Bottom Up Parsing Techniques

Bottom-Up Parsing

• Bottom-up parsing constructs a parse tree from the leaves (input symbols) to the root (starting symbol).
• Utilizes a stack to hold grammar symbols during the parsing process and processes input tokens one at a time.
• Shift-Reduce Parsing is a prevalent technique where the input symbols are shifted onto the stack until a reduction can be made.
• Pruning is used to eliminate paths in the parse tree that do not lead to a valid parse.

Shift-Reduce Parsing

• Conflicts may arise during shift-reduce parsing, specifically Shift-Reduce and Reduce-Reduce conflicts.
• Shift-Reduce conflicts occur when both shifting and reducing are possible; the parser must have a strategy to resolve these.
• Reduce-Reduce conflicts arise when multiple reductions are applicable, making it necessary to choose one, leading to ambiguity.

Introduction to LR Parsing

• LR parsing is a deterministic bottom-up parsing technique that processes the input from Left to Right and constructs a Rightmost derivation.
• Simple LR (SLR) parsing involves constructing LR(0) items, which represent the current status of the parsing process.
• LR(0) Automaton consists of states that reflect the progress of parsing, capturing the necessary items at each state.

LR Parsing Algorithm

• The LR parsing algorithm works based on a deterministic finite automaton built from the grammar.
• Each state in the LR automaton corresponds to a set of LR(0) items representing potential parsing actions.
• Transitions between states are defined by reading input symbols and handling reductions based on a parsing table.

Construction of SLR Parsing Table

• The SLR parsing table is constructed using the closure and goto operations on LR(0) items.
• First sets and follow sets are utilized to determine when to shift or reduce during parsing based on the current state and the next input symbol.

More Powerful LR Parsers

• Canonical LR(1) parsers extend SLR parsing by incorporating lookahead symbols for better decision-making during parsing.
• Canonic LR(1) items include a lookahead symbol, allowing more precise conflict resolution.
• The construction of LR(1) sets of items involves augmenting LR(0) items with lookahead information.

Constructing LALR Parsing Tables

• LALR (Look-Ahead LR) parsing combines properties of LR(0) and LR(1) by merging similar states to minimize the size of the parse table.
• LALR parsers are more efficient than canonical LR parsers while maintaining power to parse a larger class of grammars.

Parser Generator Yacc

• Yacc (Yet Another Compiler Compiler) is a widely used parser generator that produces LALR parsers from formal grammar specifications.
• Yacc simplifies the creation of syntax analyzers by automating the construction of parsing tables and the parsing code necessary for interpreters and compilers.