-
-// ## 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).
-
-// Conditionals are also just expressions. You can compare this 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.)
-enum NumberOrNothing {
-    Number(i32),
-    Nothing
-}
-use self::NumberOrNothing::{Number,Nothing};
-fn number_or_default(n: NumberOrNothing, default: i32) -> i32 {
-    match n {
-        Nothing => default,
-        Number(n) => n,
-    }
-}
-
-// Let us now refactor `vec_min`.
-fn vec_min(v: Vec<i32>) -> 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 be very useful.
-    fn min_i32(a: i32, b: i32) -> i32 {
-        if a < b { a } else { b }
-    }
-
-    let mut min = Nothing;
-    for e in v {
-        // 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.
-    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.
-
-// ## 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 {
-            Nothing => println!("The number is: <nothing>"),
-            Number(n) => println!("The number is: {}", n),
-        };
-    }
-}
-// 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<i32> {
-    vec![18,5,7,2,9,27]
-}
-pub fn main() {
-    let vec = read_vec();
-    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 01.1**: Write a funtion `vec_sum` that computes the sum of all values of a `Vec<i32>`.
-
-// **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)