Question

I trying to write a function – noPrime(X,Result) which gives a list of all the non-primary number in range [X-1,2] .

For example –

noPrime(9,Result).
Result = [8,6,4]

So far I tried the follow –

nprimes(4,[]) :- !.
nprimes(X,[H|Rest]) :- H is X-1,nprimes(H,NewRest),
    ( isPrime(H) -> Rest = NewRest; Rest = [H|NewRest]).

But it gives –

Result = [8, 8, 7, 6, 6, 5, 4, 4].

Seems like it duplicate the non-primary and gives one time the primary .

isPrime – is my own function which works well .

Can you detect what is the wrong here and how to correct it ?

Answer 1

You are always adding the element at the head of the second argument of nprimes/2.
Then, if the element is prime the second argument will still have the element; and if the element was not prime it will be added twice.

It should read:

nprimes(X,Rest) :- H is X-1,nprimes(H,NewRest),
    ( isPrime(H) -> Rest = NewRest; Rest = [H|NewRest]).