My solution to CS50x Problem Set 2 - Caesar. Feel free to critique or ask questions.

caesar.c

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


int main(int argc, string argv[])
{
    //checks if user input exactly 1 command line argument, if not prints out error message
    if (argc == 2)
    {
        //checks if argument is a number, if not prints out error message
        for (int i = 0, n = strlen(argv[1]); i < n; i++)
        {
            if (isdigit(argv[1][i]) == 0)
            {
                printf("Usage: ./caesar key\n");
                return 1;
            }
        }
    }
    else
    {
        printf("Usage: ./caesar key\n");
        return 1;
    }

    //gets the text the user wants to encode and stores it as plaintext
    string plaintext = get_string("plaintext: ");

    //stores the command line argument inputed earlier to use as key for encryption
    int key = atoi(argv[1]);

    for (int i = 0, n = strlen(plaintext); i < n; i++)
    {
        //creates variable "bound_check" to later check whether the program should wrap from z or not
        int bound_check = (plaintext[i] + key);

        //makes sure the program will only change letters
        if (isalpha(plaintext[i]) != 0)
        {
            //seperate algorithm for uppercase letters ie: "A,B,C..."
            if (isupper(plaintext[i]) != 0)
            {
                //if there is no need to wrap back to A, simply add key to character in "i" position in "plaintext"
                if (bound_check >= 65 && bound_check < 90)
                {
                    plaintext[i] = (plaintext[i] + key);
                }
                //if there is a need to wrap back to A, add key to character in "i" position in "plaintext" and then wrap to A
                else if (bound_check > 90)
                {
                    plaintext[i] = ((((plaintext[i] + key) - 65) % 26) + 65);
                }
            }
            //seperate algorithm for lowercase letters ie: "a,b,c..."
            else if (islower(plaintext[i]) != 0)
            {
                //if there is no need to wrap back to a, simply add key to character in "i" position in "plaintext"
                if (bound_check >= 97 && bound_check < 122)
                {
                    plaintext[i] = (plaintext[i] + key);
                }
                //if there is a need to wrap back to a, add key to character in "i" position in "plaintext" and then wrap to a
                else if (bound_check > 122)
                {
                    plaintext[i] = ((((plaintext[i] + key) - 97) % 26) + 97);
                }
            }
        }
    }

    //outputs the encoded text
    printf("ciphertext: %s\n", plaintext);
}