problem_set6c.py

from cs50 import get_string


# obtain card_number then do checks one-by-one
def main():
    card_number = get_string("Number: ")

    if checksum(card_number) == False:
        print("INVALID")
    elif checkamex(card_number) == True:
        print("AMEX")
    elif checkmastercard(card_number) == True:
        print("MASTERCARD")
    elif checkvisa(card_number) == True:
        print("VISA")
    else:
        print("INVALID")


# takes card number. returns true if it passes checksum else fale
def checksum(card_number):

    # initialise variables
    sum = 0
    length = len(card_number)

    # sum_of_double(i) = sum of digits in 2i. defined for i in range(10)
    sum_of_double = [0, 2, 4, 6, 8, 1, 3, 5, 7, 9]

    # loop through card_number and add the digits contribution to checksum
    for i in range(length):
        ith_digit_from_right = int(card_number[length-1-i])

        # if i is even, then we are in odd position from right, so just add ith digit
        if i % 2 == 0:
            sum += ith_digit_from_right

        # if i is odd, we are at even position from right, so add double of ith digit
        else:
            sum += sum_of_double[ith_digit_from_right]

    # if final digit in sum is 0, then passes checksum test
    if sum % 10 == 0:
        return True
    else:
        return False


# if card_number satisfies amex conditions, return True otherwise false
def checkamex(card_number):
    if len(card_number) == 15 and int(card_number[:2]) in [34, 37]:
        return True
    else:
        return False


# if card_number satisfies mastercard conditions, return True otherwise false
def checkmastercard(card_number):
    if len(card_number) == 16 and int(card_number[:2]) in [51, 51, 53, 54, 55]:
        return True
    else:
        return False


# if card_number satisfies visa conditions, return True otherwise false
def checkvisa(card_number):
    if len(card_number) in [13, 16] and int(card_number[0]) == 4:
        return True
    else:
        return False


main()