def windchill(T,V):
    """
   A function that computes the windchill factor using temperature and windspeed.

   >>> windchill(5,10)
   -9.737344294197483
   >>> windchill(20,20)
   4.242781599820461
   >>> windchill(-10,50)
   -45.32064601872968
   """
    wc_factor = (35.74 + 0.6215 * T - 35.75 * V**0.16 + 0.4275 * T * V**0.16) #formula for windchill
    return(wc_factor)

def main():
        try:
                while True:
                        # asks the user for a windspeed start value, if the value is negative it
                        # will re-ask until value is positive
                        while True:
                                wind_start = int(input("Windspeed start value?: "))
                                if wind_start < 0:
                                        print("Windspeed can't be negative!")
                                else:
                                        break
                        # asks the user for a windspeed endvalue, if the value is negative it
                        # will re-ask until value is positive
                        while True:
                                wind_stop = int(input("Windspeed stop value?: "))
                                if wind_stop < 0:
                                        print("Windspeed can't be negative!")
                                else:
                                        break
                        #asks for windspeed increment value
                        wind_increment = int(input("Windspeed increment value?: "))
                        print ("")

                        # asks for all temperature values
                        t_start = int(input("Temperature start value?: "))
                        t_stop = int(input("Temperature stop value?: "))
                        t_increment = int(input("Temperature increment value?: "))
                        print("")

                        print("")

                        # prints header of table-- first line; prints temperature heading
                        # and all windspeed values
                        vel = " T(degF)"
                        for V in range(wind_start, wind_stop, wind_increment):
                                velocity = str(V) + " mph"
                                vel = vel + " {0:>7s}".format(velocity)
                        print(vel)

                        # prints a divider
                        spacer = ""
                        for i in range(wind_start, (wind_stop + 1), wind_increment):
                                spacer = spacer + " {0:7s}".format("=======")
                        print(spacer)

                        # prints temperature and each row of windchill index for the temperature
                        for T in range(t_start, t_stop, t_increment):
                                temperature = " {0:7d}".format(T)
                                for V in range(wind_start, wind_stop, wind_increment):
                                        temperature = str(temperature) + " {0:7.2f}".format(round(windchill(T,V), 2))
                                print(temperature)

                        # lets user quit program if they press ENTER
                        leave = str(input("Press ENTER to quit: "))
                        if leave == "":
                                break
                        else:
                                continue

        except:
                # checks if user input for windspeed/temperature are number values
                print("That's not a valid number, try again.")

if __name__ == "__main__":
    import doctest
    doctest.testmod()
    main()