How can I flatten a list in Prolog, for example:
from A = [["a","b"],["c"]]
to B = ["a", "b", "c"] ?
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This too: https://stackoverflow.com/questions/9059572/flatten-a-list-in-prolog – rajashekar Nov 12 '21 at 09:21
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The only clean way to do it is to use `flatten/2`. For example, in SWI-Prolog: `?- flatten([[a,b],[c,[d]]], R). R = [a, b, c, d]. ` – User9213 Nov 13 '21 at 10:55
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https://www.swi-prolog.org/pldoc/doc/_SWI_/library/lists.pl?show=src#flatten/2 – User9213 Nov 13 '21 at 11:00
1 Answers
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A list can be flattened using maplist and append, in the following way.
A two-level list may be flattened as shown.
?- A=[["a","b"],["c"]],maplist(append,[A],[B]).
A = [["a", "b"], ["c"]],
B = ["a", "b", "c"].
A three-level may be flattened with two passes of the command.
?- A=[[["a","b"]],[["c"]]],maplist(append,[A],[B]),maplist(append,[B],[C]).
A = [[["a", "b"]], [["c"]]],
B = [["a", "b"], ["c"]],
C = ["a", "b", "c"].
Mixtures of two and three levels in a list results in an error from two passes of the command.
?- A=[[["a","b"]],[["c"]],["d"]],maplist(append,[A],[B]),maplist(append,[B],[C]).
false.
Mixtures of two and three levels in a list can only be partially flattened with one pass of the command.
?- A=[[["a","b"]],[["c"]],["d"]],maplist(append,[A],[B]).
A = [[["a", "b"]], [["c"]], ["d"]],
B = [["a", "b"], ["c"], "d"].
Lucian Green
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It is a terrible answer but since you have answered it yourself I am wondering if you really _want_ any help at all. You seem to _need_ help but this is a different topic. – User9213 Nov 13 '21 at 10:56
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You can also use the custom predicate foldr to partially flatten lists, as follows: foldr(Function,A,L,B):- reverse(A,C),foldl(Function,C,L,B),!. and foldr(append,A,[],B). – Lucian Green Nov 13 '21 at 23:24