Question

Having – myMultiply/3

myMultiply([],X,[]) :- !.
myMultiply([H|T],X,Result) :-
    myMultiply(T,X,NewRest),
    Elem is H*X,
    Result = [Elem|NewRest].

Which which mulply each element at the left list to the nun at 2nd arg and return Result .

For example –

myMultiply([1,4,5],4,R).
R = [4,16,20].

And tlm/3

tlm(List1,[],_) :- !.
tlm(List1,[H2,Rest2],Result):-
    myMultiply(List1,H2, Elem),
    tlm(List1,Rest2,NewResult),
    Result = [Elem|NewResult].

Which there I trying to make myMultiply/3 to the left list on each element of the right list and retun Result .

For example –

?- tlm([3,4,2],[4,7,8],R).
R = [[12,16,8],[21,28,14],[24,32,16]].

Generally the idea is the get the Elem list and concat him to NewResult .

My problem is that tlm/3 returns false –

 ?- tlm([3,4,2],[4,7,8],R).
false. 

Instead of the above example .

What I did wrong here ?

Answer 1

There are 2 problems with your code.

Here I’ve done a basic correction, and commented about useless cuts.

myMultiply([],X,[]) :- !. % the cut is useless here
myMultiply([H|T],X,Result) :-
    myMultiply(T,X,NewRest),
    Elem is H*X,
    Result = [Elem|NewRest].

tlm(List1,[],[]) :- !. % the cut is useless here
tlm(List1,[H2|Rest2],Result):-
    myMultiply(List1,H2, Elem),
    tlm(List1,Rest2,NewResult),
    Result = [Elem|NewResult].

and the result is…

?- tlm([3,4,2],[4,7,8],R).
R = [[12, 16, 8], [21, 28, 14], [24, 32, 16]].

Could you spot why cuts are useless?