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Flatten a list of lists in Prolog

I have a list of lists:

 L = [[[a,b],[d,e]],[[m,f],[p,o]],[[r,l],[v,d]]].

I want to write a function using Prolog that flattens it in such a way that it looks like this:

 L = [[a,b],[c,d],[m,f],[p,o],[r,l],[v,d]].

Any suggestions? Thank.

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2 answers


Using a predicateappend/2

:

? - L = [[[a, b], [d, e]], [[m, f], [p, o]], [[r, l], [v, d]]], append ( L, R).
L = [[[a, b], [d, e]], [[m, f], [p, o]], [[r, l], [v, d]]],
R = [[a, b], [d, e], [m, f], [p, o], [r, l], [v, d]].

You should look at the implementation of SWI-Prolog and copy it if you need. Leave must_be/2

if you have to do this in GNU-Prolog.



But if you need it because of findall/3

, keep in mind that there may also be available findall/4

(not for GNU-Prolog, but it has SWI-Prolog):

$ swipl
Welcome to SWI-Prolog (Multi-threaded, 64 bits, Version 7.3.2-25-gf8c39d8)
Copyright (c) 1990-2015 University of Amsterdam, VU Amsterdam
SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software,
and you are welcome to redistribute it under certain conditions.
Please visit http://www.swi-prolog.org for details.

For help, use? - help (Topic). or? - apropos (Word).

? - findall (X, between (1,3, X), Xs, Rest), findall (Y, between (7,11, Y), Rest).
Xs = [1, 2, 3, 7, 8, 9, 10, 11],
Rest = [7, 8, 9, 10, 11].

Just about any situation where you need to flatten a list can be avoided with diff lists.

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possible_moves(Tray,PossibleMoves):-
    findall([J,1,X,Y],possible_move(Tray,[J,1,X,Y]),T1),
    findall([J,2,X,Y],possible_move(Tray,[J,2,X,Y]),T2),
    append(T1,T2,Res),
    findall([J,3,X,Y],possible_move(Tray,[J,3,X,Y]),T3),
    append(Res,T3,PossibleMoves).


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