-// Rust-101, Part 03: Input, Formatting
-// ====================================
+// Rust-101, Part 03: Input, Testing
+// =================================
+// 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.
+
+// 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
+// directly available.
use std::io::prelude::*;
use std::io;
+// Let's now go over this function line-by-line.
fn read_vec() -> Vec<i32> {
let mut 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() {
+ // 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()`?
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?"),
}
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.
+
enum SomethingOrNothing<T> {
Something(T),
Nothing,
fn min(a: Self, b: Self) -> Self;
}
-fn vec_min<T: Minimum>(v: &Vec<T>) -> SomethingOrNothing<T> {
+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
}
+// `::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)
}
}
-use std::fmt;
-impl<T: fmt::Display> fmt::Display for SomethingOrNothing<T> {
- fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+impl SomethingOrNothing<i32> {
+ fn print(self) {
match self {
- &Something(ref t) => t.fmt(f),
- &Nothing => "Nothing".fmt(f),
- }
+ Nothing => println!("The number is: <nothing>"),
+ Something(n) => println!("The number is: {}", n),
+ };
}
}
+// 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 part_main() {
let vec = read_vec();
- let min = vec_min(&vec);
- println!("The minimum is: {}", min);
+ 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,
+ }
+ }
+}
+
+// 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`.
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