Pset2: vigenere. Problem with key "hello, world!"

Question

I can't solve the problem with key "hello, world!" and understand why it appears.

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

int main (int argc, string argv[])
{
if (argc > 2 || argc < 2)
{
    printf("Eror\n");
    return 1;
}

for (int i = 0; i < strlen(argv[1]); i++)
{
    if (isalpha(argv[1][i]) == 0)
    {
        printf("Eror\n");
        return 1;
    }
}

printf("Give me your msg\n");
string msg = get_string();
int j=0;
printf("ciphertext:");
    for (int i = 0, lengthmsg = strlen(msg); i < lengthmsg; i++)
    {
        j = j % strlen(argv[1]);

        if (isupper(msg[i]) && isupper(argv[1][j]))
            printf("%c", (( (msg[i] - 65) + (argv[1][j] - 65) ) % 26) + 65);
        else if (islower(msg[i]) && islower(argv[1][j]))
            printf("%c", (( (msg[i] - 97) + (argv[1][j] - 97) ) % 26) + 97);
        else if (islower(msg[i]) && isupper(argv[1][j]))
            printf("%c", (( (msg[i] - 97) + (argv[1][j] - 65) ) % 26) + 97);
        else if (isupper(msg[i]) && islower(argv[1][j]))
            printf("%c", (( (msg[i] - 65) + (argv[1][j] - 97) ) % 26) + 65);
        else
            printf("%c",msg[i]);
        j++;
    }
    printf("\n");
    return 0;
}

Answer 1

This is a somewhat common error for this exercise.

The code uses j as an index for the key. The problem is that j is incremented for EVERY character that is processed. The key index should ONLY be incremented when a letter is processed and never when a non-letter such as a space or punctuation is processed.

Because of this, the key gets out of synch with the letters to be encoded, so the wrong key letter is being used.

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