Collins cs50 Substitution solution

substitution.c

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

int main(int argc, string argv[])
{
    if (argc != 2)  //if command line argument is not equal to 2
    {
        printf("Usage: ./substitution key\n"); //print error message "Usage ./substitution" is commnand is not equal to 2
        return 1;
    }

    int argv_length = strlen(argv[1]); //to get the lenth of argv[1], the second command after ./substitution on command line
    if (argv_length != 26)      //to make sure argv length must 26 characters
    {
        printf("Key must be up to 26\n"); //if argv length is not equal to 26 (print the error message )
        return 1;
    }
    for (int i = 0; i < argv_length; i++) //iterate over argv_length in order to get its character
    {
        if (!isalpha(argv[1][i]))   //to check if argv[1] is not alphabethic
        {
            printf("All Key must be an Alphabeth\n"); //print this error message if all character in argv[1] is not an alphabeth
            return 1;
        }
        for (int j = i + 1; j < argv_length; j++) //declaring another iteration on argv_length to check for repeated alphabeth
        {
            if (argv[1][i] == argv[1][j])  //checking repeated alphabeth in argv[1]
            {
                printf("Key must not contain repeated alphabeth");  //if repeated alphabeth found, print the below error message
                return 1;
            }
        }
    }
    string plaintext = get_string("plaintext: ");   //Getting user's input as plaintext
    printf("ciphertext: ");     //to print the ciphertext
    int plaintext_length = strlen(plaintext);       //get the strlen of plaintext (user's input)
    for (int i = 0; i < plaintext_length; i++)      //iterate over the plaintext_Length
    {
        if (isupper(plaintext[i]))                  //check if plaintext character is uppercase
        {
            printf("%c", toupper(argv[1][plaintext[i] - 65]));  //calculation to print the encipher text amd make sure it is Uppercase (case doesn't change)
        }
        else if (islower(plaintext[i]))  //check if plaintext character is lowercase
        {
            printf("%c", tolower(argv[1][plaintext[i] - 97]));  ///calculation to print the encipher text amd make sure it is lowercase(case doesn't change)
        }
        else   //if plaintext is anything else, print it like that without changing
        {
            printf("%c", plaintext[i]);
        }

    }
    printf("\n");  //print new line
}