Caesar.c

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

// plaintext (p) vs ciphertex (c), and a key (k)
// “rotate” each (alphabetical) character by k positions
// Your program must accept a single command-line argument, a non-negative integer
// Your program must preserve case
// lowercase: 65(a) - 90(z)
// uppercase: 97(a) - 122(z)
// isalpha, islower, isupper, isdigit

int main(int argc, string argv[])
{

  if (argc == 2)
  {
    for (int i = 0; i < strlen(argv[1]); i++)
    {
      if (isdigit(argv[1][i]) == 0)
      {
        printf("Usage: ./caesar k\n");
        return 1;
      }
    }
    int k = atoi(argv[1]);

    string p = get_string("plaintext: ");
    printf("ciphertext: ");

    for (int i = 0, len = strlen(p); i < len; i++)
    {
      if (islower(p[i]))
      {
        printf("%c", (((p[i] - 'a') + k) % 26) + 'a');
      }

      else if (isupper(p[i]))
      {
        printf("%c", (((p[i] - 'A') + k) % 26) + 'A');
      }

      else
      {
        printf("%c", p[i]);
      }
    }

    printf("\n");
    return 0;
  }

  else
  {

    printf("Usage: ./caesar k\n");
    return 1;
  }
}