73 lines
1.7 KiB
Markdown
73 lines
1.7 KiB
Markdown
# Luhn
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Given a number determine whether or not it is valid per the Luhn formula.
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The [Luhn algorithm](https://en.wikipedia.org/wiki/Luhn_algorithm) is
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a simple checksum formula used to validate a variety of identification
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numbers, such as credit card numbers and Canadian Social Insurance
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Numbers.
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The task is to check if a given string is valid.
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Validating a Number
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------
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Strings of length 1 or less are not valid. Spaces are allowed in the input,
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but they should be stripped before checking. All other non-digit characters
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are disallowed.
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## Example 1: valid credit card number
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```
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4539 1488 0343 6467
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```
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The first step of the Luhn algorithm is to double every second digit,
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starting from the right. We will be doubling
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```
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4_3_ 1_8_ 0_4_ 6_6_
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```
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If doubling the number results in a number greater than 9 then subtract 9
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from the product. The results of our doubling:
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```
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8569 2478 0383 3437
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```
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Then sum all of the digits:
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```
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8+5+6+9+2+4+7+8+0+3+8+3+3+4+3+7 = 80
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```
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If the sum is evenly divisible by 10, then the number is valid. This number is valid!
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## Example 2: invalid credit card number
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```
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8273 1232 7352 0569
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```
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Double the second digits, starting from the right
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```
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7253 2262 5312 0539
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```
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Sum the digits
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```
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7+2+5+3+2+2+6+2+5+3+1+2+0+5+3+9 = 57
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```
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57 is not evenly divisible by 10, so this number is not valid.
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## Source
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The Luhn Algorithm on Wikipedia [http://en.wikipedia.org/wiki/Luhn_algorithm](http://en.wikipedia.org/wiki/Luhn_algorithm)
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## Submitting Incomplete Solutions
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It's possible to submit an incomplete solution so you can see how others have completed the exercise.
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