FATIH UNIVERSITY Department of Computer Engineering Programming Style and Technique Notes for Ch.8 of Bratko For CENG421&553 Fall03

FATIH UNIVERSITY Department of Computer Engineering Use

FATIH UNIVERSITY Department of Computer Engineering Use of Recursion: maplist Recursion is always used. maplist is an example of a “higher- order function”: a function that applies to another function In the original LISP (McCarthy, 1960): ((label maplist (lambda (x f) (cond ((null x) ‘( )) (‘t (cons (f (car x)) (maplist (cdr x) f)))))) maplist( List, F, NewList) if NewList contains the result of applying F to each element of List, in order The function F the binary relation representing the function we want to apply

FATIH UNIVERSITY Department of Computer Engineering maplist II Boundary (base) case: List = [ ] –if List = [ ] then NewList = [ ] General case: List = [ X|Tail] –To transform a list of the form [ X|Tail], do transform the item X by F, obtaining New X, and transform the list Tail obtaining NewTail the whole transformed list if [ NewX|NewTail] See ch8_1.pl Prolog  recursion  objects recursive

FATIH UNIVERSITY Department of Computer Engineering /* maplist( List,F,NewList) if NewList contains the result of applying the function F (represented by a binary relation ) to each entry of NewList, in order. */ maplist( [],_,[]). maplist( [ X|Tail],F,[ NewX|NewTail]) :- G =.. [ F,X,NewX], G, % some Prologs require call( G) instead of G maplist( Tail,F,NewTail). /* Square function, for example query ?- maplist( [2,6,5],square,Squares) */ square( X,Y) :- Y is X*X.

FATIH UNIVERSITY Department of Computer Engineering Generalization Generalization allows us to use recursion Example: eight queens without generalization, no integer to recur on! Define nqueens( Pos,N) –if N queens are safe in position Pos Base case: N = 0: trivial General case: N > 0 achieve a safe configuration of N-1 queens add the remaining queen so that it does attack eightqueens( Pos) :- nqueens( Pos,8).

FATIH UNIVERSITY Department of Computer Engineering Debugging Most Prolog interpreters and compilers use a four- port model for goals foo/n call succeed retry fail

FATIH UNIVERSITY Department of Computer Engineering Improving Efficiency Eight queens, again: using a more informed initial ordering of queen coordinate values helps, as described in the comments at the end of ex4_6.pl Map coloring –start with countries that have many neighbors Warnsdorff’s heuristic for the knight’s tour

FATIH UNIVERSITY Department of Computer Engineering % Ex 4.6[B] % Figure 4.7 Program 1 for the eight queens problem. % solution( BoardPosition) if % BoardPosition is a list of non-attacking queens solution( [] ). solution( [X/Y | Others] ) :- % First queen at X/Y, other queens at Others solution( Others), member( Y,[1,2,3,4,5,6,7,8]), % Order of alternatives defined here! noattack( X/Y, Others). % First queen does not attack others noattack( _, [] ). % Nothing to attack noattack( X/Y, [X1/Y1 | Others] ) :- Y =\= Y1, % Different Y-coordinates Y1-Y =\= X1-X, % Different diagonals Y1-Y =\= X-X1, noattack( X/Y, Others). % A solution template template( [1/Y1,2/Y2,3/Y3,4/Y4,5/Y5,6/Y6,7/Y7,8/Y8] ). solution1(S) :- template(S), solution(S). % When computing all solutions, all orders are equivalent: /* Original Order: member( Y,[1,2,3,4,5,6,7,8]) 4 ?- time(setof(S,solution1(S),L)), length(L,N). % 171,051 inferences in 0.22 seconds ( Lips) */ /* More informed order: member( Y,[1,3,5,7,2,4,6,1]) 16 ?- time(setof(S,solution1(S),L)), length(L,N). % 171,051 inferences in 0.22 seconds ( Lips) */ % When computing one solution, the informed order is much better: /* More informed order: member( Y,[1,3,5,7,2,4,6,1]) 18 ?- time(solution1(S)). % 368 inferences in 0.00 seconds (Infinite Lips) */ /* Original Order: member( Y,[1,2,3,4,5,6,7,8]) 19 ?- time(solution1(S)). % 9,382 inferences in 0.01 seconds ( Lips) */

FATIH UNIVERSITY Department of Computer Engineering colors( []). colors( [ Country/Color|Rest]) :- colors( Rest), member( Color,[yellow,blue,red,green]), not( (member( Country1/Color,Rest),neighbor( Country,Country1)) ). % Note the extra parentheses, to make a single conjunctive goal of two neighbor( Country,Country1) :- ngb( Country,Neighbors), member( Country1,Neighbors). makelist( List) :- collect( [germany],[],List). collect( [],Closed,Closed). % No more candidates for closed collect( [ X|Open],Closed,List) :- member( X,Closed),!, % If X has already been collected, collect( Open,Closed,List). % discard it collect( [X|Open],Closed,List) :- ngb( X,Ngbs), conc( Ngbs,Open,Open1), % built-in version of concat collect( Open1,[ X|Closed],List). country( X) :- ngb( X,_). % Incomplete ngb relation below gives only six countries

FATIH UNIVERSITY Department of Computer Engineering % Neighbors for the 6 original EU countries (Treaty of Rome, 1957) % Andorra, Monaco, San Marino, and Vatican City are not considered % Only land (bridge and tunnel included) borders are considered /* Reorder the ngb relation so that the country that has the largest amount of neighbour will be at the top. So start coloring from that, continue with its neighbours, then the neighbours of the neighbours etc. */ ngb( germany, [denmark,poland,czech,austria,switzerland,france,luxembourg,belgium,netherlands]). ngb( netherlands, [germany,belgium,france]). ngb( belgium, [netherlands,germany,luxembourg,france]). ngb( france, [belgium,luxembourg,germany,switzerland,italy,spain]). ngb( luxembourg, [belgium,germany,france]). ngb( italy, [france,austria,slovenia]). ngb( denmark, [germany]). ngb( poland, [germany]). ngb( czech, [germany]). ngb( austria, [germany, italy]). ngb( switzerland, [germany,italy,france]). ngb( spain, [france]). ngb( slovenia, [italy]).

FATIH UNIVERSITY Department of Computer Engineering Difference Lists The difference list L-E contains the elements of L that occur before E. E is a suffix of L. E.g., [a,b,c,d] – [c,d] is [a,b] Trick: [a,b,c,d|E] – E is [a,b,c,d]! Difference lists allow concatenation of lists in constant time, as in imperative languages.

FATIH UNIVERSITY Department of Computer Engineering Concatenation and Difference Lists Difference lists can be concatenated in constant time by using conc_dl: ?conc_dl([a,b|A] – A, [c,d|C] – C, Result) unify with conc_dl(X – Y, Y – Z, X – Z): {X / [a,b|A]} {Y / A} {A / [c,d|C]} {Z / C} {Result / X – Z}, i.e. {Result / [a,b|A] – C}, i.e. {Result / [a,b,c,d|C} – C}

FATIH UNIVERSITY Department of Computer Engineering Some Caveats While the result of conc_dl has an uninstatiated tail, the first argument does not. –You cannot use it as an argument to conc_dl in the same clause. “Difference lists are powerful but counterintuitive. They appear most often in procedures that were first implemented with conventional lists and then carefully rewritten for efficiency” [Covington, p.183].

FATIH UNIVERSITY Department of Computer Engineering Improveing efficiency by asserting derived facts fib and fib2. Execution trees Page 204 More difficult but more efficient one:forwardfib