-// 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 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. 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 the function `io::stdin`.
let stdin = io::stdin();
- println!("Enter a list of numbers, one per line. End with Ctrl-D.");
+ 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() {
+ // 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<String>`.
+ //@ The problem with I/O is that it can always go wrong. The type of `line` 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](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();
- match line.parse::<i32>() {
- 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 its
+ //@ 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.trim().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) /*@*/
+ },
+ // We don't care about the particular error, so we ignore it with a `_`.
+ Err(_) => {
+ println!("What did I say about numbers?") /*@*/
+ },
}
}
vec
}
-enum SomethingOrNothing<T> {
- Something(T),
- Nothing,
-}
-use self::SomethingOrNothing::{Something,Nothing};
-
-trait Minimum : Copy {
- fn min(a: Self, b: Self) -> Self;
-}
+//@ 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.
-fn vec_min<T: Minimum>(v: Vec<T>) -> SomethingOrNothing<T> {
- let mut min = Nothing;
- for e in v {
- min = Something(match min {
- Nothing => e,
- Something(n) => T::min(n, e)
- });
- }
- min
-}
+// 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 Minimum for i32 {
- fn min(a: Self, b: Self) -> Self {
- ::std::cmp::min(a, b)
- }
+// 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<i32> {
- fn print(self) {
- match self {
- Nothing => println!("The number is: <nothing>"),
- Something(n) => println!("The number is: {}", n),
- };
- }
+// **Exercise 03.1**: The goal is to write a generic version of `SomethingOrNothing::print`.
+// To this end, define a trait `Print` that provides (simple) generic printing, and implement
+// that trait for `i32`. Then define `SomethingOrNothing::print2` to use that trait, and change
+// `main` above to use the new generic `print2` function.
+// 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.
+pub trait Print {
+ /* Add things here */
}
-pub fn part_main() {
- let vec = read_vec();
- let min = vec_min(vec);
- min.print();
+impl Print for i32 {
+ /* Add things here */
}
-
-impl SomethingOrNothing<i32> {
- fn equals(self, other: Self) -> bool {
- match (self, other) {
- (Nothing , Nothing ) => true,
- (Something(n), Something (m)) => n == m,
- _ => false,
- }
+impl<T: Print> SomethingOrNothing<T> {
+ fn print2(self) {
+ unimplemented!()
}
}
-#[test]
-fn tes_vec_min() {
- assert!(vec_min(vec![6,325,33,532,5,7]).equals(Something(5)));
- assert!(vec_min(vec![]).equals(Nothing));
-}
+// **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)
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