some more exercises

[rust-101.git] / src / part03.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 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<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),
            // 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<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!()
    }
}

// **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.

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