The non-alphabetic problem in Vigenere problem set - c

Question

I just have done with Vigenere problem in CS50, but still, there's one wrong only, the non-alphabetic characters, when you write in plaintext anything without spaces, comma, any non-alphabetic, the program will run well, but if you wrote any non-alphabetic character, like space, the next character will take the wrong key, this my code :
#include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
int main(int argc, string argv[])
{
// Make sure there is a command-line argment
if (argc != 2)
{
printf("Error\n");
return 1;
}
// Variables
int key[strlen(argv[1])];
string plaintext;
// Make sure the comman-line argment is Alphabets then make the key
for (int i = 0, n = strlen(argv[1]); i < n; i++)
{
if (!isalpha(argv[1][i]))
{
printf("Error 2\n");
return 1;
}
if (islower(argv[1][i]))
{
key[i] = argv[1][i] - 'a';
}
else if (isupper(argv[1][i]))
{
key[i] = argv[1][i] - 'A';
}
}
// Ask the user to write the message
plaintext = get_string("plaintext: ");
printf("ciphertext: ");
// Make sure the plaintext doesn't equal NULL
if (plaintext != NULL)
{
for (int i = 0, n = strlen(plaintext); i < n ; i++)
{
// Print in slower case
if (islower(plaintext[i]))
{
printf("%c", (((plaintext[i] + key[i % strlen(argv[1])]) - 'a') % 26) + 'a');
}
// Print in upper case
else if (isupper(plaintext[i]))
{
printf("%c", (((plaintext[i] + key[i % strlen(argv[1])]) - 'A') % 26) + 'A');
}
// Print the non alphabetic
else if (!isalpha(plaintext[i]))
{
printf("%c", plaintext[i]);
}
}
// Print a new line
printf("\n");
}
}

Answer 1

The problem is because you are using the same index for the plaintext and the key in this
for (int i = 0, n = strlen(plaintext); i < n ; i++) loop. The key will advance one position every time plaintext does. Obviously that is not what you want. You need to manage the key index independently of the plaintext index within that loop.
Suggest you rewatch the walkthrough and perhaps write out an example much the way Zamyla does the panda example. And it's never too soon to learn how to use debug50. If I recall correctly, there is a short for it in Week 2.
CS50x has a stack forum dedicated to questions and answers about CS50x and the psets.