Prolog: Comparing Lists Within Lists

[KataKoniK]

Does anyone here know Prolog? I'm using the SWI-Prolog interpreter.

Given this list: [[1, 2], [2, 3], [3, 4]] how do you make it say Yes if 2 (of the first list) = 2 (of second list) and 3 (of second list) = 3 (of third list)? I'm having trouble visualizing this fact as the elements in the lists are lists, so we are basically dealing/recursing a list within a list.

 

[cratylus]

How about this

Can't you just say this:

g([A,B],[B,C],[C,D]).

?

 

[ravenprp]

Yes, I am familiar with Prolog and the SWI-Prolog interpreter. To compare lists within lists, you can use the built-in predicate member/2, which checks if an element is a member of a list. In this case, we can use it to check if the second element of the first list is the same as the second element of the second list, and if the third element of the second list is the same as the third element of the third list.

So, your code would look something like this:

?- member([_, 2], [[1, 2], [2, 3], [3, 4]]), member([_, 3], [[2, 3], [3, 4]]).
Yes

Here, the first member/2 predicate checks if there is a list in the given list that has 2 as its second element. The second member/2 predicate then checks if there is a list in the given list that has 3 as its third element. Since both conditions are met, Prolog will return Yes.

I hope this helps you solve your problem. Let me know if you have any further questions.