CODEWITHSUNDEEP
listloopsindexingprolog
user3009573
user3009573

Reputation: 23

Get Index of an element/ element by index in List of Lists

How could I achieve something similar to the nth0/3 predicate on a list of lists ?

I would like to be able to tell what is the index of an element in a list of lists but only interested in the inner list index. What I mean is lets say I have the following predicate indexOf([[a,b,c], [d,e,f], [g,h]], d, Index) and it returns Index=0 because d is the first element of the inner list. indexOf([[a,b,c], [d,e,f], [g,h]], c, Index) should return Index=2 since it is the 3rd element of the inner list.

What I have done so far is the following:

iterateOuter([]).

iterateOuter([[H|T]|LIST], Elem,Idx):-indexOf([H|T],Elem, Idx2),(Idx is Idx2;iterateOuter(LIST, Elem,Idx)).

indexOf([_|Tail], Element, Index):-indexOf(Tail, Element, Index1), Index is Index1+1. 

indexOf([Element|_], Element, 0).

if I invoke iterateOuter([[a,b,c],[r,t,o]],c,Ind).

It will give me Ind = 2 and that's fine but if I say: iterateOuter([[a,b,c],[r,t,o]],r,Ind).

it will simply give me false since it is only giving results on the first inner list.

Upvotes: 2

Views: 128

Answers (2)

false
false

Reputation: 10122

If you have nth0/3 already, you have all you need:

?- Xss = [[a,b,c],[r,t,o]], nth0(I, Xss, Xs), nth0(J, Xs, c).
   Xss = ["abc","rto"], I = 0, Xs = "abc", J = 2
;  false.
?- Xss = [[a,b,c],[r,t,o]], nth0(I, Xss, Xs), nth0(J, Xs, r).
   Xss = ["abc","rto"], I = 1, Xs = "rto", J = 0
;  false.

Upvotes: 1

Shon
Shon

Reputation: 4098

Since you're only interested in the index of the inner list, you can just use member/2 instead of nth0/3 to get the sublist, as False suggested. Each sublist is a member of the list Ls, so you only need to find if E is the nth element of a member: 

nth_of_a_sublist(N, Ls, E) :-
    member(L, Ls),
    nth0(N, L, E).

To check for the nth_of_a_sublist on all lists, you can use findall/3.

But False's solution is nicer, I think, because it gives you the index of the list that contains the element, which you can ignore if you don't need it.

Upvotes: 1

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