program.c

#include <cs50.h>
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
string message;
int main(int argc, string argv[]){// checks to make sure argument is correct and valid
if (argc < 2 | argc > 2)
{ printf("Not enough arguments\n"); return 1;}
string key = argv[1]; for (int i = 0, n = strlen(key); i < n; i++) {if (!isalpha(key[i])){printf("Key is not entirely aphabetical");
return 1;}}
//printf("Please enter your message here: ");
message = GetString();
int length = strlen(message);
for (int i = 0, y = 0, n = strlen(message); i < n; i++)
    // the life and soul of the party
    {
        
        int KeyForLetter = tolower(key[y % length]) - 'a';

        // gets the key for each indivudal letter
        
        if (isupper(message[i]))
        {
            printf("%c", 'A' + (message[i] - 'A' + KeyForLetter) % 26);
            // if message is upper, this keeps it al in same case
            // charecter is equal to ascii of A plus the indiviudal letter added
            // to the key mod 26 to reserve wrap aroudn
            
            y++;
            
        }
        else if (islower(message[i]))
        {
            // for message lower
            printf("%c", 'a' + (message[i] - 'a' + KeyForLetter) % 26);
            
            y++;
        }
        else
        {
            printf("%c", message[i]);
        }
    }
    printf("\n");
    return 0;
    
    
}