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 documentation of if

User · Answer

You should read Learn Prolog Now  Chapter 10 2 Using Cut   This provides an example   max X Y Z    -  X    lt   Y     Y     Z    to be said    Z is equal to Y IF   is true  which it always is  AND X is  lt   Y

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I found this helpful for using an if statement in a rule   max X Y Z   -        X   lt  Y     - gt  Z   Y        Z   X          Thanks to http   cs union edu  striegnk learn-prolog-now html node89 html

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Prolog predicates  unify  -  So  in an imperative langauge I d write  function bazoo integer foo       if foo    5         doSomething       else        doSomeOtherThing        In Prolog I d write  bazoo 5   -  doSomething  bazoo Foo   - Foo     5  doSomeOtherThing    which  when you understand both styles  is actually a lot clearer   I m bazoo for the special case when foo is 5   I m bazoo for the normal case when foo isn t 5

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First  let s recall some classical first order logic       If P then Q else R  is equivalent to   P and Q  or  non P and R        How can we express  if-then-else  like that in Prolog   Let s take the following concrete example      If X is a member of list  1 2  then X equals 2 else X equals 4    We can match above pattern   If P then Q else R   if       condition P is list member  1 2  X   negated condition non P is non member  1 2  X   consequence Q is X 2  and alternative R is X 4    To express list  non- membership in a pure way  we define    list memberd  E Es  X   -       E   X       dif E X         list memberd Es X         non member Es X   -    maplist dif X  Es       Let s check out different ways of expressing  if-then-else  in Prolog     P Q   non P R     -       list memberd  1 2  X   X 2   non member  1 2  X   X 4   X   2   X   4   - X 2   list memberd  1 2  X   X 2   non member  1 2  X   X 4   X 2  X   2   false   -       list memberd  1 2  X   X 2   non member  1 2  X   X 4   X 2  X   2   false   - X 4   list memberd  1 2  X   X 2   non member  1 2  X   X 4   X 4  X   4   -       list memberd  1 2  X   X 2   non member  1 2  X   X 4   X 4  X   4    Correctness score 5 5  Efficiency score 3 5   P - gt  Q   R     -       list memberd  1 2  X  -  X 2   X 4   false                                                   WRONG  - X 2   list memberd  1 2  X  -  X 2   X 4   X 2  X   2   -       list memberd  1 2  X  -  X 2   X 4   X 2  false                                                   WRONG  - X 4   list memberd  1 2  X  -  X 2   X 4   X 4  X   4   -       list memberd  1 2  X  -  X 2   X 4   X 4  false                                                   WRONG   Correctness score 2 5  Efficiency score 2 5   P  - gt  Q   R     -       list memberd  1 2  X   -  X 2   X 4   X   2   false                                           WRONG  - X 2   list memberd  1 2  X   -  X 2   X 4   X 2  X   2   false   -       list memberd  1 2  X   -  X 2   X 4   X 2  X   2   false   - X 4   list memberd  1 2  X   -  X 2   X 4   X 4  X   4   -       list memberd  1 2  X   -  X 2   X 4   X 4  false                                                   WRONG   Correctness score 3 5  Efficiency score 1 5       Preliminary  summary     P Q   non P R  is correct  but needs a discrete implementation of non P   P - gt  Q   R  loses declarative semantics when instantiation is insufficient   P  - gt  Q   R  is  less  incomplete than  P - gt  Q   R   but still has similar woes      Luckily for us  there are alternatives  Enter the logically monotone control construct if  3   We can use if  3 together with the reified list-membership predicate memberd t 3 like so     -      if  memberd t X  1 2    X 2  X 4   X   2   X   4   - X 2  if  memberd t X  1 2    X 2  X 4   X 2  X   2   -      if  memberd t X  1 2    X 2  X 4   X 2  X   2   false   - X 4  if  memberd t X  1 2    X 2  X 4   X 4  X   4   -      if  memberd t X  1 2    X 2  X 4   X 4  X   4    Correctness score 5 5  Efficiency score 4 5

User · Answer

A    B - gt       writeln  ok          writeln  nok        The else part is required

User · Answer

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   - gt  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    - gt  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    - gt         A  b      - gt         B other                 A  b        - gt           B ab                        A b         B ab               dif A b          B other                    A  a      - gt         B ab                  A a       B ab           dif A a          A  b      - gt         B other                 A  b        - gt           B ab                        A b         B ab               dif A b          B other

User · Answer

Yes  there is such a control construct in ISO Prolog  called - gt    You use it like this     condition - gt  then clause   else clause     Here is an example that uses a chain of else-if-clauses       X  lt  0 - gt      writeln  X is negative   That s weird   Failing now         fail     X     0 - gt      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

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A standard prolog predicate will do this        isfive 5      will evaluate to true if you call it with 5 and fail return false  if you run it with anything else   For not equal you use     isNotEqual A B  - A  B    Technically it is does not unify  but it is similar to not equal   Learn Prolog Now is a good website for learning prolog   Edit  To add another example   isEqual A A

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The best thing to do is to use the so-called cuts  which has the symbol       if then else Condition  Action1  Action2   - Condition     Action1    if then else Condition  Action1  Action2   - Action2    The above is the basic structure of a condition function   To exemplify  here s the max function      max X Y X  -X gt Y      max X Y Y  -Y  lt X    I suggest reading more documentation on cuts  but in general they are like breakpoints  Ex   In case the first max function returns a true value  the second function is not verified     PS  I m fairly new to Prolog  but this is what I ve found out

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Prolog program actually is big condition for  if  with  then  which prints  Goal is reached  and  else  which prints  No sloutions was found   A  Bmeans  A is true and B is true   most of prolog systems will not try to satisfy  B  if  A  is not reachable  i e  X 3  write  X is 3   nl will print  X is 3  when X 3  and will do nothing if X 2

[prolog] 'if' in prolog?

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