C program "caesar's cipher" for course work

caesar.c

// This program encrypts a message using Caesar's cipher

#include <stdio.h>

#include <cs50.h>

#include <string.h>

#include <ctype.h>

#include <stdlib.h>

// first, get key from command line argument

int main(int argc, string argv[])

{

    // only accept a single command-line argument (k)

    if (argc == 2)

    {

        // turn key into an integer (l)

        int l = atoi(argv[1]);

        // prompt user for plaintext

        string p = get_string("plaintext: ");

        /*--- for each character in the plaintext string,

        if alphabetic, preserve case and shift plaintext

        character by key (l) ---*/

        char d;

        char c[strlen(p) + 1];

        int ai;

        int cai;

        for (int i = 0; i < strlen(p); i++)

        {

            // if the character is not alphabetic, output the character unchanged

            // add the character to array c[i]

            if (isalpha(p[i]) == false)

            {

                d = p[i];

                c[i] = d;

            }

            // if the character is uppercase, preserve case and add character to array c[i]

            else if (isupper(p[i]))

            {

                // convert to alphabetical index

                ai = p[i] - 65;

                do

                {

                    if (ai + l < 0)

                    {

                        ai += 26;

                    }

                    if (ai + l > 25)

                    {

                        ai -= 26;

                    }

                }

                while ((ai + l) < 0 || (ai + l) > 25);

                cai = (ai + l) % 26;

                // convert back to ascii index

                cai += 65;

                d = cai;

                // push each encoded letter into the c array

                c[i] = d;

            }

            // if the character is lowercase, preserve case and add character to array c[i]

            else

            {

                // convert to alphabetical index

                ai = p[i] - 97;

                do

                {

                    if (ai + l < 0)

                    {

                        ai += 26;

                    }

                    if (ai + l > 25)

                    {

                        ai -= 26;

                    }

                }

                while ((ai + l) < 0 || (ai + l) > 25);

                cai = (ai + l) % 26;

                // convert back to ascii index

                cai += 97;

                d = cai;

                // push each encoded letter into the c array

                c[i] = d;

            }

        }

        // print ciphertext

        printf("ciphertext: %s\n", c);

        return 0;



    }

    /*--- if program is executed without any comand-line argument or more than one argument

    is entered, print error message and return from main value of 1 ---*/

    else

    {

        printf("ERROR!! You must enter a key (number). Only one key is accepted.\n");

        return 1;

    }

}