Programming Style and Technique Notes for Ch.8 of Bratko For CSCE 580 Sp03 Marco Valtorta

Use of Recursion: maplist 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

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 renamed maplist1, because maplist is built in, with slightly different meaning call/2 can be used for a more functional (less relational) style

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).

Debugging Most Prolog interpreters and compilers use a four-port model for goals call foo/n succeed fail retry

Debugging Example ch3_1.pl1 [trace] 1 ?- del( a,[a,b,a],L) See Section 2.9 of SWI-Prolog manual for some extensions

Improving Efficiency Eight queens, again: Map coloring 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

Difference Lists concat( A1-Z1,Z1-Z2,A1-Z2). See UseDefK4.ppt

Accumulators sumlist See UseDefK4.ppt