Prolog count list elements higher than n
prolog
Solution
If you want to make a tail-recursive version of your code, you need (as CapelliC points out) an extra parameter to act as an accumulator. You can see the issue in your first clause:
count_elems(L, N, Count) :- count_elems(L,N,0).
Here, `Count` is a singleton variable, not instantiated anywhere. Your recursive call to `count_elems` starts count at `0`, but there's no longer a variable to be instantiated with the total. So, you need:
count_elems(L, N, Count) :-
count_elems(L, N, 0, Count).
Then declare the `count_elem/4` clauses:
count_elems([H|T], N, Acc, Count) :-
H > N, % count this element if it's > N
Acc1 is Acc + 1, % increment the accumulator
count_elems(T, N, Acc1, Count). % check the rest of the list
count_elems([H|T], N, Acc, Count) :-
H =< N, % don't count this element if it's <= N
count_elems(T, N, Acc, Count). % check rest of list (w/out incrementing acc)
count_elems([], _, Count, Count). % At the end, instantiate total with accumulator
You can also use an "if-else" structure for `count_elems/4`:
count_elems([H|T], N, Acc, Count) :-
(H > N
-> Acc1 is Acc + 1
; Acc1 = Acc
),
count_elems(T, N, Acc1, Count).
count_elems([], _, Count, Count).
Also as CapelliC pointed out, your stated error message is probably due to not reading in your prolog source file.
Problem
I'm kinda new to Prolog so I have a few problems with a certain task. The task is to write a tail recursive predicate `count_elems(List,N,Count)` condition `List_Element > N, Count1 is Count+1`. My approach: ``` count_elems( L, N, Count ) :- count_elems(L,N,0). count_elems( [H|T], N, Count ) :- H > N , Count1 is Count+1 , count_elems(T,N,Count1). count_elems( [H|T], N, Count ) :- count_elems(T,N,Count). ``` Error-Msg: ``` ERROR: toplevel: Undefined procedure: count_elems/3 (DWIM could not correct goal) ``` I'm not quite sure where the problem is. thx for any help :)