Prolog wildcard for completing a string

问题:

I am currently stuck on a prolog problem.

So far I have:

film(Title) :- movie(Title,_,_). (Where 'movie(T,_,_,)' is a reference to my database)

namesearch(Title, Firstword) :- film(Title), contains_term(Firstword, Title).

It is hard to explain what I need help on, but basically is there a wildcard I can use to search for all films starting with a specific word, for example, if I were to search for all films beginning with the word "The".

Is there a wildcard which would allow me to input as such: namesearch(X,'The*') ?

I have tried using the asterisk like this and it does not work,

Thank your for your help

回答1:

It all depends how the title is represented.

Atom

If it is represented as an atom, you need sub_atom(Atom, Before, Length, After, Sub_atom)

?- Title = 'The Third Man', sub_atom(Title, 0, _, _, 'The').
Title = 'The Third Man'.

List of codes

If it is a list of codes which is called a string in Prologs in Edinburgh tradition, you can either "hard code" it with append/3 or you might use Definite Clause Grammars for general patterns.

?- set_prolog_flag(double_quotes,codes).
true.

?- append("The",_, Pattern), Title = "The Third Man", Pattern = Title.
Pattern = Title, Title = [84, 104, 101, 32, 84, 104, 105, 114, 100|...].

?- Title = "The Third Man", phrase(("The",...), Title).
Title = [84, 104, 101, 32, 84, 104, 105, 114, 100|...] ;
false.

Note that 84 is the character code of T etc.

phrase/2 is "the entry" to grammars. See dcg for more. Above used the following definition:

... --> [] | [_], ... .

List of characters

Similar to list of codes, list of characters provide a more readable representation that has still the advantages of being compatible with list predicates and Definite Clause Grammars:

?- set_prolog_flag(double_quotes,chars).
true.

?- append("The",_, Pattern), Title = "The Third Man", Pattern = Title.
Pattern = Title, Title = ['T', h, e, ' ', 'T', h, i, r, d|...].

?- Title = "The Third Man", phrase(("The",...), Title).
Title = ['T', h, e, ' ', 'T', h, i, r, d|...] ;
false.

See also this answer.