Prolog: get and append every third item from a list to a new list -


with following piece of code in prolog managed extract every third item list:

third([_,_,z|l], z).     third([_,_,c|l], y) :-    third(l, y).

however, still need append every item extracting new list, , create list "every third" items. appreciate hint or help! thank you.

how doing this?

list_thirds([]        , []). list_thirds([_]       , []). list_thirds([_,_]     , []). list_thirds([_,_,e|es], [e|xs]) :-    list_thirds(es, xs).

sample queries using sicstus prolog 4.3.2:

| ?- list_thirds([], xs). xs = [] ? ; no  | ?- list_thirds([a,b,c], xs). xs = [c] ? ; no  | ?- list_thirds([a,b,c,d,e,f], xs). xs = [c,f] ? ; no  | ?- list_thirds([a,b,c,d,e,f,g], xs). xs = [c,f] ? ; no

how going "other direction"?

| ?- list_thirds(list, [x,y]). list = [_a,_b,x,_c,_d,y]       ? ; list = [_a,_b,x,_c,_d,y,_e]    ? ; list = [_a,_b,x,_c,_d,y,_e,_f] ? ; no                                     % terminates universally

last, @ answer sequence general query:

| ?- list_thirds(es, xs). es = []                 , xs = []      ? ; es = [_a]               , xs = []      ? ; es = [_a,_b]            , xs = []      ? ; es = [_a,_b,_c]         , xs = [_c]    ? ; es = [_a,_b,_c,_d]      , xs = [_c]    ? ; es = [_a,_b,_c,_d,_e]   , xs = [_c]    ? ; es = [_a,_b,_c,_d,_e,_f], xs = [_c,_f] ? ; ...

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