X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/315bf91eb0b309b29c732ca7726df1f6ca9f567e..a0ae4ec8a5da0e171cb2d2f68621fa98f5ea610b:/src/part03.rs diff --git a/src/part03.rs b/src/part03.rs index a7cfb1a..b4bf2f3 100644 --- a/src/part03.rs +++ b/src/part03.rs @@ -1,166 +1,119 @@ -// Rust-101, Part 03: Input, Testing -// ================================= +// 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 pretty - but well, -// let's get that behind us. +//@ 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. -// 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 +// 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. +//@ 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(); - // The central handle to the standard input is made available by `io::stdin()`. + 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."); - // 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.) + 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() { - // 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()`? + // 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(); - // 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?"), + // 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 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.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 } -// 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. +//@ 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. -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 -} - -// `::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) - } -} - -impl SomethingOrNothing { - fn print(self) { - match self { - Nothing => println!("The number is: "), - Something(n) => println!("The number is: {}", n), - }; - } -} +// 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}; // 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(); + 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, - } +// **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 */ +} +impl SomethingOrNothing { + fn print2(self) { + unimplemented!() } } -// 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`. +// **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)