'if' in prolog?

Question

Is there a way to do an if in prolog, e.g. if a variable is 0, then to do some actions (write text to the terminal). An else isn\'t even needed, but I can\'t find any docume

Answer 1

I found this helpful for using an if statement in a rule.

max(X,Y,Z) :-
    (  X =< Y
    -> Z = Y
    ;  Z = X
    ).

Thanks to http://cs.union.edu/~striegnk/learn-prolog-now/html/node89.html

Answer 2

Yes, there is such a control construct in ISO Prolog, called ->. You use it like this:

( condition -> then_clause ; else_clause )

Here is an example that uses a chain of else-if-clauses:

(   X < 0 ->
    writeln('X is negative.  That's weird!  Failing now.'),
    fail
;   X =:= 0 ->
    writeln('X is zero.')
;   writeln('X is positive.')
)

Note that if you omit the else-clause, the condition failing will mean that the whole if-statement will fail. Therefore, I recommend always including the else-clause (even if it is just true).

Answer 3

You should read Learn Prolog Now! Chapter 10.2 Using Cut. This provides an example:

max(X,Y,Z) :- X =< Y,!, Y = Z.

to be said,

Z is equal to Y IF ! is true (which it always is) AND X is <= Y.

Answer 4

There are essentially three different ways how to express something like if-then-else in Prolog. To compare them consider char_class/2. For a and b the class should be ab and other for all other terms. One could write this clumsily like so:

char_class(a, ab).
char_class(b, ab).
char_class(X, other) :-
   dif(X, a),
   dif(X, b).

?- char_class(Ch, Class).
   Ch = a, Class = ab
;  Ch = b, Class = ab
;  Class = other,
   dif(Ch, a), dif(Ch, b).

To write things more compactly, an if-then-else construct is needed. Prolog has a built-in one:

?- ( ( Ch = a ; Ch = b ) -> Class = ab ; Class = other ).
   Ch = a, Class = ab.

While this answer is sound, it is incomplete. Just the first answer from ( Ch = a ; Ch = b ) is given. The other answers are chopped away. Not very relational, indeed.

A better construct, often called a "soft cut" (don't believe the name, a cut is a cut is a cut), gives slightly better results (this is in YAP):

?- ( ( Ch = a ; Ch = b ) *-> Class = ab ; Class = other ).
   Ch = a, Class = ab
;  Ch = b, Class = ab.

Alternatively, SICStus has if/3 with very similar semantics:

?- if( ( Ch = a ; Ch = b ), Class = ab , Class = other ).
   Ch = a, Class = ab
;  Ch = b, Class = ab.

So the last answer is still suppressed. Now enter library(reif) for SICStus, YAP, and SWI. Install it and say:

?- use_module(library(reif)).

?- if_( ( Ch = a ; Ch = b ), Class = ab , Class = other ).
   Ch = a, Class = ab
;  Ch = b, Class = ab
;  Class = other,
   dif(Ch, a), dif(Ch, b).

Note that all the if_/3 is compiled away to a wildly nested if-then-else for

char_class(Ch, Class) :-
   if_( ( Ch = a ; Ch = b ), Class = ab , Class = other ).

which expands in YAP 6.3.4 to:

char_class(A,B) :-
   ( A\=a
   ->
     ( A\=b
     ->
       B=other
     ;
       ( A==b
       ->
         B=ab
       )
     ;
       A=b,
       B=ab
     ;
       dif(A,b),
       B=other
     )
   ;
     ( A==a
     ->
       B=ab
     )
   ;
     A=a,
     B=ab
   ;
     dif(A,a),
     ( A\=b
     ->
       B=other
     ;
       ( A==b
       ->
         B=ab
       )
     ;
       A=b,
       B=ab
     ;
       dif(A,b),
       B=other
     )
   ).