Luhn check a card number
Before a card ever reaches the payment gateway, the checkout runs the Luhn checksum so an obvious typo is caught in the browser.
- Working from the right, double every second digit; if doubling gives more than 9, subtract 9.
- The number is valid when the resulting sum divides by 10.
- Spaces and dashes are formatting and are ignored.
- Any other non-digit makes it invalid, as does anything shorter than two digits.
LuhnValid(digits: string) → bool
C# needs a compiler and Drill does not host one yet, so this page is the reference rather than an exercise: the problem, worked examples, and the solution in full. To type it out, the same problem runs in Python.
Where you start
public bool LuhnValid(string digits) {
}Worked examples
| Call | Result |
|---|---|
LuhnValid("79927398713") | true |
LuhnValid("79927398710") | false |
LuhnValid("4111 1111 1111 1111") | true |
LuhnValid("4111-1111-1111-1112") | false |
Hint
Strip the formatting into a clean string first. Then walk it backwards with an index so "every second" is easy to say.
Reference solution in C#
public bool LuhnValid(string digits) {
var clean = new StringBuilder();
foreach (char c in digits) {
if (c == ' ' || c == '-') continue;
if (c < '0' || c > '9') return false;
clean.Append(c);
}
if (clean.Length < 2) return false;
int sum = 0;
for (int i = 0; i < clean.Length; i++) {
int d = clean[clean.Length - 1 - i] - '0';
if (i % 2 == 1) { d *= 2; if (d > 9) d -= 9; }
sum += d;
}
return sum % 10 == 0;
}