X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/8fcdbed310c53f621fba0401399659ed1a1ec446..83e8dbfff2a6fdce785af58dbb13f007c4234cf2:/workspace/src/part10.rs

diff --git a/workspace/src/part10.rs b/workspace/src/part10.rs
index 570e851..9fd7642 100644
--- a/workspace/src/part10.rs
+++ b/workspace/src/part10.rs
@@ -1,15 +1,17 @@
-// Rust-101, Part 09: Closures (WIP)
-// =================================
+// Rust-101, Part 10: Closures
+// ===========================
 
 use std::io::prelude::*;
-use std::io;
-
+use std::{fmt,io};
 use part05::BigInt;
 
+
+// So, let us define a trait that demands that the type provides some method `do_action` on digits.
 trait Action {
     fn do_action(&mut self, digit: u64);
 }
 
+// Now we can write a function that takes some `a` of a type `A` such that we can call `do_action` on `a`, passing it every digit.
 impl BigInt {
     fn act_v1<A: Action>(&self, mut a: A) {
         for digit in self {
@@ -23,30 +25,76 @@ struct PrintWithString {
 }
 
 impl Action for PrintWithString {
+    // Here we perform performs the actual printing of the prefix and the digit. We're not making use of our ability to
+    // change `self` here, but we could replace the prefix if we wanted.
     fn do_action(&mut self, digit: u64) {
         println!("{}{}", self.prefix, digit);
     }
 }
 
+// Finally, this function takes a `BigInt` and a prefix, and prints the digits with the given prefix.
 fn print_with_prefix_v1(b: &BigInt, prefix: String) {
     let my_action = PrintWithString { prefix: prefix };
     b.act_v1(my_action);
 }
 
+// Here's a small main function, demonstrating the code above in action. Remember to edit `main.rs` to run it.
 pub fn main() {
     let bignum = BigInt::new(1 << 63) + BigInt::new(1 << 16) + BigInt::new(1 << 63);
     print_with_prefix_v1(&bignum, "Digit: ".to_string());
 }
 
+// ## Closures
+
+// This defines `act` very similar to above, but now we demand `A` to be the type of a closure that mutates its borrowed environment,
+// takes a digit, and returns nothing.
 impl BigInt {
     fn act<A: FnMut(u64)>(&self, mut a: A) {
         for digit in self {
+            // We can call closures as if they were functions - but really, what's happening here is translated to essentially what we wrote above, in `act_v1`.
             a(digit);
         }
     }
 }
 
+// Now that we saw how to write a function that operates on closures, let's see how to write a closure.
 pub fn print_with_prefix(b: &BigInt, prefix: String) {
     b.act(|digit| println!("{}{}", prefix, digit) );
 }
+// You can change `main` to call this function, and you should notice - nothing, no difference in behavior.
+// But we wrote much less boilerplate code!
+
+// Remember that we decided to use the `FnMut` trait above? This means our closure could actually mutate its environment.
+// For example, we can use that to count the digits as they are printed.
+pub fn print_and_count(b: &BigInt) {
+    let mut count: usize = 0;
+    b.act(|digit| { println!("{}: {}", count, digit); count = count +1; } );
+    println!("There are {} digits", count);
+}
+
+// ## Fun with iterators and closures
+
+// Let's say we want to write a function that increments every entry of a `Vec` by some number, then looks for numbers larger than some threshold, and prints them.
+fn inc_print_even(v: &Vec<i32>, offset: i32, threshold: i32) {
+    for i in v.iter().map(|n| n + offset).filter(|n| *n > threshold) {
+        println!("{}", i);
+    }
+}
+
+// Sometimes it is useful to know both the position of some element in a list, and its value. That's where the `enumerate` function helps.
+fn print_enumerated<T: fmt::Display>(v: &Vec<T>) {
+    for (i, t) in v.iter().enumerate() {
+        println!("Position {}: {}", i, t);
+    }
+}
+
+// And as a final example, one can also collect all elements of an iterator, and put them, e.g., in a vector.
+fn filter_vec_by_divisor(v: &Vec<i32>, divisor: i32) -> Vec<i32> {
+    v.iter().filter(|n| *n % divisor == 0).collect()
+}
+
+// **Exercise 10.1**: Look up the [documentation of `Iterator`](http://doc.rust-lang.org/stable/std/iter/trait.Iterator.html) to learn about more functions
+// that can act on iterators. Try using some of them. What about a function that sums the even numbers of an iterator? Or a function that computes the
+// product of those numbers that sit at odd positions? A function that checks whether a vector contains a certain number? Whether all numbers are
+// smaller than some threshold? Be creative!