loops - Vigenere Cipher in C: Incomplete Encryption -

a general explanation of vigenere cipher:

the vigenere cipher method of encryption similar caesar cipher. cipher takes in word argument , interprets alphabets of word follows- 0, b 1, c 2 , on.

so if input key abc , want "hi hello" encrypted, output entail h remaining same, shifting 1 place, h shifting 2 places, e again remaining same (as being shifted 0), l shifting 1 place, other l 2 , on forth.

the basic idea each letter shifts corresponding letter in argument , spaces , other punctuation marks ignored. if argument shorter message (as in cases), argument loops around message.

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my problem:

my message being encrypted first alphabet of vignere cipher.

for example, ./vc bc ----> message: abcde abcde becomes bcdef bcdef. in other words, entire message being shifted value of b when should instead shifted value of bc (+1 first alphabet , +2 every other alphabet.)

i don't understand why happening despite being within loop.

code:

# include <cs50.h>  # include <stdio.h>  # include <stdlib.h> # include <string.h> # include <ctype.h>  int main(int argc, string argv[]) {     string word = argv[1];     int = 0;     int j = 0;      if (argc != 2 || isalpha(word[j]) == false)     {         printf("please enter valid command line argument! \n");         return 1;     }      else if (isalpha(word[j]))     {         printf("message: ");          string message = getstring();          (int n = strlen(message); < n; i++)         {                int plaintext = message[i];                int ciphertext = word[j];                 int uppcb = (plaintext - 65 + ciphertext - 65);                int upcipher1 = (uppcb) % 26;                int uplc = (plaintext - 65 + ciphertext - 97);                int upcipher2 = (uplc) % 26;                int lopcb = (plaintext - 97 + ciphertext - 97);                int locipher1 = (lopcb) % 26;                int lolp = (plaintext - 97 + ciphertext - 65);                int locipher2 = (lolp) % 26;                  if (isupper(word[j]) && isupper(message[i]))                  {                     j = (j+1)%strlen(word);                     int upcode = (upcipher1 + 65);                      printf("%c", upcode);                 }                 else if (isupper(message[i]) && islower(word[j]))                 {                     j = (j+1)%strlen(word);                     int upcode1 = (upcipher2 + 65);                     printf("%c", upcode1);                 }                 else if (islower(message[i]) && islower(word[j]))                 {                     j = (j+1)%strlen(word);                     int locode = (locipher1 + 97);                      printf("%c", locode);                 }                 else if (islower(message[i]) && isupper(word[j]))                 {                     j = (j+1)%strlen(word);                     int locode1 = (locipher2 +97);                     printf("%c", locode1);                 }                 else                  {                     printf("%c", message[i]);                 }                   }         printf("\n");          }  }