Wrapping around array causing unexpected output in c - c

Question

I would expect that the values in the array would repeat in every 3rd character but I do not understand why to out put are different if they are referencing the same index.
Input:
Command line argument: baz
User input: barfoo
Output: caqmut
Expected output: caqgon
Values of key_number[] after first for loop:
key_number[0] = 1
key_number[1] = 0
key_number[2] = 25
key_number[3] = 32767
key_number[4] = 32
key_number[5] = 57
Code used:
#include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(int argc, string argv[])
{
if(argc<=1)
{
printf("Usage: ./vigenere k");
exit(1);
}
// convert command line input from string to int varible keyword
string keyword = argv[1];
//prompt user input and initialized
printf("plaintext: ");
string plaintext = get_string();
printf("ciphertext: ");
//create lower_case_array
char lower_case[26] = "abcdefghijklmnopqrstuvwxyz";
//create upper_case_array
char upper_case[26] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
//convert key from string to int[]
//int array_length =strlen(plaintext);
int key_number[6]; // [array_length]
int key_length = strlen(keyword);
//loop though key_number for the length of plaintext
for (int i = 0; i < (strlen(plaintext)); i++)
{
//if char is uppercase
if (keyword[i] >= 65 && keyword[i] < 91 )
{
key_number[i] = (keyword[i % key_length]) - 65;
}
// if char is lowercase
else if (keyword[i] >= 97 && keyword[i] < 123)
{
key_number[i] = (keyword[i % key_length]) - 97;
}
}
//print out letters accoring to ceasar cryptography
for (int i = 0; i < strlen(plaintext); i++)
{
//branch if letter is upper case
if (plaintext[i] >= 65 && plaintext[i] < 91 )
{
printf("%c", upper_case[((plaintext[i] - 65 + key_number[i]) % 26)]);
}
//branch if letter is lower case
else if (plaintext[i] >= 97 && plaintext[i] < 123)
{
printf("%c", lower_case[((plaintext[i] - 97 + key_number[i]) % 26)]);
}
//branch if any other character
else
{
printf("%c", plaintext[i]);
}
}
printf("\n");
}

Answer 1

what puzzled me when I read your code is that you're using keyword[i % key_length] sometimes and sometimes keyword[i].
So when you're using i, you can go past the keyword table: undefined behaviour because plaintext is probably longer than keyword
You could have avoided that by rewriting your code more cleanly:
for (int i = 0; i < (strlen(plaintext)); i++)
{
char kc = keyword[i % key_length];
//if char is uppercase
if (kc >= 'A' && kc <= 'Z' ) // aka isupper(kc)
{
key_number[i] = kc - 'A';
}
// if char is lowercase
else if (kc >= 'a' && kc <= 'z') // aka islower(kc)
{
key_number[i] = kc - 'a';
}
}