How to filter in Prolog with SWISH

Question

I need to make a predicate called fAtomPairs so that given an atom (first argument), and a list of pairs (each pair is in turn a list of two atoms), unify a third parameter with the filtered list of pairs by selecting only those pairs that have as their first component the atom which is the first argument.

For example :

fAtomPairs(sA,[[basA,absAb],[ab,bbsA],[sA,abbsB],[bsA,sAsB],[sA,bb]],X)

must result in

X = [[sA,abbsB],[sA,bb]]

How can I do this?, I'm currently working in SWISH

1

What have you tried so far? – lurker Jun 14 '18 at 11:19

false · Accepted Answer · 2018-06-16T16:03:01.850

5

:- use_module(library(reif)).   % SICStus|SWI
:- use_module(library(lambda)). % SICStus|SWI

fAtomPairs(Sel, DEs, Es) :-
   tfilter(\[A,_]^ ( Sel = A ), DEs, Es).

edited Jun 16 '18 at 16:03

answered Jun 15 '18 at 20:10

false

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1

This is a better answer than mine, but you can not use those libraries directly in Swish by just saying ` :- use_module(library(reif)). ` – user27815 Jun 22 '18 at 20:31
1

You can use `library(reif)` in SWISH (not immediately obvious). – Rob Grzywinski Mar 20 '21 at 16:11

score 4 · Answer 2 · answered Jun 14 '18 at 07:04

4

You can use a version of tfilter/3 and if_/3. I also changed the representation from lists of two elements to pairs.

fAtomPairs(sA,[basA-absAb,ab-bbsA,sA-abbsB,bsA-sAsB,sA-bb]).


tfilter(_CT_2,    [], []).
tfilter(CT_2, [E-Thing|Es], Fs0) :-
   if_(call(CT_2,E), Fs0 = [E-Thing|Fs], Fs0 = Fs ),
   tfilter(CT_2, Es, Fs).

=(X,X,true).
=(X,Y,false) :- dif(X,Y).


if_(If_1, Then_0, Else_0) :-
   call(If_1, T),
   (  T == true -> call(Then_0)
   ;  T == false -> call(Else_0)
   ;  nonvar(T) -> throw(error(type_error(boolean,T),_))
   ;  /* var(T) */ throw(error(instantiation_error,_))
   ).

Then query:

?-fAtomPairs(SA,Pairs),tfilter(=(SA),Pairs,Filtered).
Filtered = [sA-abbsB, sA-bb],
Pairs = [basA-absAb, ab-bbsA, sA-abbsB, bsA-sAsB, sA-bb],
SA = sA
false

answered Jun 14 '18 at 07:04

user27815

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what does "CT" in `CT_2` stand for, please? (and "_2", for that matter.) – Will Ness Jun 14 '18 at 12:37
Its copied from the answer by false that I linked.. I believe it stands for closure/continuation and the fact it will take two arguments. – user27815 Jun 14 '18 at 13:04
thanks. I thought C was for "callable", but what's T?.. something to do with "truth"?.. – Will Ness Jun 14 '18 at 18:35
yea I think it is for truth. – user27815 Jun 14 '18 at 22:53
2

@WillNess: continuation, criterion. – false Jun 15 '18 at 18:25
@user27815: Why do you change `tfilter/3`? — user another name. – false Jun 15 '18 at 20:00

How to filter in Prolog with SWISH

2 Answers2