some factorialing

tutorial1/src/main.rs

@@ -1,5 +1,7 @@
 // some cut-and-paste and transcription from http://science.raphael.poss.name/rust-for-functional-programmers.html
 
+// defining a bunch of functions, but only trying a few at a time.
+
 #![allow(dead_code)]
 
 extern crate rand;
@@ -11,12 +13,30 @@ use rand::Rng;
 fn f (x:i32, y:i32) -> i32 { x + y }
 
 // fact : |int| -> int
-fn fact (n:i64) -> i64 {
+// this will overflow the int
+fn fact_rec (n:u64) -> u64 {
    if n == 1 { 1 }
    else { n * fact(n-1) }
 }
 
 
+// fact : |int| -> int
+// the fold also overflows, but faster
+fn fact (n:u64) -> u64 {
+    (1..n+1).fold(1, |p,n| p*n)
+
+    // (1..n+1) -- right-exclusive range interval
+    // inclusive range is available as "experimental", and looks like: (1...n+1)
+    // you can '#![feature(inclusive_range_syntax)]'
+    // but 'feature' doesn't work on the stable releases, you need a nightly
+
+    // fold - defined in the std::iter::Iterator, which is exposed
+    // by the rust prelude: https://doc.rust-lang.org/std/prelude/
+
+    // fold takes 2 arguments: initial value, and closure
+    // |foo| expr -- this is a closure. can also be |foo| { block }
+}
+
 
 fn collatz(n:i32) {
     let v = match n % 2 {
@@ -34,6 +54,17 @@ fn main() {
     println!("{}", start);
     collatz(start);
 
-    println!("fact: {}", fact(i64::from(start)));
+    // println!("fact: {}", fact(i64::from(start)));
+    // there is no i -> u 'from'
+    // println!("fact: {}", fact(start as u64));
+
+    for i in 1..10 {
+        println!("{}, {}", i, fact_rec(i))
+    }
+
+    for i in 1..10 {
+        println!("{}, {}", i, fact(i))
+    }
+
 }