CSE 452: Programming Languages Logical Programming Languages Part 3
Acknowledgements • P. Tan • http://www.csupomona.edu/~jrfisher/www/prolog_tutorial/contents.html • http://cs.wwc.edu/~cs_dept/KU/PR/Prolog.html • http://pauillac.inria.fr/~diaz/gnu-prolog/
Prolog Example • Delete an element from a list • deleteElem /3 ( /3 means there are 3 arguments) • Example: • deleteElem(4, [2,3,4,5], X) • X = [2,3,5]. • deleteElem(6, [2,3,4,5],X) • X = [2,3,4,5].
Delete an Element from a List • Functor: deleteElem • Arguments • Input: • An element • List of elements • Output: • Result list
Delete an Element from a List • Base step: • deleteElem(_, [], []). %% underscore means any symbol • deleteElem(P,[P|Tail],Tail). • Recursive step: • deleteElem(P,[H|Tail],[H|Res]) :- P \= H, deleteElem(P,Tail,Res).
Combining Lists • Write a predicate combine/3 that takes three lists as arguments and combines the elements of the first two lists into the third as follows: ?- combine([a,b,c],[1,2,3],X). X = [a,1,b,2,c,3] ?- combine([foo,bar,yip,yup],[glub,glab,glib,glob],Result). Result = [foo,glub,bar,glab,yip,glib,yup,glob]
Combining Lists combine([],List,List). combine(List,[],List). combine([H|T],[H2|T2],[H,H2|Res]) :- combine(T,T2,Res).
Sublist • Sublist/2 is true if the first argument is a sublist of the second argument. For example: sublist([c,d,e],[a,b,c,d,e,f]) is true sublist([c,,e], [a,b,c,d,e,f]) is false sublist(S,L):- append(L1,L2,L), append(S,L3,L2).
Sublist • use it in different ways: sublist([c,d,e],[a,b,c,d,e,f]). sublist([c,,e], [a,b,c,d,e,f]). sublist(S,[a,b,c]). • uses backtracking to find all possible sublists.
Quick Sort p Partition < p p >= p Sort Sort
Quick Sort • What are the functors? • quickSort • partition • How many arguments? • quickSort • Input: Unsorted list • Output: Sorted list • partition • Input: • pivot • List of numbers (besides the pivot) to be partitioned • Output: • Left list (list of numbers less than pivot) • Right list (list of numbers greater than or equal to pivot)
Quick Sort • Base step: • quickSort([], []). • Recursive step: • quickSort([P | T], Result) :- partition(T, P, L, B), quickSort(L, SortLow), quickSort(B, SortBig), append(SortLow, [P | SortBig], Result).
Partition • Base step: • partition([ ], P, [ ], [ ]). • Recursive step: • partition([H|T],P,[H|L],B) :- H < P, partition(T, P, L, B). • partition([H|T],P,L,[H|B]) :- H >= P, partition(T, P, L, B).
Quick Sort (Summary) quickSort([ ], [ ]). quickSort([P | T], Y) :- partition(T, P, L, B), quickSort(L, SortLow), quickSort(B, SortBig), append(SortLow, [P | SortBig], Y). partition([ ], P, [ ], [ ]). partition([H | T], P, [H | L], B) :- H < P, partition(T, P, L, B). partition([H | T], P, L, [H | B]) :- H >= P, partition(T, P, L, B).
