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I'm having issues figuring out how to ignore the spaces in the user string when changing to ciphertext. I thought the isalpha() check accounted for that but I guess not.

Any help would be appreciated! Thanks.

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


#define ASCII_TO_ALPHA 65
#define LOWER_ASCII_TO_ALPHA 6


int main(int argc, string argv[])
{
    int key_length;
    int string_length;
    int key_index;
    if(argc == 2)
    {
        string cipher_key = argv[1];
        key_length = strlen(cipher_key);

        // Check if every character is an alphabet
        for(int k = 0; k < key_length; k++)
        {
            if(!isalpha(cipher_key[k]))
            {
                printf("Usage ./vingenere OneStringOnlyLetters\n");
                return 1;
            }
        }

        string plain_text = get_string("plaintext: ");
        string_length = strlen(plain_text);
        char cipher_text[strlen(plain_text) + 1];

        for (int i = 0; i < string_length; i++)
        {
            if (isalpha(plain_text[i]))
            {
                key_index = i % key_length;
                if (isupper(plain_text[i]))
                {
                    cipher_text[i] = (char)(((((int) plain_text[i] - ASCII_TO_ALPHA) + (toupper(cipher_key[key_index]) - ASCII_TO_ALPHA)) % 26) + ASCII_TO_ALPHA);
                }
                else if(islower(plain_text[i]))
                {
                    cipher_text[i] = tolower((char)(((((int) plain_text[i] - LOWER_ASCII_TO_ALPHA - ASCII_TO_ALPHA) + (toupper(cipher_key[key_index]) - ASCII_TO_ALPHA)) % 26) + ASCII_TO_ALPHA));
                }

            }
            else
            {
                cipher_text[i] = plain_text[i];
            }
        }
        cipher_text[strlen(plain_text)] = '\0';
        printf("ciphertext: %s\n", cipher_text);
        return 0;
    }
    else
    {
        printf("Usage ./vingenere OneStringOnlyLetters\n");
        return 1;
    }
}
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The isalpha check should work fine and spaces (as well as all other non alphabetic input) should be printet unchanged - (does it?). The problem I see in your code is that key_index = i % key_length; doesn't account for that whenever isalpha(plain_text[i]) is false key_index should not change. Try to count the times isalpha is false and subtract that number from i. Or use a counter that only counts whenever is alpha is true starting from 0.

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