// 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
// 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
}
}
-// With this fresh knowledge, let us now refactor `vec_min`.
+// 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<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) => 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 {
};
}
}
-// 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<i32> {
pub fn main() {
let vec = read_vec();
let min = vec_min(vec);
- min.print();
+ 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_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.
-// [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)
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