CODEWITHSUNDEEP
listprolog
STee
STee

Reputation: 13

Making two lists from a given list

I am struggling to get my head around Prolog lists.

Here's my problem: I need to take in a list and two variables and store the odd elements in list A and the even elements in list B.

I got this but its not giving the results im looking for 

store(X, [], []).
store([X,Y|Z],[X|_],[Y|_]):-store(Z,X,Y).

Result should be: 

where ?- store ([a,b,c,1,2,3], A, B).
         A = [b,1,3].
         B = [a,c,2].

Upvotes: 1

Views: 1649

Answers (1)

CapelliC
CapelliC

Reputation: 60034

to build additional lists from a list argument do a recursive visit of it, storing elements where appropriate. See if you can complete this snippet

odd_even([], [], []).
odd_even([N|Ns], [N|Odds], Evens) :-
   odd(N),
   !, odd_even(Ns, Odds, Evens).
% here another odd_even for evens

odd(N) :-
  ... % use rem/2 to find the integer remainder of N is 1

edit: to move elements from even places in a list and elements at odd places in another (usually called a split), the visit just places them on two additional arguments: we need one more termination rule, because now we consider two arguments together from the list to be splitted.

split([], [], []).
split([X], [], [X]).
split([X,Y|R], [X|Xs], [Y|Ys]) :-
   split(R,Xs,Ys).

there is a case to consider: if odd vs even must be 'counted' from the end of list. Then we should swap (eventually) the lists when done...That would require to add 2 more arguments to the recursive visit, or count the num.of.elements to decide beforehand where to aplce them...

Upvotes: 1

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