// Rust-101, Part 01: Expressions, Inherent methods
// ================================================
-use std;
-
// 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 } }
}
}
-// With this fresh knowledge, let us now refactor `vec_min`. First of all, we are doing a small change
-// to the type: `&Vec<i32>` denotes a *reference* to a `Vec<i32>`. 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<i32>`.
-fn vec_min(v: &Vec<i32>) -> NumberOrNothing {
+// 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 {
- 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 as follows:
+ // can express this uniformity.
min = Number(match min {
Nothing => e,
- Number(n) => std::cmp::min(n, e)
+ Number(n) => min_i32(n, e)
});
}
// The `return` keyword exists in Rust, but it is rarely used. Instead, we typically
// 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* as follows:
+// by providing an *inherent implementation*.
impl NumberOrNothing {
fn print(self) {
match self {
fn read_vec() -> Vec<i32> {
vec![18,5,7,2,9,27]
}
-pub fn part_main() {
+pub fn main() {
let vec = read_vec();
- let min = vec_min(&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**: Write a funtion `vec_avg` that computes the average value of a `Vec<i32>`.
-// *Hint*: `vec.len()` returns the length of a vector `vec`.
+// **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.
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