CODEWITHSUNDEEP
prolog
Rydzio
Rydzio

Reputation: 35

Prolog lists inside the lists

I'm new to Prolog and what I'm trying to achieve is to define the tower-like type which contains lists of natural numbers. Each "floor" is supposed to have the same amount of "apartments" and I don't know how can I check it.

Upvotes: 1

Views: 61

Answers (1)

Duda
Duda

Reputation: 3736

Assuming with "type" you mean a structure which can hold your data, I suggest using a list of lists, like this:

[[101,102,103],[201,202,203],[301,302,303]]

So now you need to check if each "floor" (sublist), has the same amount of elements. Counting the elements in a list is rather simple:

len([],0).
len([H|T],N1) :- 
    len(T,N), 
    N1 is N+1.

Which reads: an empty list [] has 0 elements. A list which can be splittet into a head element H and a rest list T has the length N1, if the length of T is N and N1 is calculated by N+1.

So what is missing? You need to go through all your sublists and get the length of the sublist. If the length of two sublists are different, then fail. If you reach the end with allways the same number, success. So the pattern would look like this:

isHotel(R):-
    isHotel(R,_).

Add a placeholder for the counting, actual number of the second argument does not matter at this point.

isHotel([],_) .

If you got to a point where there are no floors left, accept for any number.

isHotel([CurrentFloor|T],N) :- 
    len(CurrentFloor,N),
    isHotel(T,N).

If there are floors left (floorlist can be devided into the current floor CurrentFloor and all floors above T), count the rooms at this floor and go check with the left over floors T. The first time this is runs, the number N of rooms will be calculated and forwarded to every other call - therefore for the first time the calculation is just a calculation, in every other ecxecution it is a check as well. This leads to rejects if two lusts do not match in size. Please note this code does not check if the input is a list of lists of numbers.

Checked with SWISH:

?- isHotel([[101,102,103],[201,202,203],[301,302,303]]).
true.

?- isHotel([[101,102,103],[201,202,203],[301,302,3,03]]).
false.

?- isHotel([]).
true.

Upvotes: 1

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