X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/605add03bc0fe32a35fcb41d9019bdc9b306bdae..8f2ba670de8b8b29f9bbb95ba8fa6ac382e2b745:/src/part03.rs diff --git a/src/part03.rs b/src/part03.rs index cb0807b..b9b7ce8 100644 --- a/src/part03.rs +++ b/src/part03.rs @@ -1,17 +1,56 @@ -// Rust-101, Part 03: Input, Formatting -// ==================================== +// Rust-101, Part 03: Input, Testing +// ================================= +// 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 pretty - but well, +// let's get that behind us. + +// IO/ 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. fn read_vec() -> Vec { let mut 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() { + // The `line` we have here is not yet of type `String`. 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 halts the program (with an appropriate error message) otherwise. Can you find the documentation + // of `Result::unwrap()`? 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. So I use this opportunity to + // introduce the syntax for explicitly giving the type parameter of a generic function: `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), Err(_) => println!("What did I say about numbers?"), } @@ -20,6 +59,12 @@ fn read_vec() -> Vec { vec } +// So much for `read_vec`. If there are any questions left, the documentation of the respective function +// should be very helpful. I will not always provide the links, as the documentation is quite easy to navigate +// and you should get used to that. +// +// The rest of the code dosn't change, so we just copy it. + enum SomethingOrNothing { Something(T), Nothing, @@ -30,10 +75,9 @@ trait Minimum : Copy { fn min(a: Self, b: Self) -> Self; } -fn vec_min(v: &Vec) -> SomethingOrNothing { +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) @@ -42,26 +86,81 @@ fn vec_min(v: &Vec) -> SomethingOrNothing { min } +// `::std::cmp::min` is a way to refer to this function without importing `std`. +// We could also have done `use std::cmp;` and later called `cmp::min`. Try that! impl Minimum for i32 { fn min(a: Self, b: Self) -> Self { ::std::cmp::min(a, b) } } -use std::fmt; -impl fmt::Display for SomethingOrNothing { - fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { +impl SomethingOrNothing { + fn print(self) { match self { - &Something(ref t) => t.fmt(f), - &Nothing => "Nothing".fmt(f), - } + Nothing => println!("The number is: "), + Something(n) => println!("The number is: {}", n), + }; } } +// 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 part_main() { let vec = read_vec(); - let min = vec_min(&vec); - println!("The minimum is: {}", min); + let min = vec_min(vec); + min.print(); +} + +// After all this nit-picking about I/O details, let me show you quickly something unrelated, +// but really nice: Rust's built-in support for testing. +// Now that the user can run our program on loads of inputs, we better make sure that it is correct. +// To be able to test the result of `vec_min`, we first have to write a function that +// is able to test equality if `SimethingOrNothing`. So let's quickly do that. + +// `equals` performs pattern-matching on both `self` and `other` to test the two for being +// equal. Because we are lazy, we want to write only one `match`. so we group the two into a +// pair such that we can match on both of them at once. You can read the first arm of the match +// as testing whether `(self, other)` is `(Nothing, Nothing)`, which is the case exactly if +// both `self` and `other` are `Nothing`. Similar so for the second arm. +impl SomethingOrNothing { + fn equals(self, other: Self) -> bool { + match (self, other) { + (Nothing , Nothing ) => true, + (Something(n), Something(m)) => n == m, + // `_` is the syntax for "I don't care", so this is how you add a default case to your `match`. + _ => false, + } + } +} + +// Now we are almost done! Writing a test in Rust is shockingly simple. Just write a function +// that takes no arguments as returns nothing, and add `#[test]` right in front of it. +// That's called an *attribute*, and the `test` attribute, well, declares the function to +// be a test. + +// Within the function, we can then use `panic!` to indicate test failure. Helpfully, there's +// a macro `assert!` that panics if its argument becomes `false`. +// Using `assert!` and our brand-new `equals`, we can now call `vec_min` with some lists +// and make sure it returns The Right Thing. +#[test] +fn test_vec_min() { + assert!(vec_min(vec![6,325,33,532,5,7]).equals(Something(5))); + assert!(vec_min(vec![6,325,33,532]).equals(Something(6))); } +// To execute the test, run `cargo test`. It should tell you that everything is all right. +// Now that was simple, wasn't it? +// +// **Exercise**: Add a case to `test_vec_min` that checks the behavior on empty lists. +// +// **Exercise**: Change `vec_min` such that everything still compiles, but the test fails. +// +// **Bonus Exercise**: Because `String::parse` is itself generic, you can change `read_vec` to +// be a generic function that works for any type, not just for `i32`. However, you will have to add +// a trait bound to `read_vec`, as not every type supports being parsed.
+// Once you made `vec_min` generic, copy your generic `print` from the previous part. Implement all +// our traits (`Minimum` and `Print`) for `f32` (32-bit floating-point numbers), and change `part_main()` +// such that your program now computes the minimum of a list of floating-point numbers.
+// *Hint*: You can figure out the trait bound `read_vec` needs from the documentation of `String::parse`. +// Furthermore, `std::cmp::min` works not just for `i32`, but also for `f32`. // [index](main.html) | [previous](part02.html) | next