-// 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::<i32>::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<i32> {
- let mut vec = Vec::new();
-
+ let mut vec: Vec<i32> = Vec::<i32>::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<String>`. This is a lot like `Option<String>` ("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::<i32>` is `parse` with its generic type set to `i32`.
match line.parse::<i32>() {
+ // `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?"),
}
vec
}
-enum SomethingOrNothing<T> {
- 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 the other module to make this possible: Only
+// items declared public can be imported elsewhere.
+use part02::{SomethingOrNothing,Something,Nothing,vec_min};
-fn vec_min<T: Minimum>(v: &Vec<T>) -> SomethingOrNothing<T> {
- 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**: 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<T>`".
+//
+// 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 */
}
-
-use std::fmt;
-impl<T: fmt::Display> fmt::Display for SomethingOrNothing<T> {
- fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
- match self {
- &Something(ref t) => t.fmt(f),
- &Nothing => "Nothing".fmt(f),
- }
+impl<T: Print> SomethingOrNothing<T> {
+ fn print2(self) {
+ panic!("Not yet implemented.")
}
}
-pub fn part_main() {
- let vec = read_vec();
- let min = vec_min(&vec);
- println!("The minimum is: {}", min);
-}
-
-// [index](main.html) | [previous](part02.html) | next
+// [index](main.html) | [previous](part02.html) | [next](part04.html)
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