pangram check solution only alphabet

pangram.c

int is_pan(const char *str)
{
 int arr[26] = { 0 };
 int i=0;

 while (str[i] != '')
 {
  ++arr[(str[i++] | 32) - 'a'];
 }

 for (i = 0; i < 26; i++)
 {
  if (arr[i] == 0)
   return 1;
 }

 return 0;
}