my problem is: While learning Prolog i wanted to make a NxN Sudoku solver. This solver will get the input like
[[1,2,3,4],[3,4,1,2],[2,3,4,1],[4,1,2,3]]
Where some of them might be variables. The solver has to solve that Sudoku. The problem is way smaller:
firstElementsOf([],_).
firstElementsOf([[X|_]|Rest2],Y) :-
firstElementsOf(Rest2,Y2),
append([X],[Y2],NotFlat),
flatten(NotFlat,Y).
This should be the beginning of checking, if every column has distinct numbers. The Y from firstElementsOf should contain only the first elements of the given rows. In the Example:
[1,3,2,4]
Sadly, thanks to append, it always adds another empty space to the Y list.
It gives:
[1,3,2,4,_1320]
Question1: Is there a way to get rid of that _1320?
Question2: Is this even right? Will there be a way to get the 2nd and 3rd elements of the Input with that?
For question 1: I suppose the error is in
firstElementsOf([],_).
I think should be
firstElementsOf([],[]).
Off topic: are you sure that you can't simply write the other clause as follows?
firstElementsOf([[X|_]|Rest2],[X|Y]) :-
firstElementsOf(Rest2,Y).
For question 2: I propose a more general predicate: the following getPosList/3 with support of getPosElem/3
getPosElem([H | _], 1, H).
getPosElem([_ | T], Pos, H) :-
Pos > 1,
Pm1 is Pos - 1,
getPosElem(T, Pm1, H).
getPosList([], _, []).
getPosList([H | T], Pos, [E | L]) :-
getPosElem(H, Pos, E),
getPosList(T, Pos, L).
It extract a list of all elements in position Pos, so
getPosList([[1,2,3,4],[3,4,1,2],[2,3,4,1],[4,1,2,3]], 1, L),
it's equivalent to firstElementOf([[1,2,3,4],[3,4,1,2],[2,3,4,1],[4,1,2,3]], L) and extract [1, 3, 2, 4],
getPosList([[1,2,3,4],[3,4,1,2],[2,3,4,1],[4,1,2,3]], 2, L),
extract [2, 4, 3, 1],
getPosList([[1,2,3,4],[3,4,1,2],[2,3,4,1],[4,1,2,3]], 3, L),
extract [3, 1, 4, 2],
getPosList([[1,2,3,4],[3,4,1,2],[2,3,4,1],[4,1,2,3]], 4, L),
extract [4, 2, 1, 3] and
getPosList([[1,2,3,4],[3,4,1,2],[2,3,4,1],[4,1,2,3]], 5, L),
or a number greather than 5, return false