I literally can't find the answer to this:
L = {vw | v element of {a,b) , w element of {b,c} , number of a's <= number of c's}
V --> aV | bV | e
W --> bW | cW | e
But I cannot think of how to combine the construction of the words v and w after one another and keeping in mind the mentioned restriction.. Anyone who could lend me a hand?
As I explained in my answers tips for writing cfg , correct approach is first understand all possible patterns of strings in language then write rules.
What is this language L? In strings of language L:
1. All 'a'
s comes before any 'c'
s
2. Number of 'a'
are less than or equals to number of 'c'
, So in grammar of L, if a production rule adds one symbol 'a'
then it must also add one or more 'c'
.
3. There is no restriction on occurrence of symbol 'b'
, it can appear anywhere any number of times. 4. Empty (null) string is also belongs to L as number of 'a'
= number if 'c'
== 0. And for same reason a string consists of only symbol 'b'
is also acceptable.
5. Any string without 'a'
also belongs L (in other words (c + b)*
is subset of L).
Now writing grammar rules are easy (read comments to understand each production rules):
S → BaBSBCB | ^ // add `a` add `C` also, B can be any where so added B
Z → CZ | BZ | ^ // to create `(c + b)*`
C → cC | c // C always generates one or more `c`s
B → bB | ^ // there is no restriction on B it generates `b`s or ^
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