# This program prompts for and evaluates a prefix expression.

# Returns True if the given string can be converted
# into a float successfully, or False if not.
def is_float(s):
    try:
        n = float(s)
        return True
    except ValueError:
        return False

# pre : operator is one of +, -, *, /, or %
# post: returns the result of applying the given operator
#  to the given operands
def apply(operator, operand1, operand2):
    if operator == "+":
        return operand1 + operand2
    elif operator == "-":
        return operand1 - operand2
    elif operator == "*":
        return operand1 * operand2
    elif operator == "/":
        return operand1 / operand2
    elif operator == "%":
        return operand1 % operand2
    else:
        raise ValueError("bad operator: " + operator)

# pre : tokens represent a legal prefix expression
# post: expression is evaluated and the result is returned
def evaluate(tokens):
    # extract first token
    first = tokens.pop(0)
    if is_float(first):
        # base case: a numeric token
        return float(first)
    else:
        # recursive case: an operator
        operand1 = evaluate(tokens)
        operand2 = evaluate(tokens)
        return apply(first, operand1, operand2)
    
def main():
    print("This program evaluates prefix expressions that")
    print("include the operators +, -, *, /, and %.")
    print()
    
    expr = input("Expression? ")
    while expr != "":
        tokens = expr.split(" ")
        value = evaluate(tokens)
        print("value =", round(value, 2))
        expr = input("Expression? ")
    
    print("Exiting.")
    
main()