CODEWITHSUNDEEP
listrecursionprologsublist
Einar Johnsen
Einar Johnsen

Reputation: 95

Prolog list within list

I have some trouble when im trying to create a list with multiple sublists. I fail to see why my code is not working as expected.

solves(_, _, 0, _).
solves(Length, List, Stopcond, NList):- length(NL, Length),
                         append([NL], List, NList),
                         write(NList), nl,
                         N is Stopcond-1, 
                         solves(Length, NList, N, NList2).

?- solves(3, [], 3, B).

I want B to be a list with three sublists, each with three intern variables. When I write Nlist, it shows:

 1. [[_G3207,_G3210,_G3213]]
 2. [[_G3222,_G3225,_G3228],[_G3207,_G3210,_G3213]]
 3. [[_G3237,_G3240,_G3243],[_G3222,_G3225,_G3228],[_G3207,_G3210,_G3213]]
 4. B = [[_G3207, _G3210, _G3213]] .

But I don't understand why B becomes only the last element of the list. I want it to be the list on line 3. Can anyone see why and what i'm doing wrong? I'm pretty new to prolog.

Upvotes: 1

Views: 390

Answers (1)

2bigpigs
2bigpigs

Reputation: 462

I think this is what you intended

solves(_, List, 0, List).
solves(Length, List, Stopcond, NList2):- length(NL, Length),
                         append([NL], List, NList),
                         %write(NList), nl,
                         N is Stopcond-1, 
                         solves(Length, NList, N, NList2).

What does your predicate try to do ( Ignoring the 3 element sublists for now )? It appends a list of length Stopcond to the 2nd argument passed (List).

How do you recursively construct a list of length TL Given a list of length L? You append a list of Length L with a list of Length 1 and try to construct a list of length TL-1 from our new list of length L+1. As a base case, You know that to construct a list of length TL from a list of length TL, you just need to return the list as is.

Your predicate returns a bigger-list consisting of Stopcond smaller-lists of length Length each. Recursively, A bigger-list of length Stopcond is a smaller-list of Length elements appended to a bigger-list of length Stopcond-1. Base case: A bigger-list of size 0 is simply an empty list.

If we were just to consider 0's instead of the 3 element sublists, 

solves(FinalList, 0, FinalList). % The result of appending a list of length 0 to the given list is the given list itself.
solves(List, StopCond, FinalList):- % Appends A list of length StopCond to List
    Length1List = [0], % Create a list of length 1
    append( Length1List, List, LengthPlus1List),
    N is N-1,
    solves( LengthPlus1List, N, FinalList).

Since you're new, It might help to write english comments against each line. 

% Base case: stopping condition satisfied. List is the final List we need
solves(_, List, 0, List). 

% Recursive case: We need to make one more list of length L and append/prepend it to List
solves(Length, List, Stopcond, NList2):- 
                         length(NL, Length), % Create smaller-list of length Length
                         append([NL], List, NList), % Append it to the list we're building up
                         %write(NList), nl,
                         N is Stopcond-1,  
                         solves(Length, NList, N, NList2). % Build Stopcond-1 more lists

Upvotes: 1

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