X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/943c00ca03ddc76177b4a16e19e8b831247e03f8..0b126b3637b97200f23be4e753d245852e0355ee:/workspace/src/part03.rs?ds=inline diff --git a/workspace/src/part03.rs b/workspace/src/part03.rs index 8d1c8b6..7d022e8 100644 --- a/workspace/src/part03.rs +++ b/workspace/src/part03.rs @@ -1 +1,116 @@ - +// ***Remember to enable/add this part in `main.rs`!*** + +// 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 import that with `use`. +// 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 = 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?"), + } + } + + vec +} + +// 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. + +// 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(); +} + +// **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 */ +} +impl SomethingOrNothing { + fn print2(self) { + unimplemented!() + } +} + +// **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](part04.html)