Answers for standard_library_types

iterators4.rs was particularly instructive.

I started out by searching for the initial C recursion and for loop
versions and tried to use a range iteration but couldn't figure the
syntax. So I created the first 7 answers and then used hint.

I then seached for: how to use iterator for side effects only
 https://www.google.com/search?q=how+to+use+iterator+for+side+effects+only+in+rust

And saw this:
 rust-lang/rust#44546 (comment)
And:
 rust-lang/rust#44546 (comment)

And with that I implemented Answer 8

But still wasn't happy as I felt there must be a better way where I didn't
need to throw away things. So I looking at the iterator documentation:
 https://doc.rust-lang.org/std/iter/trait.Iterator.html

I saw "fold" and low and behold that was exactly what I needed! Then I
saw "fold_first" and it was "simpler" as the first value in the range
is the initial value for the accumulator so fold_first only has one
parameters not two. But fold_first returns an Option which means I need
to return the unwrap(). Anyway both Answers 9 and 10 are simpler then
any other solution!

I do wonder which is fastest.

exercises/standard_library_types/arc1.rs

@@ -4,18 +4,20 @@
 // somewhere. Try not to create any copies of the `numbers` Vec!
 // Execute `rustlings hint arc1` for hints :)
 
-// I AM NOT DONE
+// I AM DONE
 
 #![forbid(unused_imports)] // Do not change this, (or the next) line.
 use std::sync::Arc;
 use std::thread;
 
 fn main() {
-    let numbers: Vec<_> = (0..100u32).collect();
-    let shared_numbers = // TODO
+    let numbers: Vec<_> = (0..100u64).collect();
+    let shared_numbers = Arc::new(numbers);
     let mut joinhandles = Vec::new();
 
+    // Answer 1
     for offset in 0..8 {
+        let child_numbers = shared_numbers.clone();
         joinhandles.push(thread::spawn(move || {
             let mut i = offset;
             let mut sum = 0;
@@ -26,6 +28,25 @@ fn main() {
             println!("Sum of offset {} is {}", offset, sum);
         }));
     }
+
+    //// Answer 2
+    //let threads = 10;
+    //let shard_size = shared_numbers.len() / threads;
+    //if shared_numbers.len() % threads != 0 {
+    //    panic!("shard_size isn't a multiple of numbers.len");
+    //}
+    //for offset in 0..threads {
+    //    let child_numbers = shared_numbers.clone();
+    //    joinhandles.push(thread::spawn(move || {
+    //        let base = shard_size * offset;
+    //        let mut sum = 0;
+    //        for i in 0..shard_size {
+    //            sum += child_numbers[base + i];
+    //        }
+    //        println!("Sum of offset {} is {}", offset, sum);
+    //    }));
+    //}
+
     for handle in joinhandles.into_iter() {
         handle.join().unwrap();
     }

exercises/standard_library_types/box1.rs

@@ -16,11 +16,11 @@
 //
 // Execute `rustlings hint box1` for hints :)
 
-// I AM NOT DONE
+// I AM DONE
 
 #[derive(PartialEq, Debug)]
 pub enum List {
-    Cons(i32, List),
+    Cons(i32, Box<List>),
     Nil,
 }
 
@@ -33,11 +33,12 @@ fn main() {
 }
 
 pub fn create_empty_list() -> List {
-    unimplemented!()
+    List::Nil
 }
 
 pub fn create_non_empty_list() -> List {
-    unimplemented!()
+    //List::Cons(1, Box::new(List::Nil))
+    List::Cons(1, Box::new(List::Cons(2, Box::new(List::Cons(3, Box::new(List::Nil))))))
 }
 
 #[cfg(test)]

exercises/standard_library_types/iterators1.rs

@@ -8,17 +8,19 @@
 // 
 // Execute `rustlings hint iterators1` for hints :D
 
-// I AM NOT DONE
+// I AM DONE
 
 fn main () {
     let my_fav_fruits = vec!["banana", "custard apple", "avocado", "peach", "raspberry"];
 
-    let mut my_iterable_fav_fruits = ???;   // TODO: Step 1
+    let mut my_iterable_fav_fruits = my_fav_fruits.iter();  // TODO: Step 1
 
     assert_eq!(my_iterable_fav_fruits.next(), Some(&"banana"));
-    assert_eq!(my_iterable_fav_fruits.next(), ???);     // TODO: Step 2
+    assert_eq!(my_iterable_fav_fruits.next(), Some(&"custard apple"));     // TODO: Step 2
     assert_eq!(my_iterable_fav_fruits.next(), Some(&"avocado"));
-    assert_eq!(my_iterable_fav_fruits.next(), ???);     // TODO: Step 2.1
+    assert_eq!(my_iterable_fav_fruits.next(), Some(&"peach"));     // TODO: Step 2.1
     assert_eq!(my_iterable_fav_fruits.next(), Some(&"raspberry"));
-    assert_eq!(my_iterable_fav_fruits.next(), ???);     // TODO: Step 3
+    assert_eq!(my_iterable_fav_fruits.next(), None);     // TODO: Step 3
+    // iterator next() continues to return no
+    assert_eq!(my_iterable_fav_fruits.next(), None);     // TODO: Step 3
 }

exercises/standard_library_types/iterators2.rs

@@ -7,16 +7,20 @@
 //         Try to ensure it returns a single string.
 // As always, there are hints if you execute `rustlings hint iterators2`!
 
-// I AM NOT DONE
+// I AM DONE
 
 pub fn capitalize_first(input: &str) -> String {
     let mut c = input.chars();
     match c.next() {
         None => String::new(),
-        Some(first) => first.collect::<String>() + c.as_str(),
+        Some(first) => first.to_uppercase().collect::<String>() + c.as_str(),
     }
 }
 
+pub fn capitalize_words(input: Vec<&str>) -> Vec<String> {
+    input.iter().map(|w| { capitalize_first(w) }).collect()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -37,14 +41,14 @@ mod tests {
     #[test]
     fn test_iterate_string_vec() {
         let words = vec!["hello", "world"];
-        let capitalized_words: Vec<String> = // TODO
+        let capitalized_words: Vec<String> = capitalize_words(words);
         assert_eq!(capitalized_words, ["Hello", "World"]);
     }
 
     #[test]
     fn test_iterate_into_string() {
         let words = vec!["hello", " ", "world"];
-        let capitalized_words = // TODO
+        let capitalized_words: String = capitalize_words(words).join("");
         assert_eq!(capitalized_words, "Hello World");
     }
 }

exercises/standard_library_types/iterators3.rs

@@ -7,7 +7,7 @@
 // Execute `rustlings hint iterators3` to get some hints!
 // Have fun :-)
 
-// I AM NOT DONE
+// I AM DONE
 
 #[derive(Debug, PartialEq, Eq)]
 pub enum DivisionError {
@@ -24,7 +24,15 @@ pub struct NotDivisibleError {
 // This function should calculate `a` divided by `b` if `a` is
 // evenly divisible by b.
 // Otherwise, it should return a suitable error.
-pub fn divide(a: i32, b: i32) -> Result<i32, DivisionError> {}
+pub fn divide(a: i32, b: i32) -> Result<i32, DivisionError> {
+    if b == 0 {
+        return Err(DivisionError::DivideByZero);
+    }
+    if (a % b) != 0 {
+        return Err(DivisionError::NotDivisible(NotDivisibleError {dividend: a, divisor: b}));
+    }
+    Ok(a / b)
+}
 
 #[cfg(test)]
 mod tests {
@@ -46,7 +54,6 @@ mod tests {
             }))
         );
     }
-
     #[test]
     fn test_divide_by_0() {
         assert_eq!(divide(81, 0), Err(DivisionError::DivideByZero));
@@ -56,23 +63,22 @@ mod tests {
     fn test_divide_0_by_something() {
         assert_eq!(divide(0, 81), Ok(0));
     }
-
     // Iterator exercises using your `divide` function
-    /*
+
     #[test]
     fn result_with_list() {
         let numbers = vec![27, 297, 38502, 81];
+        //let division_results = numbers.into_iter().map(|n| divide(n, 27)).collect::<Result<i32, DivisionError>>();
         let division_results = numbers.into_iter().map(|n| divide(n, 27));
-        let x //... Fill in here!
+        let x: Result<Vec<i32>, _>  = division_results.collect(); //... Fill in here!
         assert_eq!(format!("{:?}", x), "Ok([1, 11, 1426, 3])");
     }
 
     #[test]
     fn list_of_results() {
         let numbers = vec![27, 297, 38502, 81];
         let division_results = numbers.into_iter().map(|n| divide(n, 27));
-        let x //... Fill in here!
+        let x: Vec<Result<i32, _>> = division_results.collect();
         assert_eq!(format!("{:?}", x), "[Ok(1), Ok(11), Ok(1426), Ok(3)]");
     }
-    */
 }

exercises/standard_library_types/iterators4.rs

@@ -1,6 +1,147 @@
+#![feature(iterator_fold_self)]
 // iterators4.rs
 
-// I AM NOT DONE
+// I AM DONE
+
+pub fn recursive(num: u64) -> u64 {
+    let mut fact = num;
+    if num > 1 {
+        fact = num * recursive(num - 1)
+    }
+    println!("recursive: num={} fact={}", num, fact);
+    fact
+}
+
+pub fn simplest_recursive(num: u64) -> u64 {
+    if num > 1 {
+        num * simplest_recursive(num - 1)
+    } else {
+        num
+    }
+}
+
+pub fn match_recursive(num: u64) -> u64 {
+    match num {
+        n if n > 1 => n * match_recursive(n - 1),
+        n => n,
+    }
+}
+
+pub fn for_in(num: u64) -> u64 {
+    let mut fact = 1;
+    for i in 1..num+1 {
+        fact *= i;
+        println!("for_in: num={} fact={}", num, fact);
+    }
+    fact
+}
+
+// Create iterable version of Factorial
+
+struct Factorial {
+    num: u64,
+    factorial: u64,
+}
+
+impl Factorial {
+    fn new(num: u64) -> Factorial {
+        Factorial {
+            num,
+            factorial: 1
+        }
+    }
+}
+
+impl Iterator for Factorial {
+    type Item = u64;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        println!("Factorial::next:+ self.num={}", self.num);
+        if self.num > 0 {
+            self.factorial *= self.num;
+            self.num -= 1;
+            println!("Factorial::next:- Some self.factorial={}", self.factorial);
+            return Some(self.factorial)
+        } else {
+            println!("Factorial::next:- None self.factorial={}", self.factorial);
+            return None
+        }
+    }
+}
+
+pub fn loop_next_iterator(num: u64) -> u64 {
+    let mut f = Factorial::new(num);
+    loop {
+        let n = f.next();
+        if n == None {
+            break;
+        }
+        println!("iterator: f.next()={}", n.unwrap());
+    }
+    println!("iterator:- f.factorial={}", f.factorial);
+    f.factorial
+}
+
+pub fn for_in_iterator(num: u64) -> u64 {
+    println!("iterator:+ num={}", num);
+    let f = Factorial::new(num);
+    let mut x: u64 = 1;
+    for curf in f {
+        println!("iterator: curf={}", curf);
+        x = curf;
+    }
+    println!("iterator:- x={}", x);
+    x
+}
+
+pub fn while_next_iterator(num: u64) -> u64 {
+    let mut f = Factorial::new(num);
+    while f.next() != None {}
+    f.factorial
+}
+
+pub fn range_for_each_iterator(num: u64) -> u64 {
+    // "rf" is a a reference to mutable fact
+    // which is captured by the map closure.
+    //let mut fact: u64 = 1;
+    //let rf = &mut fact;
+    //(1..num+1).map(|n| {
+    //    *rf *= n;
+    //    println!("n={} *rf={}", n, *rf);
+    //} ).for_each(drop);
+    //fact
+
+    // Turns out you don't need "rf", I'm guessing because fact
+    // is on the left side of "=" and therfore is areference.
+    let mut fact: u64 = 1;
+    (1..num+1).map(|n| {
+        fact *= n;
+        //println!("n={} fact={}", n, fact);
+    } ).for_each(drop);
+    fact
+}
+
+pub fn range_fold_iterator(num: u64) -> u64 {
+    (1..num+1).fold(1, |fact, n| fact * n)
+
+    //// For debugging add a println
+    //(1..num+1).fold(1, |fact, n| {
+    //    let f = fact * n;
+    //    println!("n={} f={}", n, f);
+    //    f
+    //} )
+}
+
+pub fn range_fold_first_iterator(num: u64) -> u64 {
+    (1..num+1).fold_first(|fact, n| fact * n).unwrap()
+
+    //// For debugging add a println
+    //(1..num+1).fold_first(|fact, n| {
+    //    let f = fact * n;
+    //    println!("n={} f={}", n, f);
+    //    f
+    //} ).unwrap()
+}
 
 pub fn factorial(num: u64) -> u64 {
     // Complete this function to return the factorial of num
@@ -12,7 +153,40 @@ pub fn factorial(num: u64) -> u64 {
     // For an extra challenge, don't use:
     // - recursion
     // Execute `rustlings hint iterators4` for hints.
+
+    // Answer 1 recursive
+    //recursive(num)
+
+    // Answer 2 simplest recursive
+    //simplest_recursive(num)
+
+    // Answer 3 match recursive
+    //match_recursive(num)
+
+    // Answer 4 for_in no recursion
+    //for_in(num)
+
+    // Answer 5 iterate using loop
+    //loop_next_iterator(num)
+
+    // Answer 6 iterate using for in
+    //for_in_iterator(num)
+
+    // Answer 7 iterate using while
+    //while_next_iterator(num)
+
+    // Answer 8 Iterate using range and for_each
+    //range_for_each_iterator(num)
+
+    // Answer 9 Iterate using range and fold
+    // THIS IS THE BEST ANSWER, or maybe fold_first below.
+    //range_fold_iterator(num)
+
+    // Answer 10 Iterate using range and fold_first
+    // THIS IS THE BEST ANSWER :)
+    range_fold_first_iterator(num)
 }
+
 
 #[cfg(test)]
 mod tests {