hangman in prolog

Question

I am a beginner to prolog. How can I convert letters into * (asterisk)? I know that it is a character codes in prolog to represent strings. The problem is if it is a letter, convert it to " *", if it is a underscore, just keep it.

So how do I know whether it is a underscore or not?

Answer 1

So, the task is to replace letters with asterisks. At first, you should google some material about strings in prolog. Than it's easy to have something like that

string_replace( [], [] ).
string_replace( [H | Tail], StringNew ) :-
    ( not(underscore(H)), asterisk(A), StringNew = [A | StringTail], string_replace( Tail, StringTail) )
    ;
    ( underscore(H), StringNew = [H | StringTail], string_replace( Tail, StringTail) )
    .

letter( X ) :-
    ( X >= 97, X =< 122 )
    ; 
    ( X >=65, X =<90 ).

underscore( 95 ).
asterisk( 42 ).

main :-
    string_replace( "test_string", S1 ),
    writef( "%s", [S1] ), nl,
    string_replace( "another string", S2 ),
    writef( "%s", [S2] ), nl,
    !
    .

It works like that

?- main.
****_******
**************
true.

It don't use any builtins predicates, but it's could be useful to understand how all it really works.

Answer 2

check each character in string:

...maplist(convstar, String, Converted), ...

convstar(0'_, 0'_).
convstar(_, 0'*).

Another way, using an 'if-then' construct:

convstar(X, Y) :-
  (  X == 0'_
  -> Y = 0'_
  ;  Y = 0'*
  ).