Designing LR(O) Parser Part-2
Duration: 7 min
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The video features an educational lecture by Sanchit Jain from Knowledge Gate on constructing a parsing table for a specific context-free grammar. The grammar presented is A' -> A and A -> aA / b. The instructor begins by displaying a state diagram (DFA) that represents the LR(0) items for the grammar. He systematically explains the transitions between states I0 through I4. He then proceeds to construct a parsing table with columns for 'Action' (terminals a, b, $) and 'Goto' (non-terminal A). The lecture covers filling in shift, reduce, and goto actions based on the items in each state, culminating in a completed table and a brief mention of a sample string "abab".
Chapters
0:00 – 2:00 00:00-02:00
The instructor starts by reviewing the state diagram. State I0 contains items A' -> .A, A -> .aA, and A -> .b. He points out transitions: 'A' leads to I1, 'a' leads to I2, and 'b' leads to I3. State I1 contains A' -> A.. State I2 contains A -> a.A, A -> a.aA, and A -> a.b. State I3 contains A -> b.. State I4 contains A -> aA.. The instructor highlights the transitions and the items within each box to establish the foundation for the parsing table. He specifically points to the items in I0 and traces the arrows to I1, I2, and I3.
2:00 – 5:00 02:00-05:00
He draws a table with headers 'Action' (sub-columns a, b, $) and 'Goto' (sub-column A). He starts filling the row for I0. On input 'a', he writes S2 (shift to state 2). On input 'b', he writes S3 (shift to state 3). On input 'A' in the Goto column, he writes 1. For row I1, since it contains A' -> A., he marks the '$' column as "Accept". He explains that this indicates successful parsing of the start symbol. He fills the first row completely before moving to the next.
5:00 – 6:53 05:00-06:53
He continues filling the table. For row I2, on 'a' he writes S4, and on 'b' he writes S3. For row I3, which has A -> b., he writes R2 (reduce by production 2) in the action columns. For row I4, which has A -> aA., he writes R1 (reduce by production 1) in the action columns. Finally, he writes "abab" at the bottom as an example string to be parsed using the constructed table. The table is now mostly complete with shift and reduce actions filled in.
The lecture demonstrates the direct mapping between the LR(0) state diagram and the parsing table. Shift actions correspond to transitions on terminals, Goto actions correspond to transitions on non-terminals, and Reduce actions correspond to states containing completed items (dot at the end). The process transforms the theoretical state machine into a practical parsing mechanism. The instructor uses the example string "abab" to hint at the next step of parsing the string.