caesar.c

/**
 * caesar.c
 *
 * Rafael Beraldo
 * rberaldo@cabaladada.org
 *
 * Encrypts text with the Caesar cipher 
 */
 
#include <stdio.h>
#include <cs50.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>

/* In order for the encryption function to work, we must first get the
alphabetical index of a character by subtracting 65 if it is uppercase and 97
if it is lowercase. That's because in ASCII, they are not 0-indexed. */
#define UPPERCASE_ASCII_SHIFT 65
#define LOWERCASE_ASCII_SHIFT 97
#define SIZE_ALPHABET 26

// Functions
char encrypt(char letter, int key, int shift);

int main(int argc, string argv[])
{
   // Exits if program has less or more than one argument
   if (argc != 2)
   {
      printf("Usage: %s <key>\n", argv[0]);
      return 1;
   }
   
   int key = atoi(argv[1]);
   string message = GetString();
   
   // Cycle though all characters in message
   for (int i = 0, message_length = strlen(message); i < message_length; i++)
   {
      if (!isalpha(message[i]))
      {
         printf("%c", message[i]);
      }
      else if (isupper(message[i]))
      {
         printf("%c", encrypt(message[i], key, UPPERCASE_ASCII_SHIFT));
      }
      else
      {
         printf("%c", encrypt(message[i], key, LOWERCASE_ASCII_SHIFT));
      }
   }
   printf("\n");
   
   return 0;
}

/////////////////////////////////////////////

/**
 * Encrypts a character using a key, shifting to correct for the ASCII table
 */
char encrypt(char letter, int key, int shift)
{
      letter -= shift;
      return (letter + key) % SIZE_ALPHABET + shift;
}

@mike168m isalpha() is part of the ctype.h library and returns true if a character is an alphabetic letter. I found it here: https://reference.cs50.net/ctype.h/isalpha