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Is there something like a checksum attached to Credit Card Numbers. Or better: Is there a way to determine for a given number N if -this _might_ be a valid number -this can't be a valid number

This was posted by Wayne D. Hoxsie Jr. <hoxsiew@crl.com>: #include <stdio.h> v(char *s) { int i=0,j=0,k; k=!(!s[16]); for(;*s;*s++) i+=(++j%2-k)?(*s-'0')*2>9?((*s-'0')*2)-9:(*s-'0')*2:*s-'0'; return (i%10); } main() { char s[160]; printf("Enter credit card number\n"); scanf("%s",s); printf("Credit card number is %svalid\n",v(s)?"in":""); } --- Dr. Dimitri Vulis Brighton Beach Boardwalk BBS, Forest Hills, N.Y.: +1-718-261-2013, 14.4Kbps