end_scripts = {"quit", "end", "q"}
help_scripts = {"help", "h"}

help_doc = """
A reverse polish or postfix calculator evaluates two numbers by the next opertion.
For example, the sum of 4 and 5 can be found by entering: 5 4 +

Available operations include

+ addition
- subtraction
* multiplication
/ division
% modulo
^ exponential

Try finding the answer to 20 3 + 9 * 2 /
"""

operations = {
    "+" : (lambda a, b: a+b),
    "-" : (lambda a, b: a-b),
    "*" : (lambda a, b: a*b),
    "/" : (lambda a, b: a/b),
    "%" : (lambda a, b: a%b),
    "^" : (lambda a, b: a**b)
}

acceptable_inputs = [str(i) for i in range(0,10)] +list(operations.keys()) +[" "]

def ok_input(instructions):
    for character in instructions:
        if character not in acceptable_inputs:
            return False
    return True

def interpret(raw_instructions):
    instructions_as_list = raw_instructions.strip().split()
    return instructions_as_list

def solve(instructions):
    stack = []

    for token in instructions:
        if token in operations:
            if len(stack) < 2:
                print("ERROR: Each operation requires at least 2 operands.")
                return None
            b = stack.pop()
            a = stack.pop()
            stack.append(operations[token](a,b))
        else:
            stack.append(float(token))

    return stack.pop()

def simplify_format(output_number):
    if output_number == None:
        return ""
    decmials = 0
    if int(output_number) == float(output_number):
        decimals = 0
    else:
        decimals = 4
    return "{0:.{1}f}".format(output_number, decimals)

if __name__ == "__main__":
    print("REVERSE POLISH CALCULATOR\nFor help, type help[ENTER]\nTo quit, type quit[ENTER]")
    n = 0
    while True:
        instructions = input("rpn >> ")
        if instructions in end_scripts:
            break
        elif instructions in help_scripts:
            print(help_doc)
        elif not ok_input(instructions):
            print("ERROR: Please use only numbers and operators for calculations.")
        elif instructions == "":
            pass
        else:
            print(simplify_format(solve(interpret(instructions))))
        n += 1
    print("Bye! You preformed {} calculations.".format(n))