Question

How to write a predicate doublelist(L, R) with L is list of numbers, R is also a list and has the same length as L and each element is doubled up.

Example:

?- doublelist([-1, 0, 3], L).  
L = [-2, 0, 6].

I’m a prolog beginner, I confuse to write one myself. Please help.

Answer 1

Start by writing down your example:

double_list([-1,0,3],[-2,0,6]).

This is a perfectly valid Prolog. You can use it in all kinds of directions:

?- double_list([-1,0,3],L).
L = [-2,0,6].

?- double_list([-1,0,3],[-2,0,6]).
Yes.

So, now you need to make it more general. As a first step, write down what it means:

double_list([-1,0,3],[-2,0,6]):-
    (-2) is 2*(-1),
    0    is 2*???,
    6    is ?????.

Next step, substitute variables for numbers:

double_list([-1,0,3],[-2,0,6]):-
    A1 is (-1), B1 is (-2), B1 is 2*A1,
    A2 ????,    B2 ????,    B2 is 2*??,
    A3 ????,    B3 ????,    B3 ???????.

Can you continue from this? Do you see the pattern?

double_list([X1,X2,X3],[Y1,Y2,Y3]):-
    ......

Now you have a predicate that works for all lists of length 3. Then,

double_list([X1 | T1], [Y1 | Z1]):-
    ......

double_list( [], ZN) :- ZN = ???? .