you may treat a string as a array of chars with length (size) of strlen() .
so, if you have a something like this:

STRING* strng="HELLO";

than function strlen(strng) will return 5.
to access first letter u use strng[0], second letter strng[1], .. etc, fifth letter strng[4];
so don't access a string like array from 1 to strlen(). access it like array from 0 to strlen()-1;