CODEWITHSUNDEEP
prolog
user236501
user236501

Reputation: 8648

Prolog how to store first 3 elements in a list into another list

I tried something like this: said path = [[1,2],[2,3],[3,4],[5,6]]

first_three(N,[H|T],[H|_]):- not(N=0),N is N-1, first_three(N,T,L).

then I call first_three(3, path, Y) but it return no.

Upvotes: 1

Views: 1082

Answers (2)

Giulio Piancastelli
Giulio Piancastelli

Reputation: 15807

First of all, N is N - 1 can't possibly succeed - N is not a variable in the imperative programming language sense, so you don't store values in it, it can be assigned only once, so to speak - so you may want to introduce a new variable and call something like N1 is N - 1, and subsequently use N1 in the recursive call to first_three. Then, the last argument to that recursive call comes out of nowhere, so you may need to pay attention to it too. As a last suggestion, try to split the predicate in two clauses: the first to manage the elementary basic case where N is 0, the second to manage the meaty cases, building on top of the other.

Oh, and since N is a variable quantity, I would suggest to use a different name rather than the misleading first_three for your predicate.

Upvotes: 1

gusbro
gusbro

Reputation: 22585

You need to add a base case (when N = 0). And you also need to instantiate a new fresh variable to assign the predecesor of N, and finally you also need to return the result of the recursion (the [H|L] in the head of the clasuse):

first_three(0,_,[).
first_three(N,[H|T],[H|L]):- 
  N\=0,
  N1 is N-1, 
  first_three(N,T,L).

Of course, you can also just write something like this:

first_three([One,Two, Three|_], [One,Two, Three]).

and call it:

first_three(Path, Y).

Upvotes: 1

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