Implementation examples
Pseudo-Code
function checkLuhn(string purportedCC) {
int sum := integer(purportedCC[length(purportedCC)-1])
int nDigits := length(purportedCC)
int parity := nDigits modulus 2
for i from 0 to nDigits - 2 {
int digit := integer(purportedCC[i])
if i modulus 2 = parity
digit := digit × 2
if digit > 9
digit := digit - 9
sum := sum + digit
}
return (sum modulus 10) = 0
}
C
#include <stdlib.h> // atoi
#include <string.h> // strlen
#include <stdbool.h> // bool
bool checkLuhn(const char *pPurported)
{
int nSum = 0;
int nDigits = strlen(pPurported);
int nParity = (nDigits-1) % 2;
char cDigit[2] = "\0";
for (int i = nDigits; i > 0 ; i--)
{
cDigit[0] = pPurported[i-1];
int nDigit = atoi(cDigit);
if (nParity == i % 2)
nDigit = nDigit * 2;
nSum += nDigit/10;
nSum += nDigit%10;
}
return 0 == nSum % 10;
}
Python
def luhn(purported):
LUHN_ODD_LOOKUP = (0, 2, 4, 6, 8, 1, 3, 5, 7, 9) # sum_of_digits (index * 2)
if not isinstance(purported, str):
purported = str(purported)
try:
evens = sum(int(p) for p in purported[-1::-2])
odds = sum(LUHN_ODD_LOOKUP[int(p)] for p in purported[-2::-2])
return ((evens + odds) % 10 == 0)
except ValueError: # Raised if an int conversion fails
return False
See also
References