You can do it even faster with the if statements: #include <stdio.h> #include <stdlib.h> int main(int argc, char *argv[]) { if (argc <

You can do it even faster with the if statements: #include <stdio.h> #include <stdlib.h> int main(int argc, char *argv[]) { if (argc < 2) { fprintf(stderr, "Usage: %s <string>\n", argv[0]); return 1; } char *s = argv[1]; int i; /* find the end of the string */ for (i = 0; s[i] != '\0'; ++i) ; /* make sure the string wasn't empty */ if (i == 0) { fprintf(stderr, "Error: empty string\n"); return 1; } /* last character is at s[i - 1] */ char d = s[i - 1]; if (d == '0') printf("even\n"); if (d == '1') printf("odd\n"); if (d == '2') printf("even\n"); if (d == '3') printf("odd\n"); if (d == '4') printf("even\n"); if (d == '5') printf("odd\n"); if (d == '6') printf("even\n"); if (d == '7') printf("odd\n"); if (d == '8') printf("even\n"); if (d == '9') printf("odd\n"); return 0; } gcc -std=c11 -Wall -Wextra -O2 -o check_digit check_digit.c ./check_digit 999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999