Create Probrem4.lean

Src/Probrem4.lean

@@ -0,0 +1,21 @@
+/-
+# Problem 4
+Find the number of elements in a list.
+-/
+
+variable {α : Type}
+
+def myLength (l : List α) : Nat :=
+  --sorry
+  List.length l
+  --sorry
+
+-- The following is a test case, you don't need to edit it.
+
+example : myLength [123, 456, 789] = 3 := by rfl
+
+example : myLength ([] : List α) = 0 := by rfl
+
+example : myLength [1,1,1,1,1,1,1,1,1,1,1,1] = 12 := by rfl
+
+example : myLength ['w','x', 'y', 'z'] = 4 := by rfl