vault backup: 2023-10-06 14:47:45

Algo/Courses/Chapter 6 - Lists (Exercises).md

@@ -44,23 +44,17 @@ val search 'a -> 'a list -> bool = <fun>
 
 ## 1.4 - $n^{th}$
 ```Ocaml
-# let rec length l = 
-	if l = [] then
-		0
+# let nth l n =
+	if n <= 0 then
+		invalid_arg "n <= 0"
 	else
-		let e::t = l in
-			1 + length t;;
-# let rec nth x n =
-	if (x = [] || n =< 0) then
-		Invalid_arg "The number of element in the list is 0"
-	else if length x < n then
-		Failwith "The list is too short"
-	else
-		let e::t = x in
-		if n = 0
-			return e
-		else
-			nth t (n-1) ;;
+		let rec nthrec = function 
+			| ([],_) -> failwith "list too short"
+			| (h::_,1)-> h
+			| (_::t, n) -> nthrec(t, n-1)
+		in
+			ntrec(l, n);;
+val nth 'a list -> 'a -> int = <fun>
 
 ```