CODEWITHSUNDEEP
listprolog
ThunderWiring
ThunderWiring

Reputation: 738

How to append number to list in prolog?

I am very new to Prolog. My goal is to append integers to a list, up to a bound: if the function receives N, it outputs a list [N, N-1, ... , 1].

Here is my code: 

myAppend(0, L) :- append([],[], L).
myAppend(N, L) :- append([N], [], L), N1 is N - 1, myAppend(N1, L).

invoking the function above returns false for every N=\=0:

51 ?- myAppend(0,L).
L = [] ;
false.

52 ?- myAppend(2,L).
false. <-------------------- was expecting L = [2, 1]

53 ?-

However, when i changed my function to (put a dot . instead of , after call to append in the 2nd rule):

myAppend(0, L) :- append([],[],L).
myAppend(N,L) :-  append([N], [], L). N1 is N - 1, myAppend(N1, L).

I got the following output:

51 ?- myAppend(0,L).
L = [] ;
false.

52 ?- myAppend(4,L).
L = [4] . <-------------- was expecting [4, 3, 2, 1]

53 ?-

I am unable to understand why in the 1st implementation I was receiving false, although the logic behind it is correct?

Upvotes: 0

Views: 1752

Answers (1)

max66
max66

Reputation: 66230

What about

myAppend(0, []).

myAppend(N, [N | L]) :-
  N > 0,
  N1 is N - 1,
  myAppend(N1, L).

?

Upvotes: 1

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