Erlang - Common items from two lists - list

Question

Erlang Newbie here . Suppose I have two lists which are like this .
L1= [{'Lady in the Water',2.5},
{'Snakes on a Plane',3.5},
{'Just My Luck',3.0},
{'Superman Returns',3.5},
{'You, Me and Dupree',2.5},
{'The Night Listener',3.0}]
and
L2 = [{'Lady in the Water',3.0},
{'Snakes on a Plane',3.5},
{'Just My Luck',1.5},
{'Superman Returns',5.0},
{'You, Me and Dupree',3.5}]
I want the common ratings in a list of tuple like
[{2.5,3.0},{3.5,3.5},{3.0,1.5},{3.5,5.0},{2.5,3.5}]
My code is like this
common_rating(R1,R2)->common_rating(R1,R2,0,0).
common_rating(_X,[],M,N) ->{M,N};
common_rating(X,[Y|Y1],A,B)->
{M,R}=X,
{N,K }= Y,
case M=/=N of
true -> common_rating(X,Y1,A,B);
false -> common_rating(X,[],A+R,B+K)
end.
common_rating_final([],_R2,J) ->J;
common_rating_final([X|X1],R2,J)->
common_rating_final(X1,R2,J++[common_rating(X,R2)]).
To better understand the code
common_rating function expects a tuple of {movie,rating} and finds the same movie and rating from another list(B) and returns {rating,rating_B}
Now common_rating_final recursively goes over a list ,lets say A, and uses common_rating to find {rating_A,rating_B} for all movies that are common in A and B both
but when I run my code
my_module:common_rating_final(L1,L2,[]).
it returns me
[{2.5,3.0},{3.5,3.5},{3.0,1.5},{3.5,5.0},{2.5,3.5},{0,0}]
I can filter the {0,0} part but I think my logic is flawed but could not able to write a code which returns only the common ratings without the {0,0} part .
Please assist .

Answer 1

TL;DR
[{X2, Y2} || {X1, X2} <- L1, {Y1, Y2} <- L2, X1 =:= Y1].
Maybe the "cracks" here can find a better (more efficient, etc.) solution, but this one works. Basically it pattern matches ("deconstruct") the Ls and compares the first element of the tuples, returning the second element if the first ones happen to be equal.
The whole stuff/proof:
gorre#uplink:~$ erl
Erlang/OTP 19 [erts-8.3] [source] [64-bit] [smp:4:4] [async-threads:10] [kernel-poll:false]
Eshell V8.3 (abort with ^G)
1> L1= [{"Lady in the Water",2.5}, {"Snakes on a Plane",3.5}, {"Just My Luck",3.0}, {"Superman Returns",3.5}, {"You, Me and Dupree",2.5}, {"The Night Listener",3.0}].
[{"Lady in the Water",2.5},
{"Snakes on a Plane",3.5},
{"Just My Luck",3.0},
{"Superman Returns",3.5},
{"You, Me and Dupree",2.5},
{"The Night Listener",3.0}]
2> L2 = [{"Lady in the Water",3.0}, {"Snakes on a Plane",3.5}, {"Just My Luck",1.5}, {"Superman Returns",5.0}, {"You, Me and Dupree",3.5}].
[{"Lady in the Water",3.0},
{"Snakes on a Plane",3.5},
{"Just My Luck",1.5},
{"Superman Returns",5.0},
{"You, Me and Dupree",3.5}]
3> [{X2, Y2} || {X1, X2} <- L1, {Y1, Y2} <- L2, X1 =:= Y1].
[{2.5,3.0},{3.5,3.5},{3.0,1.5},{3.5,5.0},{2.5,3.5}]
4>
Notice that I changed the atoms into strings (it works the same way though).