How can I construct a Context-Free Grammer for the language x^ay^bz^2(a+b) where a>=0, b>=0. Thanks for helps...
Think of it this way
x^a y^b z^2(a+b) = x^a y^b z^2a z^2b = x^a y^b (z^2)^b (z^2)^a
Therefore
S -> xSzz | S1
S1 -> yS1zz | e
Observe that for each x
and for each y
, you need to generate two z
's because of the 2( a + b ). Also, observe that each string can be viewed as an "inside" part of y
's and z
's, and an "outside" part of x
's and z
's.
Since for each y
you need two z
's, the inside part can be described by (using capitals to denote non-terminal symbols and []
for the empty string):
I --> []
I --> y I z z
Now write a grammar for the outside part in the same way, but referring to I
in the base case.
There are essentially two cases that you need to treat:
x
at the beginning of the string, in which case you need to add two z
's at the end. y
in the middle, in which case you also need to add two z
's at the end. Try reducing either of these descriptions to a simpler grammer (eg a^nb^n
) for which you know the solution.
This hint should be enough to deduce the generative grammer.
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.