simple-linked-list, doubly-linked-list: Add is_empty

https://rust-lang.github.io/rust-clippy/master/index.html#len_without_is_empty

This alleges that for certain types, len() may be expensive, but
is_empty() is likely to always be cheap, and therefore it's good custom
to have both. I find this sufficiently persuasive. On the principle that
the track instill good habits in students, we will add and test such a
method.

I understand that for the example solutions, len() is actually cheap.
In addition, I understand that for a list it's always possible to be
able to cheaply determine the `len()` simply by keeping an extra field
that gets updated on every insert.

Nevertheless I think it's a good idea anyway.

Helps address #1011
Helps address #1012

Author: petertseng

exercises/doubly-linked-list/.meta/description.md

@@ -33,7 +33,7 @@ private functions.
 
 Implement the functionality for adding and removing elements (pushing and popping)
 at the front and back. This is enough to use the list as a double-ended queue.
-Also implement the `len` function.
+Also implement the `len` and `is_empty` functions.
 
 In the finished implementation, all modifications of the list should be done through the cursor struct
 to minimize duplication. The `push_*` and `pop_*` methods on `LinkedList`

exercises/doubly-linked-list/README.md

@@ -35,7 +35,7 @@ private functions.
 
 Implement the functionality for adding and removing elements (pushing and popping)
 at the front and back. This is enough to use the list as a double-ended queue.
-Also implement the `len` function.
+Also implement the `len` and `is_empty` functions.
 
 In the finished implementation, all modifications of the list should be done through the cursor struct
 to minimize duplication. The `push_*` and `pop_*` methods on `LinkedList`

exercises/doubly-linked-list/example.rs

@@ -77,6 +77,10 @@ impl<T> LinkedList<T> {
         }
     }
 
+    pub fn is_empty(&self) -> bool {
+        self.len == 0
+    }
+
     pub fn len(&self) -> usize {
         self.len
     }

exercises/doubly-linked-list/src/lib.rs

@@ -14,6 +14,15 @@ impl<T> LinkedList<T> {
         unimplemented!()
     }
 
+    // You may be wondering why it's necessary to have is_empty()
+    // when it can easily be determined from len().
+    // It's good custom to have both because len() can be expensive for some types,
+    // whereas is_empty() is almost always cheap.
+    // (Also ask yourself whether len() is expensive for LinkedList)
+    pub fn is_empty(&self) -> bool {
+        unimplemented!()
+    }
+
     pub fn len(&self) -> usize {
         unimplemented!()
     }

exercises/doubly-linked-list/tests/doubly-linked-list.rs

@@ -15,6 +15,7 @@ fn is_generic() {
 fn basics_empty_list() {
     let list: LinkedList<i32> = LinkedList::new();
     assert_eq!(list.len(), 0);
+    assert!(list.is_empty());
 }
 
 // push / pop at back ————————————————————————————————————————
@@ -25,7 +26,11 @@ fn basics_single_element_back() {
     list.push_back(5);
 
     assert_eq!(list.len(), 1);
+    assert!(!list.is_empty());
+
     assert_eq!(list.pop_back(), Some(5));
+
+    assert!(list.is_empty());
 }
 
 #[test]
@@ -34,13 +39,15 @@ fn basics_push_pop_at_back() {
     let mut list: LinkedList<i32> = LinkedList::new();
     for i in 0..10 {
         list.push_back(i);
+        assert!(!list.is_empty());
     }
     assert_eq!(list.len(), 10);
-
     for i in (0..10).rev() {
+        assert!(!list.is_empty());
         assert_eq!(i, list.pop_back().unwrap());
     }
     assert_eq!(list.len(), 0);
+    assert!(list.is_empty());
 }
 
 // push / pop at front ———————————————————————————————————————
@@ -51,7 +58,11 @@ fn basics_single_element_front() {
     list.push_front(5);
 
     assert_eq!(list.len(), 1);
+    assert!(!list.is_empty());
+
     assert_eq!(list.pop_front(), Some(5));
+
+    assert!(list.is_empty());
 }
 
 #[test]
@@ -60,13 +71,15 @@ fn basics_push_pop_at_front() {
     let mut list: LinkedList<i32> = LinkedList::new();
     for i in 0..10 {
         list.push_front(i);
+        assert!(!list.is_empty());
     }
     assert_eq!(list.len(), 10);
-
     for i in (0..10).rev() {
+        assert!(!list.is_empty());
         assert_eq!(i, list.pop_front().unwrap());
     }
     assert_eq!(list.len(), 0);
+    assert!(list.is_empty());
 }
 
 // push / pop at mixed sides —————————————————————————————————
@@ -76,10 +89,13 @@ fn basics_push_front_pop_back() {
     let mut list: LinkedList<i32> = LinkedList::new();
     for i in 0..10 {
         list.push_front(i);
+        assert!(!list.is_empty());
     }
     for i in 0..10 {
+        assert!(!list.is_empty());
         assert_eq!(i, list.pop_back().unwrap());
     }
+    assert!(list.is_empty());
 }
 
 #[test]
@@ -88,10 +104,13 @@ fn basics_push_back_pop_front() {
     let mut list: LinkedList<i32> = LinkedList::new();
     for i in 0..10 {
         list.push_back(i);
+        assert!(!list.is_empty());
     }
     for i in 0..10 {
+        assert!(!list.is_empty());
         assert_eq!(i, list.pop_front().unwrap());
     }
+    assert!(list.is_empty());
 }
 
 // ———————————————————————————————————————————————————————————

exercises/simple-linked-list/example.rs

@@ -15,6 +15,10 @@ impl<T> SimpleLinkedList<T> {
         SimpleLinkedList { head: None, len: 0 }
     }
 
+    pub fn is_empty(&self) -> bool {
+        self.len == 0
+    }
+
     pub fn len(&self) -> usize {
         self.len
     }

exercises/simple-linked-list/src/lib.rs

@@ -11,6 +11,15 @@ impl<T> SimpleLinkedList<T> {
         unimplemented!()
     }
 
+    // You may be wondering why it's necessary to have is_empty()
+    // when it can easily be determined from len().
+    // It's good custom to have both because len() can be expensive for some types,
+    // whereas is_empty() is almost always cheap.
+    // (Also ask yourself whether len() is expensive for SimpleLinkedList)
+    pub fn is_empty(&self) -> bool {
+        unimplemented!()
+    }
+
     pub fn len(&self) -> usize {
         unimplemented!()
     }

exercises/simple-linked-list/tests/simple-linked-list.rs

@@ -28,6 +28,38 @@ fn test_pop_decrements_length() {
     assert_eq!(list.len(), 0, "list's length must be 0");
 }
 
+#[test]
+#[ignore]
+fn test_is_empty() {
+    let mut list: SimpleLinkedList<u32> = SimpleLinkedList::new();
+    assert!(list.is_empty(), "List wasn't empty on creation");
+    for inserts in 0..100 {
+        for i in 0..inserts {
+            list.push(i);
+            assert!(
+                !list.is_empty(),
+                "List was empty after having inserted {}/{} elements",
+                i,
+                inserts
+            );
+        }
+        for i in 0..inserts {
+            assert!(
+                !list.is_empty(),
+                "List was empty before removing {}/{} elements",
+                i,
+                inserts
+            );
+            list.pop();
+        }
+        assert!(
+            list.is_empty(),
+            "List wasn't empty after having removed {} elements",
+            inserts
+        );
+    }
+}
+
 #[test]
 #[ignore]
 fn test_pop_returns_head_element_and_removes_it() {