September 2017 Update

Task/Parsing-Shunting-yard-algorithm/OCaml/parsing-shunting-yard-algorithm.ocaml

@@ -0,0 +1,59 @@
+type associativity = Left | Right;;
+
+
+let prec op =
+  match op with
+  | "^" -> 4
+  | "*" -> 3
+  | "/" -> 3
+  | "+" -> 2
+  | "-" -> 2
+  | _ -> -1;;
+
+
+let assoc op =
+  match op with
+  | "^" -> Right
+  | _ -> Left;;
+
+
+let split_while p =
+  let rec go ls xs =
+    match xs with
+    | x::xs' when p x -> go (x::ls) xs'
+    | _ -> List.rev ls, xs
+  in go [];;
+
+
+let rec intercalate sep xs =
+  match xs with
+  | [] -> ""
+  | [x] -> x
+  | x::xs' -> x ^ sep ^ intercalate sep xs';;
+
+
+let shunting_yard =
+  let rec pusher stack queue tkns =
+    match tkns with
+    | [] -> List.rev queue @ stack
+    | "("::tkns' -> pusher ("("::stack) queue tkns'
+    | ")"::tkns' ->
+        let mv, "("::stack' = split_while ((<>) "(") stack in
+        pusher stack' (mv @ queue) tkns'
+    | t::tkns' when prec t < 0 -> pusher stack (t::queue) tkns'
+    | op::tkns' ->
+        let mv_to_queue op2 =
+          (match assoc op with
+            | Left -> prec op <= prec op2
+            | Right -> prec op < prec op2)
+        in
+        let mv, stack' = split_while mv_to_queue stack in
+        pusher (op::stack') (mv @ queue) tkns'
+  in pusher [] [];;
+
+
+let () =
+  let inp = read_line () in
+  let tkns = String.split_on_char ' ' inp in
+  let postfix = shunting_yard tkns in
+  print_endline (intercalate " " postfix);;