Intersection of Two Lists of Tuples in Prolog

Question

I am trying to find the intersection of two lists in Prolog using the intersection() function. Unfortunately, my code is giving undesirable results.

Here is what I have:

RESULTA = [ ('10:00 am - 11:15 am', 'TR'), ('6:00 pm - 8:50 pm', 'T'), ('6:00 pm - 8:50 pm', 'R'), ('6:00 pm - 8:50 pm', 'M')].

RESULTB = [ ('6:00 pm - 8:50 pm', 'R'), ('3:00 pm - 3:50 pm', 'TR')].

intersection(RESULTA, RESULTB, AB).

The desired output is ('6:00 pm - 8:50 pm', 'R') however, the code returns RESULTA = AB, AB = [].

Does anyone have an idea on how to fix this issue?

EDIT

RESULTA and RESULTB are actually generated from bagof() operations. My actual code is:

1 ?- bagof((TIME, DAYS), COURSE^teaches_at('Dr. J. Leidig', COURSE, TIME, DAYS), RESULTA).
RESULTA = [ ('10:00 am - 11:15 am', 'TR'), ('6:00 pm - 8:50 pm', 'T'), ('6:00 pm - 8:50 pm', 'R'), ('6:00 pm - 8:50 pm', 'M')].

2 ?- bagof((TIME, DAYS), COURSE^teaches_at('Dr. El-Said', COURSE, TIME, DAYS), RESULTB).
RESULTB = [ ('6:00 pm - 8:50 pm', 'R'), ('3:00 pm - 3:50 pm', 'TR')

Sorry for the confusion.

Answer 1

The code in your question is really queries to Prolog.

There are 3 different queries: each query ends with a period. All three queries are completely independent, even if they share variables with same names.

To do what you want just change periods to commas:

RESULTA = [ ('10:00 am - 11:15 am', 'TR'), ('6:00 pm - 8:50 pm', 'T'), ('6:00 pm - 8:50 pm', 'R'), ('6:00 pm - 8:50 pm', 'M')],
RESULTB = [ ('6:00 pm - 8:50 pm', 'R'), ('3:00 pm - 3:50 pm', 'TR')],
intersection(RESULTA, RESULTB, AB).

Update after the question updated:

bagof((TIME, DAYS), COURSE^teaches_at('Dr. J. Leidig', COURSE, TIME, DAYS), RESULTA),
bagof((TIME, DAYS), COURSE^teaches_at('Dr. El-Said', COURSE, TIME, DAYS), RESULTB),
intersection(RESULTA, RESULTB, AB).

Answer 2

you must use that variables in a rule, like

test(AB) :-
    RESULTA = [ ('10:00 am - 11:15 am', 'TR'), ('6:00 pm - 8:50 pm', 'T'), ('6:00 pm - 8:50 pm', 'R'), ('6:00 pm - 8:50 pm', 'M')],
    RESULTB = [ ('6:00 pm - 8:50 pm', 'R'), ('3:00 pm - 3:50 pm', 'TR')],
    intersection(RESULTA, RESULTB, AB).

then consult the file. The you'll get

?- test(AB).
AB = [ ('6:00 pm - 8:50 pm', 'R')].