caesar.c

#include <cs50.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>
#include <stdio.h>

char shiftCipher(char c, int k) {
    if(isupper(c)) {
        return ((c - 65 + k) % 26) + 65;
    } else if (islower(c)){
        return ((c - 97 + k) % 26) + 97;
    } else {
        return c;
    }
}

int main(int argc, string argv[]) {
    
    string text;
    
    if(argc != 2) {
        printf("Error. Please provide a cipher key.\n");
        return 1;
    }
    
    do {
        printf("plaintext: ");
        text = get_string();
    } while(strlen(text) <= 0);

    char cipher[strlen(text)];
    int key = atoi(argv[1]);
    
    for(int i = 0; i < strlen(text); i++) {
      cipher[i] = shiftCipher(text[i], key);
    }
    
    printf("ciphertext: %s", cipher);
    printf("\n");
    return 0;
}