funcprog: Finished week 2, assignment 5.4.

funcprog/week2/ass5_4.ml

-let even_len x = 
-  if (x mod 2) == 0 then
-    x
-  else
-    x - 1
-  ;;
+#load "str.cma";;
 
-let isPalindrome palin = 
-  let result = ref true in (
-  for i = 0 to ((even_len (String.length palin)) / 2) - 1 do
-    if (palin.[i] != palin.[(even_len (String.length palin)) - i - 1]) then
-      begin
-        result := false;
-      end
-    else
-      ()
-  done;
-  !result)
-  ;;
+(*
+ * The function isPalindrome checks if a given character string is a palindrome,
+ * i.e. it is identical whether being read from left to right or from right to
+ * left. Note that this function removes punctuation and word dividers before
+ * checking the given character string.
+ *)
+let isPalindrome raw =
+    (*
+     * Check if the left and right character of the string are the same. An
+     * empty string and a string with length 1 is by definition a palindrome.
+     * Check uses a character string `s' and its length `l' to recursively
+     * determine if `s' is a palindrome.
+     *)
+    let rec check s l =
+        l < 2 || (s.[0] == s.[l-1] && (check (String.sub s 1 (l-2)) (l-2)))
+    in
+    (* Remove punctuation / word dividers -> only alphanumeric chars remain. *)
+    let filter = Str.regexp "[^a-zA-Z0-9]+" in
+    let filtered = String.lowercase (Str.global_replace filter "" raw) in
+        (check filtered (String.length filtered))
+;;
 
-Printf.printf "%b\n" (isPalindrome "asddsa") 
+let test_isPalindrome str =
+    Printf.printf "isPalindrome(\"%s\") -> %b\n" str (isPalindrome str)
+;;
 
+test_isPalindrome "";;
+test_isPalindrome "a";;
+test_isPalindrome "baas saab";;
+test_isPalindrome "never odd or even";;
+test_isPalindrome "Was it a rat i saw?";;
+
+test_isPalindrome "expected failure!";;