SWI-Prolog -- string

Availability:built-in

string_code(?Index, +String, ?Code)

True when Code represents the character at the 1-based Index position in String. If Index is unbound the string is scanned from index 1. Raises a domain error if Index is negative. Fails silently if Index is zero or greater than the length of String. The mode string_code(-,+,+) is deterministic if the searched-for Code appears only once in String. See also sub_string/5.