prolog.md

Prolog tips

Using recursion

Counting occurrences in a list

count_word(_,[],0).
count_word(W,L,C) :-
	L = [H|T],
	count_word(W, T, X),
	(W = H -> ToAdd=1; ToAdd=0),
	C is ToAdd + X. % crucial line

Sum of list:

sumList([],0).
sumList(L,N) :-
	L = [H|T],
	sumList(T,X),
	N = X + H.

Factorials:

fact(X, 1).
fact(X, N) :-
	NewX is X - 1,
	fact(NewX, NewProd),
	N is NewProd * X.

Sorting:

bubblesort(L, Sorted) :-
	append(X, [X1, X2|T], L),
	X2 < X1, % Sorting condition here
	append(X,[X2,X1|T], L), Sorting),
	bubblesort(Sorting, Sorted),
	!.

bubblesort(L, L).

Using append

Followed by:

followedBy(X,Y,L) :- append(_, [X,Y|_],L).

Last:

last(E,L) :- append(_, [E], L).

Using member with setof/findall

Setof removes all duplicate elements

Syntax is findall(Element,(Conditions), List)

Good for creating lists

studentsInHouseByID(House, SortedByID) :-
	findall(SID, student(SID, _, House), UnsortedByID),

And filtering them

filterByCourseID(OldList, MustStudy, NewList) :-
	findall(Name, (member(Name, OldList), student(SID, Name, _), enrolled(SID, MustStudy)), NewList).

Other functions

• length(L,N)
• member(E,L)
• reverse(L, ReversedList)

Syntax

Negation is \+(). If adding in compound statements, make sure to wrap those in brackets as well (because negation takes only one argument).

Less than <, lte =<, greater than >, gte >=, equal to (assignment) =, evaluate, then assign is.

If/Else is ( condition -> then_clause ; else_clause ).

Cut operator is !. Add to expose base case (see bubblesort above).