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>> Zamyla Chan: Let's check your credit.

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In this problem, we'll prompt the user
for an input of a credit card number.

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Then we'll run that credit card
number to see if it's valid.

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If so, we'll print the company
that that credit card belongs to.

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Otherwise, we'll tell the user
that that card is invalid.

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>> Let's jump right in with
prompting for user input.

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The credit card number is
of data type long_long,

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so the CS50 library function
get_long_long will be quite useful.

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But this function ensures that
the user inputs any integer.

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So any positive integers, negative
integers, or zero is all fine.

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So up to you to further
validate whether or not

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the user has given us a valid
credit card type number.

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>> Now that we have a credit card
number from the user, then

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we need to calculate the checksum.

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So credit card checksums
are calculated as follows.

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Starting from the second to last digit,
we multiply every other digit by 2.

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Then we add those
products' digits together.

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From that we add the
sum of the digits that

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weren't multiplied by 2 to that
previous sum that we calculated.

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Finally, if that number ends
in 0, then the number is valid.

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>> Let's go through an example
and take this step by step.

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So our first step is to start
from the second to last digit

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and multiply every other digit by two.

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Now, if I stored the credit card
number in a variable called CC number,

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then modding that by 10 would
give me the very last digit.

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So how might you access
the second to last digit?

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>> OK, so once we've accessed
the second to last digit,

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then we'll iterate through
multiplying every other digit by 2.

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Once we have that, then
we'll take those products

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and add those products' digits together.

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So here I have single
digits, so that's fine.

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But then once I get to my last
number-- 7 multiplied by 2--

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I add the 1 and the 4 to give me 27.

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From there, we add that product--
27-- to the sum of the digits that

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weren't multiplied by 2.

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>> So here I've highlighted all
of those numbers in orange.

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Once we add those digits, then
we get our final checksum number.

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So we validate it by making
sure that the last digit is 0.

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If the checksum does not end in
0, then the credit card number

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is definitely invalid.

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Otherwise, let's go on to
check the company identifiers

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and the credit card number length.

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>> We've provided a list
of three companies,

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along with that, the number of integers
that their credit card numbers have.

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Then the first two digits that
those cards might start with.

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So up to you to keep track of the
first two digits of the credit card

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and the number of integers in that card.

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And with that, you've
finished the problem.

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My name is Zamyla, and this was Credit.

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