X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/605add03bc0fe32a35fcb41d9019bdc9b306bdae..7296568ff6faea20c9d34cd716fc82c8166df154:/src/part03.rs diff --git a/src/part03.rs b/src/part03.rs index cb0807b..3dfb2ab 100644 --- a/src/part03.rs +++ b/src/part03.rs @@ -1,67 +1,128 @@ -// Rust-101, Part 03: Input, Formatting -// ==================================== +// Rust-101, Part 03: Input +// ======================== +//@ In part 00, I promised that we would eventually replace `read_vec` by a function +//@ that actually asks the user to enter a bunch of numbers. Unfortunately, +//@ I/O is a complicated topic, so the code to do that is not exactly pretty - but well, +//@ let's get that behind us. + +// I/O is provided by the module `std::io`, so we first have to import that with `use`. +// We also import the I/O *prelude*, which makes a bunch of commonly used I/O stuff +// directly available. use std::io::prelude::*; use std::io; +//@ Let's now go over this function line-by-line. First, we call the constructor of `Vec` +//@ to create an empty vector. As mentioned in the previous part, `new` here is just +//@ a static function with no special treatment. While it is possible to call `new` +//@ for a particular type (`Vec::::new()`), the common way to make sure we +//@ get the right type is to annotate a type at the *variable*. It is this variable +//@ that we interact with for the rest of the function, so having its type available +//@ (and visible!) is much more useful. Without knowing the return type of `Vec::new`, +//@ specifying its type parameter doesn't tell us all that much. fn read_vec() -> Vec { - let mut vec = Vec::new(); - + let mut vec: Vec = Vec::::new(); + // The central handle to the standard input is made available by the function `io::stdin`. let stdin = io::stdin(); - println!("Enter a list of numbers, one per line. End with Ctrl-D."); + println!("Enter a list of numbers, one per line. End with Ctrl-D (Linux) or Ctrl-Z (Windows)."); + //@ We would now like to iterate over standard input line-by-line. We can use a `for` loop + //@ for that, but there is a catch: What happens if there is some other piece of code running + //@ concurrently, that also reads from standard input? The result would be a mess. Hence + //@ Rust requires us to `lock` standard input if we want to perform large operations on + //@ it. (See [the documentation](https://doc.rust-lang.org/stable/std/io/struct.Stdin.html) for + //@ more details.) for line in stdin.lock().lines() { + // Rust's type for (dynamic, growable) strings is `String`. However, our variable `line` + // here is not yet of that type: It has type `io::Result`. + //@ The problem with I/O is that it can always go wrong. The type of `line` is a lot like + //@ `Option` ("a `String` or nothing"), but in the case of "nothing", there is + //@ additional information about the error. Again, I recommend to check + //@ [the documentation](https://doc.rust-lang.org/stable/std/io/type.Result.html). You will + //@ see that `io::Result` is actually just an alias for `Result`, so click on that to obtain + //@ the list of all constructors and methods of the type. + + //@ We will be lazy here and just assume that nothing goes wrong: `unwrap` returns the + //@ `String` if there is one, and panics the program otherwise. Since a `Result` carries + //@ some details about the error that occurred, there will be a somewhat reasonable error + //@ message. Still, you would not want a user to see such an error, so in a "real" program, + //@ we would have to do proper error handling. + //@ Can you find the documentation of `Result::unwrap`? + //@ + // I chose the same name (`line`) for the new variable to ensure that I will never, + // accidentally, access the "old" `line` again. let line = line.unwrap(); - match line.parse::() { - Ok(num) => vec.push(num), - Err(_) => println!("What did I say about numbers?"), + // Now that we have our `String`, we want to make it an `i32`. + //@ We first `trim` the `line` to remove leading and trailing whitespace. + //@ `parse` is a method on `String` that can convert a string to anything. Try finding its + //@ documentation! + + //@ In this case, Rust *could* figure out automatically that we need an `i32` (because of + //@ the return type of the function), but that's a bit too much magic for my taste. We are + //@ being more explicit here: `parse::` is `parse` with its generic type set to `i32`. + match line.trim().parse::() { + + //@ `parse` returns again a `Result`, and this time we use a `match` to handle errors + //@ (like, the user entering something that is not a number). + //@ This is a common pattern in Rust: Operations that could go wrong will return + //@ `Option` or `Result`. The only way to get to the value we are interested in is + //@ through pattern matching (and through helper functions like `unwrap`). If we call + //@ a function that returns a `Result`, and throw the return value away, the compiler + //@ will emit a warning. It is hence impossible for us to *forget* handling an error, + //@ or to accidentally use a value that doesn't make any sense because there was an + //@ error producing it. + Ok(num) => { + vec.push(num) /*@*/ + }, + // We don't care about the particular error, so we ignore it with a `_`. + Err(_) => { + println!("What did I say about numbers?") /*@*/ + }, } } vec } -enum SomethingOrNothing { - Something(T), - Nothing, -} -use self::SomethingOrNothing::{Something,Nothing}; +//@ So much for `read_vec`. If there are any questions left, the documentation of the respective +//@ function should be very helpful. Try finding the one for `Vec::push`. I will not always provide +//@ the links, as the documentation is quite easy to navigate and you should get used to that. -trait Minimum : Copy { - fn min(a: Self, b: Self) -> Self; -} +// For the rest of the code, we just re-use part 02 by importing it with `use`. +//@ I already sneaked a bunch of `pub` in part 02 to make this possible: Only +//@ items declared public can be imported elsewhere. +use part02::{SomethingOrNothing,Something,Nothing,vec_min}; -fn vec_min(v: &Vec) -> SomethingOrNothing { - let mut min = Nothing; - for e in v { - let e = *e; - min = Something(match min { - Nothing => e, - Something(n) => T::min(n, e) - }); - } - min +// If you update your `main.rs` to use part 03, `cargo run` should now ask you for some numbers, +// and tell you the minimum. Neat, isn't it? +pub fn main() { + let vec = read_vec(); + let min = vec_min(vec); /*@*/ + min.print(); /*@*/ } -impl Minimum for i32 { - fn min(a: Self, b: Self) -> Self { - ::std::cmp::min(a, b) - } +// **Exercise 03.1**: Define a trait `Print` to write a generic version of +// `SomethingOrNothing::print`. +// Implement that trait for `i32`, and change the code above to use it. +// I will again provide a skeleton for this solution. It also shows how to attach bounds to generic +// implementations (just compare it to the `impl` block from the previous exercise). +// You can read this as "For all types `T` satisfying the `Print` trait, I provide an implementation +// for `SomethingOrNothing`". +// +// Notice that I called the function on `SomethingOrNothing` `print2` to disambiguate from the +// `print` defined previously. +// +// *Hint*: There is a macro `print!` for printing without appending a newline. +pub trait Print { + /* Add things here */ } - -use std::fmt; -impl fmt::Display for SomethingOrNothing { - fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { - match self { - &Something(ref t) => t.fmt(f), - &Nothing => "Nothing".fmt(f), - } +impl SomethingOrNothing { + fn print2(self) { + unimplemented!() } } -pub fn part_main() { - let vec = read_vec(); - let min = vec_min(&vec); - println!("The minimum is: {}", min); -} +// **Exercise 03.2**: Building on exercise 02.2, implement all the things you need on `f32` to make +// your program work with floating-point numbers. -// [index](main.html) | [previous](part02.html) | next +//@ [index](main.html) | [previous](part02.html) | [raw source](workspace/src/part03.rs) | +//@ [next](part04.html)