Why strcasestr or (strstr) function outputs (null)?

here is the code:

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

string alphabet = "abdcefghijklmnopqrstuvwxyz";
string text = "world";
string ciphertext = "";

for(int i = 0; i < strlen(text); i++)
{
     ciphertext = strstr(alphabet, &text[i]);
     printf("%s \n", ciphertext);
}

It outputs the following result:

(null) 
(null) 
(null) 
(null) 
dcefghijklmnopqrstuvwxyz 

So it looks like strstr() works only for the last character, in this case "d". Why it does not work for prior characters? strcasestr() has the same behavior

Because you want to find character in the string, not string in the string, you need to use strchr function:

for(size_t i = 0; text[i]; i++)
{
     ciphertext = strchr(alphabet, text[i]);
     printf("%s \n", ciphertext);
}

1. Use the correct type for indexes (size_t)
2. You do not have to call strlen on every iteration. It is enough to check if you did not reach the null terminating character.