crypt.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef enum { false, true } bool;

void throw_error(void) {
    printf("GM is dik en lelijk\n");
    exit(1);
}

int main(int argc, char* argv[]) {
    if (argc != 3) {
        throw_error();
    }
    int k = atoi(argv[1]);
    if (k < 0) {
        throw_error();
    }
    int i, n;
    for (i = 0, n = strlen(argv[2]); i < n; i++) {
        int c = argv[2][i];
        bool lower = false;
        // lowercase
        if (c >= 97 && c <= 122) {
            c -= 97;
            lower = true;
        }

        // uppercase
        if (c >= 65 && c <= 90) {
            c -= 65;
        }

        c += k;
        while (c > 25) {
            c -= 26;
        }
        printf("%c", c + (lower ? 97 : 65));
    }
    printf("\n");
    return 0;
}