Create 07 Credit Card Validation.md

Author: brucestull

1 Python/labs/07 Credit Card Validation.md

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+# Lab 7: Credit Card Validation
+
+
+Let's write a function `credit_card_validator` which returns whether a string containing a credit card number is valid as a boolean. The steps are as follows:
+
+1. Convert the input string into a list of ints
+2. Slice off the last digit.  That is the **check digit**.
+3. Reverse the digits.
+4. Double every other element in the reversed list (starting with the first number in the list).
+5. Subtract nine from numbers over nine.
+6. Sum all values.
+7. Take the second digit of that sum.
+8. If that matches the check digit, the whole card number is valid.
+
+Here is a valid credit card number to test with: 4556737586899855
+
+For example, the worked out steps would be:
+
+1. `4  5  5  6  7  3  7  5  8  6  8  9  9  8  5  5`
+2. `4  5  5  6  7  3  7  5  8  6  8  9  9  8  5`
+3. `5  8  9  9  8  6  8  5  7  3  7  6  5  5  4`
+4. `10 8  18 9  16 6  16 5  14 3  14 6  10 5  8`
+5. `1  8  9  9  7  6  7  5  5  3  5  6  1  5  8`
+6. 85
+7. 5
+8. `True` Valid!