%Source: https://sites.google.com/site/prologsite/prolog-problems %query:drop(g,g,f). % 1.16 (**): Drop every N'th element from a list % drop(L1,N,L2) :- L2 is obtained from L1 by dropping every N'th element. % (list,integer,list) (?,+,?) drop(L1,N,L2) :- drop(L1,N,L2,N). % drop(L1,N,L2,K) :- L2 is obtained from L1 by first copying K-1 elements % and then dropping an element and, from then on, dropping every % N'th element. % (list,integer,list,integer) (?,+,?,+) drop([],_,[],_). drop([_|Xs],N,Ys,1) :- drop(Xs,N,Ys,N). drop([X|Xs],N,[X|Ys],K) :- K > 1, K1 is K - 1, drop(Xs,N,Ys,K1).
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