Credit Card validation LUHN algorithm using C
Checks whether a string of digits is a valid credit card number according to the Luhn algorithm.
Starting with the second to last digit and moving left, double the value
of all the alternating digits. For any digits that thus become 10 or more,
add their digits together. For example, 1111 becomes 2121, while 8763
becomes 7733 (from (1+6)7(1+2)3).
Add all these digits together. For example, 1111 becomes 2121, then
2+1+2+1 is 6; while 8763 becomes 7733, then 7+7+3+3 is 20.
If the total ends in 0 (put another way, if the total modulus 10 is 0),
then the number is valid according to the Luhn formula, else it is not
valid. So, 1111 is not valid (as shown above, it comes out to 6), while
8763 is valid (as shown above, it comes out to 20).
#include <stdio.h>
#include <string.h>
static int isValidNumber(const char *);
/*
* Test harness for an implementation of the Luhn algorithm that checks the
* validity of a credit card number.
*/
int main(int argc, char *argv[])
{
int i;
if (argc < 2) {
fprintf(stderr, "Usage: luhn <number>, …\n");
return 1;
}
for (i = 1; i < argc; ++i)
printf("Number ‘%s’ is%s a valid credit card number\n",
argv[i], isValidNumber(argv[i]) ? "" : " not");
return 0;
}
static int isValidNumber(const char *number)
{
int n, i, alternate, sum;
if (!number)
return 0;
n = strlen(number);
if (n < 13 ” n > 19)
return 0;
for (alternate = 0, sum = 0, i = n – 1; i > -1; –i) {
if (!isdigit(number[i]))
return 0;
n = number[i] – ‘0′;
if (alternate) {
n *= 2;
if (n > 9)
n = (n % 10) + 1;
}
alternate = !alternate;
sum += n;
}
return (sum % 10 == 0);
}
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