Replacing white spaces in prolog

Question

Is it possible in prolog to replace all white spaces of a string with some given character? Example- If I have a variable How are you today? and I want How_are_you_today?

Answer 1

For atoms

There are may ways in which this can be done. I find the following particularly simple, using atomic_list_concat/3:

?- atomic_list_concat(Words, ' ', 'How are you today?'), atomic_list_concat(Words, '_', Result).
Words = ['How', are, you, 'today?'],
Result = 'How_are_you_today?'.

For SWI strings

The above can also be done with SWI strings. Unfortunately, there is no string_list_concat/3 which would have made the conversion trivial. split_string/4 is very versatile, but it only does half of the job:

?- split_string("How are you today?", " ", "", Words).
Words = ["How", "are", "you", "today?"].

We can either define string_list_concat/3 ourselves (a first attempt at defining this is shown below) or we need a slightly different approach, e.g. repeated string_concat/3.

string_list_concat(Strings, Separator, String):-
  var(String), !,
  maplist(atom_string, [Separator0|Atoms], [Separator|Strings]),
  atomic_list_concat(Atoms, Separator0, Atom),
  atom_string(Atom, String).
string_list_concat(Strings, Separator, String):-
  maplist(atom_string, [Separator0,Atom], [Separator,String]),
  atomic_list_concat(Atoms, Separator0, Atom),
  maplist(atom_string, Atoms, Strings).

And then:

?- string_list_concat(Words, " ", "How are you today?"), string_list_concat(Words, "_", Result).
Words = ["How", "are", "you", "today?"],
Result = "How_are_you_today?".