Mega-R1. Rule-Based Systems Video Lecture | Artificial Intelligence: A Fundamental Guide - AI & ML

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hi everyone a few of you may know me
from my tutorial but if you don't I am
Marx after I'm the senior TA here at
6.34 and these Friday mega recitations
are going to be a little bit different
than any of the other things in the
class all right so what is mega
recitation mega recitation is in this
lecture style room but I hope to make it
more like a recitation or a tutorial by
unfortunately for some of you perhaps
calling on you guys to participate
during the mega recitation as we attempt
to work with some of the material that
patrick has sort of given us the big
picture for in the lectures and if
you've gone to recitation so far you may
have received a little bit of enrichment
into how the algorithms work well here
in mega recitation we're going to figure
out how you can work the algorithms and
that's very important because in this
class being able to work the algorithms
by a hand in some of these examples I
put up here or like Patrick sometimes
does in lectures to teach you the
algorithms is a crucial way of
demonstrating that you understand how
the algorithms work and we use it on a
quiz the way we use it on the quiz is we
well this is an old quiz problem it's
from last year it tripped up a lot of
people so we're going to go over it
today I'm going to tell you all the
tricks that tripped up a lot of people
in addition to emphasizing tricks that
I've picked up over the gosh is has been
three years of teeing this course before
so that you will not fall prey to any of
these tricks
hopefully I say hopefully because some
of these tricks I talked about atmega
recitation last year and people still
got tricked by them I guess maybe some
people didn't come to mega recitation or
they were just very tricky tricks so be
sure to come to mega recitation pay
attention to the pay attention to the
tricks pay attention to how these things
are solved ask questions if you're not
sure and when we get done the hour
together I'm hoping that everyone's
going to do really well on the rules
part of the first quiz so without
further ado
ah if there are no questions on that
which I don't think there probably will
be let's move on to the problem at hand
as some of you may have noticed it is a
Harry Potter based problem one of our
days last year was big Harry Potter fan
so what we have here are a series of
rolls and a series of assertions now if
you happen to be real go-getter and have
already looked at all the past quizzes
online and you may have already seen
this problem and if you really really
have attention to detail you may notice
something a little bit different in the
way I wrote it as compared to the way it
was written online in fact you may if
you have a lot of attention to detail
but you're not really in on scheme or
Lisp you may think that my the way I
wrote it is much easier to understand
and in case you can't understand the
other way and in case one of the TAS
decides to write it in a lisp like way
again let me explain if you look at one
of my rules say if X is ambitious and X
is a squib then X has a bad term that's
rule 0 up at the top that's written in
sort of what we call in fix notation
it's easy it's simple all the operators
are in the order you would expect 2 plus
3 equals 5 there's another kind of
notation that is sometimes used on
previous quizzes and by sometimes a good
number of times actually I'm not sure
why we still use it that way because of
the fact of the classes we've done to
Python but we do sometimes and that's
called prefix notation in prefix
notation the operator comes first so for
instance plus 2 3 equals 5 the plus
comes first the function gain comes
first in this case the function is plus
so if we were writing this in prefix
notation in fact the way I received it
when it said if and X is ambitious X as
a squib then X has a bad term the and
went first and there's a parenthesis
which included everything that was
scoped under the and does anyone have
any questions about prefix notation in
case it comes up just remember if things
start being written in a really weird
way with lots and lots and lots of
parentheses whatever is outside of the
parentheses whether it be + or and or is
an operator that acts on the things that
are inside however we're not going to
deal with that now because I think the
most important thing is to understand
the rules and the less important thing
is to understand prefix notation and you
can do a problem at home that's written
in prefix notation to test yourself on
that so what rules do we have here
we've got rule 0 through 5 for some
reason they are labeled with P's
they are often labeled with either P's
or ours the sort of secret on why
they're labeled with P's is that one
year somebody labeled them with P's for
like proposition or something like that
and then other J's looked at that test
and sometimes also labeled it with P's
and it's sort of down the line continue
to be labeled with P's and sometimes
with R's
so these peas are the six rules the
first rule is if X is ambitious and X is
a squib then X has a bad term
so what's the deal T's question mark X's
the question mark is before an X or
perhaps a Y down here indicate that
there's a variable waiting to be bound
we're not assuming there's a harry
potter character named x and only if
that question mark x the mystery
character only if mystery x character
has ambitious can they can that person
possibly have a bad term what we're
seeing is any character in the Harry
Potter universe or not in the Harry
Potter universe may be a rhinoceros can
fit into that X so for instance if a
rhinoceros is ambitious and a rhinoceros
is a squib then a rhinoceros has a bad
term that rules saying for any X um this
is true and it's very important how we
treat the question mark X when how we
bind the question mark X when we do both
back tuning and forward chaining I'll
get back to that because um some people
made some very very small mistakes that
really messed up a lot of their forward
and backward chaining last year it was
actually very tricky Jeremy the TA who
wrote this was was very clever and it
makes it a really great case study for
you guys alright so rule one
if X lives in Gryffindor Tower then X is
a protagonist by the way for conciseness
I'm going to be using GT for Gryffindor
Tower when I write these later on and
I'll use SD for Slytherin dungeon
speaking of which rule 2 if X lives in
Slytherin dungeon then X is a filling
X is ambitious why are there two things
here well after the if we have what we
call the antecedent that's something
that needs to be true
in order for this rule to match after
the then we have what is called the
consequent and in this case there are
two consequence anyone who lives in
Slytherin dungeon is automatically a
villain and also ambitious so you can
sort of think of there being sort of an
end therefore for the purposes of both
of those assertions would be added to
the knowledge base Rule three if X is a
protagonist wor X is a villain and X is
ambitious then X studies a lot by the
way the scope is for the oops the scope
for this just to be sure that we're
clear is this so we need them to be a
protagonist or a villain and no matter
what they have to be on Bish's rule for
if X studies a lot and X is a
protagonist X becomes Hermione's friend
and rule 5 if X Nobbs Y and X lives in
Gryffindor Tower and y lives in
Slytherin dungeon then X has a bad term
so those are our six rules that we can
use to understand Jeremy's world of the
Harry Potter universe and we also start
off with four assertions
let me not underestimate the value of
always looking to the assertions it's
one of my white star ideas that are up
here on the board let me see no not that
it's perfect
always check the assertions before using
a rule this really trip people up last
year and you'll see why because we're
doing last year's problem our four
assertions that we start with assertion
zero Millicent lives in Slytherin
dungeon assertion one
Millicent is ambitious assertion one is
what trip people up so remember that
Millicent is ambitious assertion to
Seamus lives in Gryffindor Tower at
assertion 3 Seamus togs Millicent
so those are those are our four
assertions that we've already started
with now the two things we're going to
have to do are backward chaining and
forward chaining now when you guys learn
these two backward chaining forward
chaining raise your hand if you thought
forward chaining was harder than
backward chaining aha I knew it
I can prove whatever point I want
because no one wants to raise their hand
because I also think backward chaining
is harder than forging raise your head
if you think backward chaining is harder
than forward chaining first of all we
have a good number of people second of
all since no one wants to raise their
hand I could just s it the other way
that's a pro tip if you were ever with
the large group of people you can prove
any point you want by asking the other
direction no one will raise their hand
so I agree that backward chaining is
higher than forward chaining and I
disagree with Patrick that we're going
to get out of here early so let's start
with backward chaining first so that we
make sure that we spend the bulk of our
time with it
it's forward chaining well you know you
just go through you go through pretty
methodically and add new rules backward
chaining you have to draw this crazy
tree there's a lot of places to get lost
in the middle of the road so let's do
some backward chaining and to do that
I'll do it over here on the left side so
I when we're doing backward chaining we
have to remember a few things they're
written directly on the quiz so you're
not gonna have to worry about that but
there's still pretty important so
actually all right on here first I'm
going to read them off so when working
on a hypothesis the backward chainer
tries to find a matching assertion in
the list of assertions first if no
matching assertion is found the backward
chainer will try to find a rule with a
matching consequent a rule that has
something in two then that can prove the
assertions trying to figure out so for
instance if I was doing backward
chaining on Seamus snogs Millicent what
happens I'm immediately done because
there
Saoirse insertion 3 it says shared
Seamus Nagas Millicent I proved it I'm
done I'm happy we can live we can go
home unfortunately that's not what the
possesses to do well let's say that
instead we were supposed to say I don't
know Seamus is a protagonist well then
we wind up looking through here and we
would look at the fact that rule one can
prove that someone is a protagonist and
we'd recurse and try to prove whatever
is in the antecedent of rule 1 which is
ok well does he live in Gryffindor Tower
assertion - he does so that's some
really quick and if that was too fast
don't worry we're going to go step by
step with the actual problem but I just
wanted to give two really easy problems
really quickly how it's going to do it
let's go step by step with a real
problem let's keep in mind the backward
chainer never adds new assertions to the
list of assertions and if you have a
tie-break you always order based on rule
order first p0 do p5 and if you can fire
if the rule same rule matches with more
than one thing in your list of
assertions then you tie break based on
the order of the assertions ok of very
important disambiguation and tiebreaking
our big place to get messed up so we're
going to try to prove that Millicent
becomes Hermione's friend all right and
so I'm going to abbreviate some of the
things but not not for the very first
line so Millicent becomes Hermione's
friend we start drawing a goal tree now
you guys are going to learn exactly what
these mean very soon in fact next
lecture but for now trust me when I say
these goal trees are depth-first and
I'll explain what that means because
some people have messed themselves up
and spent more time than they needed to
by treating the goal tree in a different
way now Millicent becomes Hermione's
friend
let's pretend we're the backward chainer
we're trying to prove that this is true
or disprove and say well it's definitely
not true with what we have so let's see
I'm now going to make you guys help and
people in the back think they won't be
called on so I often like to call the
people in the back what do you think is
the first thing we should do very first
thing yes look for imagining insertion
excellent do we have a matching search
in everyone no we don't that would be
the world's easiest quiz problem we do
not have a matching insertion great so
since we don't have a matching insertion
what now that's right and you see any
rule that could prove that no it becomes
Hermione's friend right you can see in
p4x which can be anybody is capable
anyone is capable of being Hermione's
friend great so p4 is our rule of the
hour so we're going to use p4 and when
we use p4 to prove that Millicent
becomes Hermione's friend we're going to
have to add something or another to gold
tree so let's see what do we have to add
to the gold tree that's right does
everyone see that in order for her to
become Hermione's friend by rule 4 of to
see if she studies a lot and as a
protagonist question mm-hmm okay that's
a very good question you're going to get
screwed up if you look at the antecedent
in backward chaining first it's
backwards partially for a reason you
need to look at the consequent now why
is that well let's say there was a rule
rule six that said if X becomes
Hermione's friend then Hermione feeds X
Polyjuice potion or something like that
will that will help us in bat chain to
figure out if Millicent becomes
Hermione's friend
people are singing their head but think
about well that we'll be able to prove
it no now if they do become Primanti's
find it we want to do some for training
we'll figure out that they're going to
transmute into some kind of other thing
because of the apology's pushing but
it's not going to help us do the one
thing we wanted back checking which is
to prove that thing on the top so we
actually need to look for something that
has our current goal in its consequent
then we add on the antecedents as people
I've asked so far sorry I don't know
your names like Patrick have correctly
given me every time which is excellent
so notice also that she didn't say X
studies a lot needs to be added to the
gold tree she said Millicent studies a
lot Oh another question
great obvious after like putting the
Goulbourne in there should we first
check the assertions to check all the
assertions that our condition started
with them
so the question is do once we put Rule
four in there should we check to see if
those antecedents of rule four are
already in the assertions before
checking other rules the answer you're
not only correct sir you're exactly one
step ahead so we're going to assume I
called on you for the very next thing
that's exactly what we do
once we draw it onto the tree you're
you're you're exactly on the way further
question or perfect
so all right great so again we're
putting up Millicent studies a lot
Millicent is a protagonist we're not
putting up X some people did that and of
course it's it and note as we heard we
need them both to be true or we're not
going to be able to continue onward so
we have I'm gonna use M for Millicent
Millicent studies a lot also over here
Millicent is a protagonist great and
we've already heard that we need to
search in the assertions before we go
into any rules for what we're looking
for next the question is what are we
looking for next and in what order
this is where that thing I told you
about depth search is crucial we're
going to look here on the left node and
if it has any children if we have to
keep going down we're not going to look
here yet all right there's a few reasons
for that one of which is that we're lazy
the evaluation if you learned about it
is also if you learn about lazy
evaluation is also lazy and if we can
disprove this studies a lot branch we
don't have to do any work with the
protagonist branch we're done we're out
of here and it's and it's it's a it's a
loss all right so we're going to move
down the studies a lot branch and we
already heard from the audience we need
to look to see if it's in the assertions
everyone is studies a lot Millicent
studies a lot in the assertions no it's
not alright well then now we're going to
get to that question that I had before
about the antecedent or the consequent
because studies a lot is here in the
antecedent of rule floor but we're not
gonna be able to use Rule four here in
fact that's how we got here in the first
place so
question is does it um let's see the
question is do we have a rule that
matches in the cut in the consequent
that matches Millicent studies a lot yep
that's right p3 that's right so p3 is
would go smells and studies a lot and so
therefore what will we add to the goal
tree yeah she turns out to be a girl all
right so um that's exactly right we
heard that we need to add note we need
to add an end node but which also has an
or node at the bottom of it because this
is a little bit of a complicated rule so
we have an add node so we have an end
node and the first thing on the end node
is Millicent is a protagonist or
Millicent is a villain and the second
thing on the end node is Millicent is
ambitious well actually that is the
second thing on the end note I hope
that's what I said all right the second
thing on the end note is Millicent is
ambitious great okay so here's our handy
little tree now this is where it's
important as I said we're doing a
depth-first search we're going down
along the left branch so where do you
think we're going to go next on this
tree
ah that's a good guess you didn't say
we're trying to prove em as a Protoss on
the right branch which is a very common
state where you which one of the two
Millicent is a protagonist do we want to
look at yeah down left that's right so
you guys will learned up for search on
Monday at which point it'll become much
clearer what I'm saying
but yeah you always follow the left
branch far down as you can then you take
the right branch of that same branch
that you followed alright great so we're
going to try to find it Nelson as a
protagonist what do we do first
everyone yes is it in there no it's not
okay so therefore we're going to try to
find a rule as we heard about whether
Millicent is a protagonist so all the
way in the back Oh
what is their rule that matches the
imagine its consequent that Millicent is
a protagonist
there is okay well let's give it a try
which one Mulan's right so therefore we
have to add what to our gold chain right
what's on that leaf yeah endo top
perfect so our new thing that we're
searching for is pretend this connects
actually I guess it does connect we're
looking for a mole is in Gryffindor
Tower great all right and since its
stuff first that's where we're going
next great we're on a roll so what do we
do first
do we have other insertions some people
say yes but the answer is no we have in
fact Shamus living in Gryffindor Tower
most of you said no you're right the
majority rules we don't have any
assertions however do we have a rule
with that in the consequent no so what
do we do
people saying different things that are
all correct backtrack you say go to the
next we can't prove it all correct
put a big X this isn't true now we look
up we're on an or node so we're not done
yet because either of those can be true
great so now we go back up backtrack
Millicent is a villain great
what were you first check the assertions
is it in there no it's not don't worry
we're getting to one more will be we're
about 40 percent of the 30 to 40 percent
of the class lost points um very very
soon so we see that we see that
Millicent is a villain and it's not the
assertions so therefore is there any
role that has that and is consequent
we're little Millicent is a villain
oh alright I'll move it down slightly it
got a little bit off-kilter when I put
in the parentheses around the end hold
on ah there you go
okay then they Stella alright we're
trying to prove that she's a villain
though so do we have any with that in
the consequent the only thing that will
prove that she's a villain if we fire
off that rule I want to give her a
little bit of help ah
because you had to so it was a little
bit harder now some people ask what
about that ambitious thing can we like
add that to the tree or something no the
backward chainer is single-minded focus
and stupid even though it's later going
to need to prove that she's ambitious it
doesn't care it just sees the villain
there villain oh that's great as long as
we've got the antecedent we are going to
we're going to be happy this is actually
an important point not in this
particular problem but in some other
problems so by using p2 we're obviously
going to have to add the antecedents ex
lives in Slytherin dungeon when X here
is Millicent
so Millicent lives in Slytherin dungeon
alright so what's the first thing we do
when we see this new assertion you check
these her shins is it in there yes
that's not where everyone lost points
but yes it is in there check this order
node is happy now we move up to the end
node with Millicent is ambitious
so Melissa sit Millicent is ambitious
what do we do now
so let us use Rule two
actually no it's it's on the question is
do we need the do we need the X is
ambitious to be in our list of
assertions in order to use p2 for
instance if we didn't have the assertion
that most Edison bitches would P to have
a problem firing so it's a it's a very
interesting question it's sort of what
actually I was trying to what I was
trying to mention earlier when I said
it's single-minded it's focused it
doesn't care about the other one because
you might say well wait a minute p2
could never have actually fired if it
didn't have both of its consequence but
in fact the backward chainer doesn't
care all that the backward chainer is
looking to do is to prove whether it's
to prove whether or not there's a
possibility that Millicent might be a
villain it's not trying to say oh all
the results of of some certain rules are
making Millicent a villain and all the
other things that are there like
ambitious are in the database it's not
the Fortuner it actually doesn't care
this sometimes leads to unnecessary
computation but it's very simple and
it's very simple to code so it often
will probably speed you up in the long
run it like if there's for instance 100
things in then and you only care about
one of them it would be a waste almost
to add those 99 there was it up follow
up over here somewhere
you fight however then at that point
every time you check the assertions
you'd be attempting to check those 99
also you might not have used those rules
the question is could you make a hash
table with all of the 100 things that
were in the consequence and then after
having made that hash table since you
were already walking through those
anyway when you were checking it
you could add those to the assertions
the problem is that you might not
necessarily want although with the
backward chainer you might not
necessarily want to think that all those
things in the the consequent are
necessarily true however it can it you
might not care about them and then you
have to make an order and computation
every time you check that list of
assertions in the upper left or you
don't know what assertions there are you
have to check the entire list of
assertions in the upper left every time
you get to a new node in the goal tree
so each one is order one but I'm not
saying to find a specific assertion as
order is order and I'm saying every time
you make a new node in the goal tree you
have to check all the assertions you've
added yeah before but let's say that
there were 10,000 consequence of p2 um
you would have to check 10000 instead of
4 if you added them if you added them
all after proving p2 was true on the
goal tree also another important note is
that um you might not use every branch
of the goal tree if there's like an or
that's up higher and you wind up using a
different branch it was case you'd
probably have to make a separate hash
table for every sub branch that you have
and then remember which hash tables were
updated based on which sub branch and
then subbranches died it's probably more
effort than it's worth you could it's an
interest certainly an interesting
question it's a great question for for
debate and recitation and also I'm happy
to talk about it later I think that
someone who very intelligently made hash
tables and thought the problem through
as patrick said more knowledge can mean
a better
well actually maybe he hasn't said more
knowledge give me a better search
because we haven't done that lecture yet
but you're talking about implementation
decision and using hash table might well
be a good way to do this
the problem is that we're presuming some
things about how the rules are firing
and what bindings they're acquiring that
depend on the order of the assertions in
the table hash table would lose the
order and occasionally that's true
that's true but I'm thinking like if
someone wanted to make that
implementation is like do some research
in rules-based systems it's possible you
can increase the running speed in this
class we are not as not completely as
focused on the fat on the fastest
algorithms but I still think that's a
cool thing to try questions from people
in the crowd that's a good idea because
it's a little extra enrichment thing
it's not anything you would possibly
need to know for the quiz in this sense
if you want to build along with using
this you can ask ambitious when I'm
giving use after that another density
that
it says X is not finishes so how do you
resolve like that so the question is
let's say you used Rule two somewhere in
the tree to get that X as a villain
which then says that X is ambitious
then later you have X is not ambitious
somewhere somewhere in one of the other
rules that you then later need how is
that the question is how do you resolve
that what does it do
first of all okay so first of all
interestingly if it says X is not
ambitious literally like that and I'm
not being pedantic here this is one of
the tricks someone played in a previous
quiz um you can have that and extras
ambitious both in the list because it's
a dumb rules based system it doesn't
know that you can't have both of those
on the list so if you have a positive
assertion in a consequence X is not
ambitious and the consequent X is not
ambitious it'll be happy to add
Millicent is not ambitious and while
Millicent dishes is already there
because it's stupid now if it had delete
Millicent is ambitious in the consequent
and deleted something that was
previously there that would be that
would be an interesting problem it could
cause mistakes if your back channel
would probably make the mistake of
allowing this however this is exactly
why we do not I believe as a policy do
not have delete statements on the
quizzes at least previous quizzes did
not have them except for in a
hypothetical of what happens if we add a
delete statement here we I don't think
we've ever made people work things too
with the delete delete would would
possibly cause some issues question in
the back just to check this
backward chaining actually add a surgeon
never a sessions and backward change to
the assertion table it's dumb it just
keep the system is done I think it's a
dumb idea to do this it's actually
pretty fast to do it this way but the
system is dumb the so the question is do
you add assertions or you don't add
assertions you do not you simply go
through checking all the things in your
goal tree until the goal trees either
proven or disproven then you are out of
here all right so cool so far everyone
good questions from everyone these are
all questions that are things that often
trip people up so our next thing
Millicent is ambitious we sort of the
cat is out of the bag because there's a
question in mentioning that there was an
assertion that says that but I direct
you guys to our favorite rule rule -
shouldn't we use rule - to prove that
Millicent is ambitious question but the
answer is if you want to lose four
points or however many points then yes
but if you don't want to lose points
then this is already correct by this is
already correct by virtue of an
assertion one so I guess I should write
also that this over here Millicent
studies a lot rule is this rule is will
for wait a minute yeah that's rule for
with size 11 protagonist down here to
prove that she studies a lot is rule 3
and then at the bottom here we have we
have what rules here we have protect the
protagonist rule which is 1 and the
villain role which is 2 on this is
correct virtue of a zero okay great so
we've proved this whole end node we know
that Millicent studies a lot but now
we're going to have to go a little bit
more quickly based on time and so I will
do the remaining things anyone who has
not been called on is off the hook
but that doesn't mean not to pay
attention because there's some
interesting stuff still to come so
Millicent is a protagonist well good
news everyone
there's a one other reason that I can do
this and that's that you've already told
me what I need to do Millicent is a
protagonist you use rule one yeah rule
one and check if she lives in Gryffindor
Tower now will it do it again
yes it will it hasn't cashed that it's
already tried that
now a hash-table notwithstanding cashing
something you've already tried maybe a
good idea with backward chaining but we
do not do that in this class that's an
implementation detail it's another idea
of something you can do we don't cache
things that we've already tried we try
them again damn it
and we and when we try it again maybe
it'll work this time no it never it
never works this time this fails this
and node fails the whole thing fails
they're not friends at all they're
bitter enemies Millicent does not become
Hermione's friend so now they ask us a
few questions which are pretty easy to
answer based on the ordeal we've just
been gue so determine the minimum number
of additional assertions that we would
need to add for Millicent to become
Hermione's friends but you're not
allowed to add an assertion that matches
a consequent of a rule you can only add
an assertion that you can in other words
you can't add an assertion that will
prove some other rules you have to add
an assertion that directly yeah just
that's basically the the rule here
because there's a lot of choices but he
wanted one particular answer and so can
anyone think of an assertion that
doesn't match any consequence of any
rule that will make her be Hermione's
friend a lot of people say Millicent
lives in Gryffindor that's correct so
part three your solution to part two
causes an uncommon situation what is the
uncommon situation and what should you
do to the list of assertions to solve
this problem does anyone know what the
uncommon situation would be if we added
the Chi lives in Gryffindor head is
raised up here what is your answer
that's true she lives in Gryffindor and
Slytherin so how would you fix that yeah
you could pick out take out southern
from the assertions and say that she
only lived in Gryffindor a lot of people
gave the answer to this question which
is a perfectly reasonable thing to do in
a rules-based system they said well we
can add a rule that says that if you
live in X and you if you live in X and Y
is different than X then you can't live
in Y the problem with that among other
things is that we asked for a way to
change the assertions and not the rules
and you gave us a way to change the
assertions that's the right answer
great so there was and there was another
backward chaining problem this is 2009
quiz 1 and I will I will leave it off
for the moment you guys should take a
look at it in particular with variable
binding because remember you always have
to bind the variable that's relevant to
you that that doesn't mean that you
always have to bind all the variables
for instance let's say that we wanted to
prove that melissa has a bad term which
as it turns out is what we want to prove
in the other backward chaining very
quickly and without doing the problem
can anyone can anyone who's who things
are very clever at backward chaining
tell me exactly what things would be
added to the to the gold tree to prove
that Millicent has a bad term raise your
hand I won't I won't pick out a victim
because I'm asking this to happen pretty
quickly
no one all right
good add the three and see this dribble
205 which sold about specifically for
anyone for variable Y so you just have
to show that there's at least one person
who matches so what you can do I'm just
to read out what would the first thing
say with the snogs
X smells very sorry no it's not anyone
yeah no this is no sense knows why
that's crucial some people did a lot of
different things some people put Sheamus
in already you can't do that the system
stupid it doesn't know that the person
is Sheamus also some people said it
worked because look there's Longley each
other bit to the wrong it's the wrong
snog old direction up there be because
we have Seamus noggin Millicent dot know
so it's not even Seamus but yes exactly
it would be Millicent songs why
Millicent lives in Gryffindor Tower why
lives in Slytherin dungeon so let's give
forward chaining all the attention it
deserves about eight minutes which will
be more than enough okay we still got
all these rules
we've still got all these assertions
instead of doing that Carville backward
chaining oh it's not that horrible but
we'll do forward chaining it'll be
really easy we'll just add new sir
shion's as they go
remember the tiebreak order rules in
order from zero to five and if the same
rule could trigger could fire off with
multiple different assertions will use
the assertions in order from zero to
three so let's see you don't have much
time but with our four assertions let's
see what rules possibly could match with
our four sessions well I'll figure out
for you do we have an assertion about an
ambitious person yes but they're not a
squib so not rule zero do we have
someone who lives in Gryffindor we
absolutely do rule one is matches do we
have someone who lives in southern we do
rule two matches do we have a
protagonist or a villain we don't have
any protagonists or villains rule 3 is
not going to match because it's in an
end do we have someone who studies a lot
we do not have someone who studies a lot
rule 4 is not going to match do we have
some snogging we do have some snogging
so rule 1 2 & 5
match so and for matching we have run to
and file everyone which one wins the tie
right between 1 2 & 5 why because it
comes first numerically which one is
which one is rule 1 if X lives in
Gryffindor tire
the only binding for X is Seamus Seamus
lives in Gryffindor Tower we're firing
off rule 1 so new assertions the first
assertion that will add is Seamus is a
protagonist great now there's one other
thing and it's the main thing I wanted
to tell you about in for chaining about
our old friend rule 1 now that we've
done this does will one still match an
assertion in the database yes it does
but what's going to stop us from
constantly doing rule 1 every time
because it comes numerically before the
other rules what stops us there is part
of our implementation and it's a very
important part that people sometimes
forget so what I like to call the no
impotent rules implementation detail
that is if it rules completely 100%
impotent would do absolutely nothing
there's then you do not fire it you go
to the next one that's pretty important
let's say that you possibly had a delete
which you won't have on the quiz that
means that if the thing to delete is
missing its impotent if the thing to
leave is there it's not a bit 10 because
you can delete let's say you have
500,000 things that you're going to add
to your assertions four hundred ninety
nine thousand nine hundred ninety nine
are already in your database one of them
is missing is it an impotent rule no
it's not you have to fire it anyway you
can't be like oh it's mostly impotent
rule no you have to you have to fire off
the rule if it will do anything right
now one won't do anything because we've
already added Seamus as a protagonist
so what rules match down well Seamus
becoming a protagonist is nice and all
but he's not ambitious so we still don't
match rule three we still rats ruled one
two and five and as I basically just got
finished telling you the one that we're
going to fire off now is too
right so rule 2 is if X lives in
Slytherin dungeon our only binding for
that is Millicent and that will show
that she's a villain and she is
ambitious so what assertions will we add
Ellison is a villain and people stopped
that's right good job everyone
you figure you remember that Milton is
already ambitious from rule from
assertion 1 so we don't have to add them
both a lot of people did they were wrong
so Millicent is a villain
the other part of that role doesn't do
anything but it's not an impotent rule
because one of the things is not on
there great so what at what what rules
match now well as it turns out now that
Millicent is a villain and ambitious
we've matched rule 3 as well so we match
1 2 3 & 5
what rule we're going to fire everyone 3
because 1 & 2 are impotent rules
well fire rule 3 well Millicent is a
villain
she's also ambitious she's our only
match for 3 therefore what assertion do
we had Nelson studies a lot that's right
great so Nelson studies a lot however
she's not a protagonist we still don't
match rule 4 oh well well we figured
that out already ourselves from backward
chaining that we were never going to
match that so what matches it's still 1
2 3 & 5 seeing is 1 2 & 3 you're
impotent the only match is 5 and so the
only match we care about the only one
we're going to fire is 5 and the result
let's see X snogs why that's Sheamus
togs Millicent X lives in Gryffindor
that's Seamus why lives in southern
that's Millicent so what do we add
Seamus has a bad term
simple painless it works out we're done
and we got 100 on this quiz hopefully
you guys will to question the question
is if you have a new assertion which
matches a rule but it's really high
number it's all the way down at the
bottom of the rules but it's new and it
has new stuff should we do that first
maybe because it's new oh you mean that
has a lower number a lower number okay
so the question is if a rule let's say
that rule 3 made millicent be a squib
then yes you'd immediately not only
match 0 with 0 would immediately fire
because it's numerically it comes first
you'll see that on some of the other old
quizzes and you may see that in a
tutorial if you guys want to do some
more practice problems in tutorials your
tears we'll have that available as one
opportunity and tutorial but yeah that's
exactly right it happened that it went
in order this time but it will
definitely hop anywhere it can in order
to get to the lowest numbered assertion