X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/816ab35bec9dec1571988fcf97e57a38a32f5ff5..4816335a8c0e5bcb2514d9c7857596348fa72ff4:/src/part03.rs?ds=inline diff --git a/src/part03.rs b/src/part03.rs index d4805e7..0e7db1d 100644 --- a/src/part03.rs +++ b/src/part03.rs @@ -1,18 +1,70 @@ -// 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 import that. +// We also import the I/O *prelude*, which brings 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 `io::stdin()`. let stdin = io::stdin(); println!("Enter a list of numbers, one per line. End with Ctrl-D."); + // 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](http://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. The problem with I/O is that it can always go wrong, so + // `line` has type `io::Result`. This 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](http://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(); + // Now that we have our `String`, we want to make it an `i32`. `parse` is a method on `String` that + // can convert a string to anything. Try finding it's 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.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?"), } } @@ -20,61 +72,40 @@ fn read_vec() -> Vec { vec } -enum SomethingOrNothing { - Something(T), - Nothing, -} -use self::SomethingOrNothing::{Something,Nothing}; - -trait Minimum : Copy { - fn min(a: Self, b: Self) -> Self; -} - -fn vec_min(v: Vec) -> SomethingOrNothing { - let mut min = Nothing; - for e in v { - min = Something(match min { - Nothing => e, - Something(n) => T::min(n, e) - }); - } - min -} +// 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. -impl Minimum for i32 { - fn min(a: Self, b: Self) -> Self { - ::std::cmp::min(a, b) - } -} +// 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}; -impl SomethingOrNothing { - fn print(self) { - match self { - Nothing => println!("The number is: "), - Something(n) => println!("The number is: {}", n), - }; - } -} -pub fn part_main() { +// 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 SomethingOrNothing { - fn equals(self, other: Self) -> bool { - match (self, other) { - (Nothing , Nothing ) => true, - (Something(n), Something (m)) => n == m, - _ => false, - } - } +// **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. +trait Print { + /* Add things here */ } - -#[test] -fn tes_vec_min() { - assert!(vec_min(vec![6,325,33,532,5,7]).equals(Something(5))); - assert!(vec_min(vec![]).equals(Nothing)); +impl SomethingOrNothing { + fn print2(self) { + unimplemented!() + } } -// [index](main.html) | [previous](part02.html) | next +// [index](main.html) | [previous](part02.html) | [next](part04.html)