Let's consider a simple example of converting a CFG to an LBA. Suppose we have the following CFG:
CFG:
S → aSb | ε
This CFG generates strings of the form "a^n b^n" where 'n' is the same number of 'a's and 'b's. Now, let's convert this CFG to an LBA.
Linearly Bounded Automaton (LBA):
- Tape Alphabet: {a, b, $ } (including the end-of-tape marker $)
- State Set: {q0, q1, q_accept, q_reject}
- Transition Rules:
(q0, a) → (q0, a, R)
(Move right when reading 'a' in state q0)(q0, b) → (q1, $, L)
(Move left when reading 'b' in state q0, replace 'b' with $)(q0, $) → (q_accept, $, R)
(Accept when reaching the end of the string in state q0)(q1, a) → (q_reject, a, R)
(Reject if 'a' is encountered after 'b' in state q1)(q1, b) → (q1, b, L)
(Move left when reading 'b' in state q1)(q1, $) → (q0, $, R)
(Move right when reaching the end-of-tape marker in state q1, replace $ with 'a')
Acceptance Criteria:
- Accept if the LBA reaches the state q_accept.
- Reject if the LBA reaches the state q_reject.
Now, let's walk through an example derivation and simulate the LBA for the input "aaabbb":
LBA Simulation Steps:
- Initial Configuration:
q0 aaabbb$
(Start in state q0 with the input string on the tape) - Apply Transition 1:
q0 aabbb$
(Move right, read 'a') - Apply Transition 1:
q0 abbb$
(Move right, read 'a') - Apply Transition 1:
q0 bbb$
(Move right, read 'a') - Apply Transition 2:
q1 $bb$
(Move left, replace 'b' with $) - Apply Transition 5:
q1 $b$
(Move left, read 'b') - Apply Transition 5:
q1 $b$
(Move left, read 'b') - Apply Transition 5:
q1 $b$
(Move left, read 'b') - Apply Transition 3:
q_accept $b$
(Move right, reach end of the string, accept)
The LBA successfully accepts the input "aaabbb" according to the language generated by the CFG.