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Top-down parsing is a strategy of analyzing unknown data relationships by hypothesizing general parse tree structures and then considering whether the known fundamental structures are compatible with the hypothesis. It occurs in the analysis of both natural languages and computer languages. Top-down parsing can be viewed as an attempt to find ...
In computer science, an operator-precedence parser is a bottom-up parser that interprets an operator-precedence grammar.For example, most calculators use operator-precedence parsers to convert from the human-readable infix notation relying on order of operations to a format that is optimized for evaluation such as Reverse Polish notation (RPN).
Another method [8] is to build the parse forest as you go, augmenting each Earley item with a pointer to a shared packed parse forest (SPPF) node labelled with a triple (s, i, j) where s is a symbol or an LR(0) item (production rule with dot), and i and j give the section of the input string derived by this node. A node's contents are either a ...
In computer science, a recursive descent parser is a kind of top-down parser built from a set of mutually recursive procedures (or a non-recursive equivalent) where each such procedure implements one of the nonterminals of the grammar. Thus the structure of the resulting program closely mirrors that of the grammar it recognizes. [1] [2]
The parsing table instructs it to apply rule (1) from the grammar and write the rule number 1 to the output stream. The stack becomes: [ F, +, F, ), $] The parser now has an 'a' on its input stream and an 'F' as its stack top. The parsing table instructs it to apply rule (3) from the grammar and write the rule number 3 to the output stream.
When a top-down parser tries to parse an ambiguous input with respect to an ambiguous context-free grammar (CFG), it may need an exponential number of steps (with respect to the length of the input) to try all alternatives of the CFG in order to produce all possible parse trees. This eventually would require exponential memory space.
Parsing the string "1 - 2 - 3" with the first grammar in an LALR parser (which can handle left-recursive grammars) would have resulted in the parse tree: Left-recursive parsing of a double subtraction. This parse tree groups the terms on the left, giving the correct semantics (1 - 2) - 3. Parsing with the second grammar gives Right-recursive ...
Exhaustive search of the possible edges in the dependency tree, with backtracking in the case an ill-formed tree is created, gives the baseline () runtime for graph-based dependency parsing. This approach was first formally described by Michael A. Covington in 2001, but he claimed that it was "an algorithm that has been known, in some form ...