Elimination of Left Factoring Video Lecture | Compiler Design - Computer Science Engineering (CSE)

hi guys so welcome back to the compiler
construction and design lecture number
two and in this lecture let me tell you
is the most easiest lecture in this
whole series and today we are going to
study less factoring and how to
eliminate it and identify it well this
you can discuss it because it's the most
easiest part how do you identify whether
a grammar is left factoring well if it
is the if it is of the form a gives you
alpha beta slash alpha gamma that is the
initial terminals are just the same for
every this thing the option available
then it is left factoring we will be
more clear as soon as we take a few
examples
so on paper like we did in the previous
video we just see how we can eliminate
it like what the grammar is what the
format is so basically you take out this
alpha common so I take off this alpha
carbon and you write a - you add this
new variable a - and for a - you define
a - is whatever the part which is left
after taking out alpha common that is
beta and gamma is your a - so this is a
very easy rule guys and we're going to
see how we can do it with the just one
example would be enough to clear this
because this is the most easiest part so
let's just dive into the example so
example number one is a gives you a a B
slash a slash a a and b gives you B B
slash D so just for your reference I am
writing the rules once here once again
here that is a gives you alpha beta
alpha gamma so you do a gives you alpha
a - and a - gives you beta gamma the
remaining uncommon parts okay so let's
figure out this one so we for the first
one you have a and a common so these are
the parts which are being left
out so you write a gives you small a a -
and a - gives you a b / a now for this
one a this b is common okay so yeah this
b is common here so you write this
capital d gives you B B - sounds fair
enough
however for B - okay
you have the uncommon part as this B but
what is uncommon heard nothing so as you
can see just it was just B which is
taken out so when you take out B common
you're left with epsilon so this is a
catch here which if you remember so B -
gives you a capital B / Epsilon
so basically the only takeaway from this
is that whenever a suppose if I have
let's say in this example itself if I
had a giving me a a a B a a / a okay so
in this I would take out this a common
from everywhere the uncommon part would
be a B here it will be capital A or but
what about here
well if there is nothing here to take
common I after taking common if nothing
is left you put in an Epsilon so
basically the answer and this would be a
gives you a a - and a - gives you a be a
and epsilon so that's how you do it
pretty easy stuff guys and now when we
move in the next lecture we're going to
do some pretty not hard stuff but it'll
be more interesting and it gets better
as we go on and on so in the next video
lecture I'm going to be covering these
recursive descent parser so see you guys
next time do comment do subscribe and
thank you for watching