Prolog - Cartesian product calculator

Question

I need to create a Cartesian product calculator for Prolog. It should work like this: Input: `product([1,2,3], [a,b], X).` Output: `X = [[1,a],[2,a],[3,a],[1,b],[2,b],[3,b]].` I know there are examples on the Internet, but I wanted to write something myself. This is my code and I think it's pretty close, but for some reason it doesn't exactly work. Any ideas, guys? ``` % call new with 4 parameters (so we can keep List1 in memory) product(L1,L2,L3):- product(L1,L2,L3,L1). % stop when both List1 and List2 are empty product([], [], [], []). % first list is empty, recreate it and work it again with the next element of second list (and shorten memory) product([], [_|T2], List3, [H4|T4]):- product([H4|T4], T2, List3, T4). %go through first list and always first element of second list to our answer product([H1|T1], [H2|T2], [[H1,H2]|T3], List4):- product(T1, [H2|T2], T3, List4). ```

Accepted Answer

As said by Coder (+1), you should change the terminal clause from

```
product([], [], [], []).
```

to

```
product(_, [], [], _).
```

But isn't enough.

You should change the third clause from

```
product([], [_|T2], List3, [H4|T4]):-
    product([H4|T4], T2, List3, T4).
```

to

```
product([], [_|T2], List3, L4):-
    product(L4, T2, List3, L4).
```

I mean: is an error to consume the saved list 1.

With your version, from

```
product([1,2,3,4,5], [a,b,c,d], X),
```

you get only

```
[[1,a],[2,a],[3,a],[4,a],[5,a],[1,b],[2,b],[3,b],[4,b],[5,b],[2,c],[3,c],[4,c],[5,c],[3,d],[4,d],[5,d]]
```

That is: you loose `[1,c]`, `[1,d]` and `[2,d]`.

Prolog - Cartesian product calculator

Solution

Problem