text: #include <stdio.h>
  #include <cs50.h>
  #include <stdlib.h>
  #include <ctype.h>
  #include <string.h>
  
  int main(int argc, string argv[])
  {
      // Incorrect use
      if (argc != 2)
      {
          printf("Usage: ./caesar key\n");
          return 1;
      }
  
      // Get key from argv
      for (int i = 0, l = strlen(argv[1]); i < l; i++)
      {
          if (isdigit(argv[1][i]) == false)
          {
              printf("Usage: ./caesar key\n");
              return 1;
          }
      }
  
      int key = atoi(argv[1]);
  
      // Ask user for the plain text
      string plaintext = get_string("plaintext:  ");
  
      printf("ciphertext: ");
  
      // Transforms plain text in cipher text and show results
      for (int i = 0, l = strlen(plaintext); i < l; i++)
      {
          if (isalpha(plaintext[i]))
          {
              if (isupper(plaintext[i]))
              {
                  printf("%c", ((plaintext[i] - 'A' + key) % 26) + 'A');
              }
              else if (islower(plaintext[i]))
              {
                  printf("%c", ((plaintext[i] - 'a' + key) % 26) + 'a');
              }
          }
          else
          {
              printf("%c", plaintext[i]);
          }
      }
      printf("\n");
      return 0;
  }