-// 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 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::<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();
- // The central handle to the standard input is made available by `io::stdin()`.
+ 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.");
- // 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<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 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<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();
- // 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::<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?"),
+ // 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::<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
}
-// 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<T> {
- Something(T),
- Nothing,
-}
-use self::SomethingOrNothing::{Something,Nothing};
-
-trait Minimum : Copy {
- fn min(a: Self, b: Self) -> Self;
-}
-
-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
-}
-
-// `::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<i32> {
- fn print(self) {
- match self {
- Nothing => println!("The number is: <nothing>"),
- 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<i32> {
- 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<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 */
+}
+impl<T: Print> SomethingOrNothing<T> {
+ 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. <br/>
-// 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. <br/>
-// *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](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part03.rs) | [next](part04.html)
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