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listprolog

Prolog: replace the nth item in the list with the first

I'd like to have a Prolog predicate that can replace the nth item in the list with the first.

Example:

% replace(+List,+Counter,-New List, %-First Item).
?- replace([1,2,3,4,5],3,L,Z).

L = [1, 2, 1, 4, 5]
Z = 1

I don't know how to do this. Thanks for your help!

Solution

  • Try using the predicate nth1(Index, List, Item, Rest):

    ?- nth1(3, [1,2,3,4,5], Item, Rest).
Item = 3,
Rest = [1, 2, 4, 5].

?- nth1(3, List, 1, [1,2,4,5]).
List = [1, 2, 1, 4, 5].

    Putting it all together:

    replace(List, Index, NewList, First) :-
    List = [First|_],
    nth1(Index, List, _Removed, Rest),
    nth1(Index, NewList, First, Rest).

    Examples:

    ?- replace([1,2,3,4,5], 3, L, Z).
L = [1, 2, 1, 4, 5],
Z = 1.

?- replace([one,two,three,four,five], 4, NewList, First).
NewList = [one, two, three, one, five],
First = one.