Merge pull request #42 from zdyxry/master

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

diff --git a/src/part03.rs b/src/part03.rs
index 9ab153dcf4176688b6afc9565e0ea75bb57f31cc..b81bbc371cd7c3a8c2dc3d16de6c83a58d08a319 100644 (file)
--- a/src/part03.rs
+++ b/src/part03.rs
@@ -1,105 +1,121 @@
 // Rust-101, Part 03: Input
 // ========================
 
 // 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.
+//@ 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 brings a bunch of commonly used I/O stuff
+// 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;
 
 // 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.
+//@ 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();
 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()`.
+    // The central handle to the standard input is made available by the function `io::stdin`.

     let stdin = 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() {
         // Rust's type for (dynamic, growable) strings is `String`. However, our variable `line`
     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.
+        // 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.
+        //@ 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();

-        // 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!
+        // 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.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),
+        //@ 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 `_`.
             // We don't care about the particular error, so we ignore it with a `_`.

-            Err(_) => println!("What did I say about numbers?"),
+            Err(_) => {
+                println!("What did I say about numbers?")           /*@*/
+            },

         }
     }
 
     vec
 }
 
         }
     }
 
     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.
+//@ 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`.
 
 // 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.
+//@ 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();
 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();                                                    /*@*/

 }
 
 }
 

-// **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.
+// **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>`".
 // 
 // 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.
+// 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.
 // 
 // *Hint*: There is a macro `print!` for printing without appending a newline.

-trait Print {
+pub trait Print {
+    /* Add things here */
+}
+impl Print for i32 {

     /* Add things here */
 }
 impl<T: Print> SomethingOrNothing<T> {
     /* Add things here */
 }
 impl<T: Print> SomethingOrNothing<T> {


@@ -108,7 +124,8 @@ impl<T: Print> SomethingOrNothing<T> {
     }
 }
 
     }
 }
 

-// **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.
+// **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](part04.html)
+//@ [index](main.html) | [previous](part02.html) | [raw source](workspace/src/part03.rs) |
+//@ [next](part04.html)