DFA Designing sting where number of a =0(mod3)
Duration: 3 min
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The video presents a lecture on designing minimal Deterministic Finite Automata (DFA) for specific string conditions over the alphabet $\Sigma = \{a, b\}$. The instructor outlines four distinct problems involving modular arithmetic constraints on the count of 'a's and 'b's. He systematically constructs state diagrams for each condition, demonstrating how to track remainders using states and transitions. The lecture focuses on the independence of character counts in these specific modular problems, showing how to handle 'a' and 'b' transitions separately to satisfy the required modulo conditions.
Chapters
0:00 – 2:00 00:00-02:00
The instructor begins by listing four conditions on the whiteboard: i) $|w|_a = 0 \pmod 2$, ii) $|w|_b = 1 \pmod 2$, iii) $|w|_a = 2 \pmod 3$, and iv) $|w|_a = 1 \pmod 4$. He starts with the first two conditions. For condition (i), he draws two states, $q_0$ and $q_1$, representing even and odd counts of 'a'. He draws transitions $q_0 \xrightarrow{a} q_1$ and $q_1 \xrightarrow{a} q_0$, adding self-loops for 'b' on both states. Next, he addresses condition (ii), drawing states for even and odd counts of 'b'. He draws transitions $q_0 \xrightarrow{b} q_1$ and $q_1 \xrightarrow{b} q_0$, adding self-loops for 'a'. He marks the start state with an incoming arrow and the state with an odd number of 'b's as the final state with a double circle.
2:00 – 3:19 02:00-03:19
The instructor proceeds to condition (iii), where the number of 'a's must be congruent to 2 modulo 3. He draws three states labeled $q_0, q_1, q_2$ to represent remainders 0, 1, and 2. He draws a cycle of transitions for 'a': $q_0 \xrightarrow{a} q_1 \xrightarrow{a} q_2 \xrightarrow{a} q_0$. He adds self-loops for 'b' on all three states and marks $q_2$ as the final state. Finally, he tackles condition (iv), requiring the number of 'a's to be congruent to 1 modulo 4. He draws four states $q_0, q_1, q_2, q_3$ and creates a cycle for 'a' transitions. He adds self-loops for 'b' on all states and marks $q_1$ as the final state. Throughout this section, he emphasizes that these are separate problems.
The lecture effectively demonstrates the standard method for constructing DFAs based on modular arithmetic constraints. By treating the count of each character independently, the instructor shows how to create minimal state machines where states represent remainders. The visual progression from 2 states to 3 and then 4 states clearly illustrates the relationship between the modulus and the number of required states. This approach is fundamental for solving regular language problems involving counting. It highlights the direct mapping between the modulus value and the state count.