#!/usr/bin/python3
import sys

def parse(calculation):
    global operator,operand1,operand2
    tokenIndex = 0
    tokens = ["","",""]
    
    i=0
    numberBeginning = True
    while i < len(calculation):
        print("Token index: ",tokenIndex,"character[",i,": ]",calculation[i])
        # spaces are simply skipped
        if calculation[i] == " ":
            print("space")
            i+=1
        # A number consists of characters between 0 and 9 or the decimal point and possibly a preceeding minus sign
        elif numberBeginning and calculation[i] == '-':
            tokens[tokenIndex] = '-'
            numberBeginning = False
            i+=1
        elif calculation[i] >= '0' and calculation[i] <= '9':
            print("number: ",calculation[i])
            tokens[tokenIndex]=tokens[tokenIndex]+calculation[i]
            numberBeginning = False
            i += 1
        
        # a decimal point is also allowed
        elif calculation[i] == '.':
            print("dot")
            # check that the decimal point was not already given, no 2 decimal points allowed
            if tokens[tokenIndex].find('.') == -1:
                tokens[tokenIndex]= tokens[tokenIndex]+calculation[i]
                i += 1
                numberBeginning = False
            else:
                return False
                   
        # It can still be an operator. In this case the first operand must be non null 
        elif tokenIndex == 0 and not tokens[0] == "": 
            if calculation[i] == '+' or calculation[i] == '-' or calculation[i] == '*' or calculation[i] == '/' :
                print("Found operator: ",calculation[i])
                # save the operator and continue parsing the second operand
                tokens[1]=tokens[1]+calculation[i]
                tokenIndex += 2
                numberBeginning = True
                i += 1
            else:
                return False
        else:
            return False
#    print("First operand: ",tokens[0])
#    print("Second operand: ",tokens[2])
#    print("Operator: ",tokens[1])
    operator = tokens[1]
    #if the operand contains a decimal point it must also contain at least 1 digit
    if len(tokens[0]) == 0 or len(tokens[2]) == 0:
        return False
    if tokens[0].find('.') != -1 and len(tokens[0]) == 1:
        return False
    if tokens[2].find('.') != -1 and len(tokens[2]) == 1:
        return False
    operand1 = float(tokens[0])
    operand2 = float(tokens[2])
                        
    return True


def plus():
    print(operand1,' + ',operand2,' = ',operand1+operand2)
    
def minus():
    print(operand1,' - ',operand2,' = ',operand1-operand2)     
    
def multiply():
    print(operand1,' * ',operand2,' = ',operand1*operand2)     
    
def divide():
    try:
        print(operand1,' / ',operand2,' = ',operand1/operand2)
    except ZeroDivisionError:
        print('Cannot divide by zero')

operations= {'+' : plus,
             '-' : minus,
             '*' : multiply,
             '/' : divide,}

global operator,operand1,operand2
while True:
    print ('Enter the calculation to be performed in the form: "operand1 operator operand2"')
    print ('Example: "5.3 + 4.2" or "1.4 * 7.9"');
    print ('Operation: ',end='')
    # if the input is empty just try again
    calculation = input()
    if calculation=="":
        continue

#    print(calculation)

    if parse(calculation):
#        print("String ok");
        operations[operator]()
        sys.exit()
    else:
        print("Bad operation input: ",calculation)