sentimental - credit.py

from cs50 import get_string


# Function to check card number against Luhn's Algorithm
def check(number):
    # Create checksum variables
    length = len(number)
    sum1 = 0 # Var to STORE each digit in luhnstep2 after multiplying every other number, beg with second to last, by 2
    sum2 = 0 # Var to SUM numbers stored in luhnstep2
    sum3 = 0 # Var to sum numbers stored in luhnstep3 (i.e., nums not multiplied by 2 in sum2/luhnstep2)
    luhnstep2 = [] # List in which sum2 helps store each digit of the product from multiplying every other number by 2
    luhnstep3 = [] # List in which sum3 stores numbers not multiplied by 2

    # Checksum operations for even-digit cards
    if length % 2 == 0:
        for i in range(length):
            # Check if i in iteration is an even num; if so, multiply by two and store product in sum1
            if i % 2 == 0:
                sum1 = int(number[i]) * 2
                # iterate over sum1 and store each indiv digit in list luhnstep2
                for i in range(len(str(sum1))):
                    luhnstep2.append(str(sum1)[i])
            # If not an even digit in iteration, store digit in list which holds card numbers not used in "multiply by 2" portion of Luhn's algorithm
            else:
                luhnstep3.append(number[i])

    # Checksum operations for odd-digit cards
    else:
        for i in range(length):
            # Check if i in iteration is an odd num; if so, multiply by two and store product in sum1
            if i % 2 != 0:
                sum1 = int(number[i]) * 2
                # iterate over sum1 and store each indiv digit in list luhnstep2
                for i in range(len(str(sum1))):
                    luhnstep2.append(str(sum1)[i])
            # If not an odd digit in iteration, store digit in list which holds card numbers not used in "multiply by 2" portion of Luhn's algorithm
            else:
                luhnstep3.append(number[i])

    # Checksum validation
    # Add numbers multiplied by 2
    for i in range(len(luhnstep2)):
        sum2 += int(luhnstep2[i])

    # Add numbers not multiplied by 2
    for i in range(len(luhnstep3)):
        sum3 += int(luhnstep3[i])

    # Add both sets of sums together
    checksum = str(sum2 + sum3)

    # If sum result does not end in "0" then function returns "false"
    if (checksum[len(checksum) - 1]) != "0":
        return False


# List to hold valid prefixes for the three cards we'll test for
validprefix = [
    ('34', 'AMEX'),
    ('37', 'AMEX'),
    ('51', 'MASTERCARD'),
    ('52', 'MASTERCARD'),
    ('53', 'MASTERCARD'),
    ('54', 'MASTERCARD'),
    ('55', 'MASTERCARD'),
    ('4', 'VISA')
]

# Get user to input a card number and reprompt if not a full digit string
while True:
    cardnumber = get_string("Number: ")
    if cardnumber.isdigit() == True:
        break

# Create flag variable
i = 0

# Create variable to evaluate just the two prefix numbers of user input
prefix = cardnumber[0] + cardnumber[1]

# Lookup prefix numbers in validprefix List and return name of card if found
for digits, name in validprefix:
    # If prefix in card number matches one in the List, add one to flag variable then run checksum fn (above)
    # If checksum doesn't comply, print invalid; if name not in list move to next test
    if str(prefix) in digits:
        i += 1
        if check(cardnumber) == False:
            print("INVALID")
            break
        print(name)
        break

    # If card starts with a 4 then add 1 to flag var then run checksum since this is our
    # Only non-two digit validation in validprefix List; If checksum doesn't comply, print invalid
    elif digits[0] == '4':
        i += 1
        if check(cardnumber) == False:
            print("INVALID")
            break
        print(name)
        break
# If flag variable never tripped, card must be invalid; print invalid
if i == 0:
    print("INVALID")