Fix subtract operator

lexer.py

@@ -74,7 +74,7 @@ class Lexer():
 
 			elif self.current_char == '-':
 				self.advance()
-				if self.current_char.isdigit():
+				if self.current_char is not None and self.current_char.isdigit():
 					return Token(NUMBER, -self.number())
 
 				return Token(MINUS, operator.sub)

rpncalc.py

@@ -13,8 +13,11 @@ from constants import DEC
 
 def print_stack(interpreter, stack):
 	for item in stack:
-		if interpreter.mode == DEC and item.is_integer():
-			print(int(item))
+		if interpreter.mode == DEC:
+			if item.is_integer():
+				print(int(item))
+			else:
+				print(round(float(item), 4))
 		else:
 			print(interpreter.mode_func(item))