Combining lists with arithmetic

[Infty]

Greetings,

This is my first post here so I would like to greet you all.
I started learning prolog and I must admit I'm having problems since its a totally different idea of programming, totally different from imperative type of thinking.

Here is my idea, actually i made an example i would like to solve in order to improve my knowledge.

If I'm given a number X, X>10 i would like to split its digits and store them in a list.

Example:
digits_in_list([],153,X).
X=[1,5,3].

I wrote some predicate but since I lack understanding how precisely prolog recursion works i cant menage to solve this.

My predicate:

add(X,[],[X]).
add(X,L,[X|L]).

digits_in_list([],X,[X]): -X<10.
digits_in_list([],X,L): -X1 is X mod 10,add(X1,L,Y),X is X //10.

 

[kahleen]

Your first mistake is here:

X is X // 10

In Prolog this statement is different than in imperative languages. In an imperative language, X will be given a new value. In Prolog it tests if X is equal to X // 10 and it usually isn't

Your second mistake is that you don't use recursion in digits_in_list, the process stops after only one step.

To solve your problem you would need something like this:

digits_in_list(X, [X]) :- X < 10, !.
digits_in_list(X, L) :-
    Y is X mod 10,
    X1 is X // 10,
    digits_in_list(X1, L1),
    append(L1, [Y], L).

If X = 153, you will compute X1 = 15 and Y = 3, you will solve the problem for X1 and will obtain L1 = [1, 5] and then you will append Y = 3 to the result, obtaining [1, 5, 3]