"Incompatible integer to pointer conversion"

Question

The following program is refusing to compile because of these errors:

vigenere.c:52:31: error: incompatible integer to pointer conversion assigning to
  'string' (aka 'char *') from 'int' [-Werror,-Wint-conversion]
  ...ciphertext[i] = ((((plaintext[i] - 65) + keyword[num % keylength]) % 26) + 65);
                   ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
vigenere.c:56:31: error: incompatible integer to pointer conversion assigning to
  'string' (aka 'char *') from 'int' [-Werror,-Wint-conversion]
  ...ciphertext[i] = ((((plaintext[i] - 97) + keyword[num % keylength]) % 26) + 97);
                   ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

And here's the program, which is meant to implement a simple vigenere cipher:

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

int main(int argc, string argv[])
{
    if(argc != 2)
    {
        printf("Invalid input; try again!\n");
        return 1;
    }
    else if(argv[1])
    {   
        for(int i = 0, n = strlen(argv[1]); i < n; i++)
        {
            if(!isalpha(argv[1][i]))
            {
                printf("Invalid input; try again!\n");
                return 1;
            }
        }
    }

    // get plaintext from user
    string plaintext = GetString();
    string ciphertext[100];
    int num = 0;

    string keyword = argv[1];
    int keylength = strlen(keyword);

    // change key values from letters to shifts
    for(int i = 0, n = keylength; i < n; i++)
    {
        if(isupper(keyword[i]))
        {
            keyword[i] = keyword[i] - 65;
        }
        else if(islower(keyword[i]))
        {
            keyword[i] = keyword[i] - 97;
        }
    }

    for(int i = 0, n = strlen(plaintext); i < n; i++)
    {
        if(isalpha(plaintext[i]))
        {
            if(isupper(plaintext[i]))
            {   
                ciphertext[i] = ((((plaintext[i] - 65) + keyword[num % keylength]) % 26) + 65);
            }
            else if(islower(plaintext[i]))
            {
                ciphertext[i] = ((((plaintext[i] - 97) + keyword[num % keylength]) % 26) + 97);
            }
            num++;
        }
        // non-alphabetic characters
        else 
        {
            ciphertext[i] = plaintext[i];
        }
        printf("%c", ciphertext[i]);
    }
    printf("\n");
}

I've no idea why the compiler throws the error, because I have an older version of the program, compiled a few months ago (the code is exactly the same on lines 52 and 56) which works just fine.

I'll really appreciate any and all help :)

Answer 1

Variable ciphertext is array of char*, I think it should be:

char ciphertext[1000]