implement and test subtraction

[rust-101.git] / src / part00.rs

diff --git a/src/part00.rs b/src/part00.rs
index 962e801238b4f26574d91a6c8578e6b81982bb20..6469907e91203f62f856a8a8e25c229c9b7a41c1 100644 (file)
--- a/src/part00.rs
+++ b/src/part00.rs
@@ -1,21 +1,114 @@
-// [index](main.html) | previous | [next](part01.html)
+// Rust-101, Part 00: Algebraic datatypes
+// ======================================

 
 

-// Rust-101, Part 00
-// =================
+// As our first piece of Rust code, we want to write a function that computes the
+// minimum of a list.

 
 

-// As a starter, we want to write a function that computes the minimum of a list.
-// First, we need to get a list. For now, let's just hard-code some sample list.
-// Later, we will ask the user to input a list.
-// 
-// Observe how in Rust, the function type comes *after* the arguments (which, in
-// this case, the function does not have). `Vec<i32>` is the type of a growable list
-// of (signed, 32-bit) integers.
+//@ ## Getting started
+//@ Let us start by thinking about the *type* of our function. Rust forces us to give the types of
+//@ all arguments, and the return type, before we even start writing the body. In the case of our minimum
+//@ function, we may be inclined to say that it returns a number. But then we would be in trouble: What's
+//@ the minimum of an empty list? The type of the function says we have to return *something*.
+//@ We could just choose 0, but that would be kind of arbitrary. What we need
+//@ is a type that is "a number, or nothing". Such a type (of multiple exclusive options)
+//@ is called an "algebraic datatype", and Rust lets us define such types with the keyword `enum`.
+//@ Coming from C(++), you can think of such a type as a `union`, together with a field that
+//@ stores the variant of the union that's currently used.

 
 

-pub fn read_vec() -> Vec<i32> {
-    vec![0,1,2,3,4]
+// An `enum` for "a number or nothing" could look as follows:
+enum NumberOrNothing {
+    Number(i32),
+    Nothing

 }
 }

+//@ Notice that `i32` is the type of (signed, 32-bit) integers. To write down the type of
+//@ the minimum function, we need just one more ingredient: `Vec<i32>` is the type of
+//@ (growable) arrays of numbers, and we will use that as our list type.

 
 

+// Observe how in Rust, the return type comes *after* the arguments.
+fn vec_min(vec: Vec<i32>) -> NumberOrNothing {
+    //@ In the function, we first need some variable to store the minimum as computed so far.
+    //@ Since we start out with nothing computed, this will again be a 
+    //@ "number or nothing":
+    let mut min = NumberOrNothing::Nothing;
+    //@ We do not have to write a type next to `min`, Rust can figure that out automatically
+    //@ (a bit like `auto` in C++11). Also notice the `mut`: In Rust, variables are
+    //@ immutable per default, and you need to tell Rust if you want
+    //@ to change a variable later.

 
 

-pub fn part_main() {
-    
+    // Now we want to *iterate* over the list. Rust has some nice syntax for iterators:
+    for el in vec {
+        // So `el` is al element of the list. We need to update `min` accordingly, but how do we get the current
+        // number in there? This is what pattern matching can do:
+        match min {
+            // In this case (*arm*) of the `match`, `min` is currently nothing, so let's just make it the number `el`.
+            NumberOrNothing::Nothing => {
+                min = NumberOrNothing::Number(el);                  /*@*/
+            },
+            // In this arm, `min` is currently the number `n`, so let's compute the new minimum and store it.
+            //@ We will write the function `min_i32` just after we completed this one.
+            NumberOrNothing::Number(n) => {
+                let new_min = min_i32(n, el);                       /*@*/
+                min = NumberOrNothing::Number(new_min);             /*@*/
+            }
+        }
+    }
+    // Finally, we return the result of the computation.
+    return min;

 }
 }

+
+// Now that we reduced the problem to computing the minimum of two integers, let's do that.
+fn min_i32(a: i32, b: i32) -> i32 {
+    if a < b {
+        return a;                                                   /*@*/
+    } else {
+        return b;                                                   /*@*/
+    }
+}
+
+// Phew. We wrote our first Rust function! But all this `NumberOrNothing::` is getting kind of
+// ugly. Can't we do that nicer?
+
+// Indeed, we can: The following line tells Rust to take
+// the constructors of `NumberOrNothing` into the local namespace.
+// Try moving that above the function, and removing all the occurrences `NumberOrNothing::`.
+use self::NumberOrNothing::{Number,Nothing};
+
+// To call this function, we now just need a list. Of course, ultimately we want to ask the user for
+// a list of numbers, but for now, let's just hard-code something.
+
+//@ `vec!` is a *macro* (as you can tell from the `!`) that constructs a constant `Vec<_>` with the given
+//@ elements.
+fn read_vec() -> Vec<i32> {
+    vec![18,5,7,1,9,27]                                             /*@*/
+}
+
+// Of course, we would also like to actually see the result of the computation, so we need to print the result.
+//@ Of course Rust can print numbers, but after calling `vec_min`, we have a `NumberOrNothing`.
+//@ So let's write a small helper function that prints such values.
+
+//@ `println!` is again a macro, where the first argument is a *format string*. For
+//@ now, you just need to know that `{}` is the placeholder for a value, and that Rust
+//@ will check at compile-time that you supplied the right number of arguments.
+fn print_number_or_nothing(n: NumberOrNothing) {
+    match n {                                                       /*@*/
+        Nothing => println!("The number is: <nothing>"),            /*@*/
+        Number(n) => println!("The number is: {}", n),              /*@*/
+    };                                                              /*@*/
+}
+
+// Putting it all together:
+pub fn main() {
+    let vec = read_vec();
+    let min = vec_min(vec);
+    print_number_or_nothing(min);
+}
+
+//@ You can now use `cargo build` to compile your *crate*. That's Rust's name for a *compilation unit*, which in
+//@ the case of Rust means an application or a library. <br/>
+// Finally, try `cargo run` on the console to run it.
+
+//@ Yay, it said "1"! That's actually the right answer. Okay, we could have
+//@ computed that ourselves, but that's besides the point. More importantly:
+//@ You completed the first part of the course.
+
+//@ [index](main.html) | previous | [next](part01.html)