test that subtraction appropriately panics

[rust-101.git] / src / part03.rs

diff --git a/src/part03.rs b/src/part03.rs
index cb0807b87c5dcf7c0bf128f7cde976a5fd888881..d8d71b12cc138b95a477717f3a19d002b3513a5e 100644 (file)
--- a/src/part03.rs
+++ b/src/part03.rs
@@ -1,67 +1,119 @@
-// 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 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;
 
 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> {
 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.");
     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() {
     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](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();
         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 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
 }
 
         }
     }
 
     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 part 02 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 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 */

 }
 }

-
-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) {
+        unimplemented!()

     }
 }
 
     }
 }
 

-pub fn part_main() {
-    let vec = read_vec();
-    let min = vec_min(&vec);
-    println!("The minimum is: {}", min);
-}
+// **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) | [next](part04.html)