DFA Designing string contains sub string s

Duration: 6 min

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  23. UPPSC Live Class Recordings: UPPSC Polytechnic Lecturer 2025 (CS)
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AI summary & chapters

AI Summary

An AI-generated summary of this video lecture.

This educational video lecture demonstrates how to design a minimal Deterministic Finite Automaton (DFA) that accepts all strings over the alphabet {a, b} containing a specific substring. The instructor, Sanchit Jain, begins by defining the problem and listing two examples: substring 'abb' and substring 'aba'. He systematically constructs the DFA for 'abb', starting with an initial state and adding transitions for each character of the substring. He carefully handles edge cases and overlaps, such as input 'a' at state q2 (having seen 'ab'), redirecting it to state q1. He then briefly sketches the DFA for 'aba', showing similar state transitions. Finally, he concludes with a general rule for constructing such DFAs, stating that a substring of length m requires m+1 states and does not necessitate a dead state.

Chapters

  1. 0:00 – 2:00 00:00-02:00

    The video opens with the problem statement on the whiteboard: 'Design a minimal DFA that accepts all strings over the alphabet {a, b}, where every accepted string 'w' contains sub string s.' The instructor lists two examples: i) abb and ii) aba. He begins constructing the DFA for the first example, 'abb'. He draws four states labeled q0, q1, q2, and q3. He draws a transition from q0 to q1 on input 'a', from q1 to q2 on input 'b', and from q2 to q3 (the final state) on input 'b'. He adds a self-loop on q0 for input 'b' and a self-loop on q1 for input 'a'. He then addresses the case where the input is 'a' while in state q2 (having seen 'ab'), drawing a transition from q2 back to q1 to handle the overlap in strings like 'aba'.

  2. 2:00 – 5:00 02:00-05:00

    The instructor continues refining the DFA for 'abb', adding self-loops on the final state q3 for both 'a' and 'b' to indicate that once the substring is found, any subsequent characters are accepted. He writes example strings like 'abbab' and 'abba' to verify the logic. He then moves to the second example, 'aba'. He draws a new set of states q0, q1, q2, q3. He draws transitions: q0 to q1 on 'a', q1 to q2 on 'b', and q2 to q3 on 'a'. He adds a self-loop on q0 for 'b' and a self-loop on q1 for 'a'. He draws a transition from q2 to q0 on input 'b'. He adds self-loops on the final state q3 for 'a' and 'b', emphasizing that the machine remains in the accepting state once the substring is detected.

  3. 5:00 – 5:53 05:00-05:53

    The instructor presents a 'Conclusion' slide summarizing the general method. He writes the formula: 'if w = xsx, |s| = m, then no of states in the DFA is m+1'. He underlines that there is 'No need of dead state' because all transitions can be managed within the existing states. He also notes that one 'Can loop on final state'. He explains that for any substring of length m, the minimal DFA will require exactly m+1 states, covering the start state through the final accepting state, without needing a separate trap or dead state for invalid inputs.

The lecture progresses from a specific problem definition to the practical construction of DFAs for substrings 'abb' and 'aba'. The instructor demonstrates the step-by-step addition of states and transitions, highlighting the critical logic of handling overlaps where a prefix of the substring matches a suffix of the current input. The lesson culminates in a generalized rule for such problems, establishing that a substring of length m necessitates m+1 states and that a dead state is unnecessary, as the automaton can always transition to a relevant previous state.

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