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Asked: 2026-06-18T01:35:34+00:00

Say I need a predicate rep( ?List , ?Times , ?TList ) which is

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Say I need a predicate rep(?List, ?Times, ?TList) which is true iff List is repeated Times times in TList (for example, rep([a,c],2,[a,c,a,c])). It should work as long as two of the arguments are instantiated. Here a somewhat working version:

rep(_,0,[]).
rep(List,1,List).
rep(List,Times,TList) :- integer(Times), Times>1,
    succ(RemTimes,Times), append(List,RemList,TList),
    rep(List,RemTimes,RemList).
rep(List,Times,TList) :- var(Times),
    append(List,RemList,TList),
    rep(List,RemTimes,RemList), !,
    succ(RemTimes,Times).

Two questions:

  1. Isn’t there some built-in (that I am unable to find) that does that?
  2. Is there a more straight-forward way of doing this? Like getting rid of the last clause? It is necessary because I couldn’t find a way to express the relationship between Times and RemTimes when Times is not instantiated.
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1 Answer

  1. Editorial Team

    I don’t know about a specialized builtin. Here a procedure using the generative capability of length/2

    rep(List, Times, TList) :-
    ( var(Times) ; Times > 0 ),  % after joel76' comment...
    ( var(List) ; is_list(List) ), % after false' comment...
    ( var(TList) ; is_list(TList) ), % idem...
    length(List, LA), LA > 0,
    length(TList, LT), LT > 0,
    Times is LT / LA,
    findall(List, between(1, Times, _), [List|Ls]),
    append([List|Ls], TList), !.

    the final cut avoid looping when any list is free.

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