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Top-down parsing in computer science is a parsing strategy where one first looks at the highest level of the parse tree and works down the parse tree by using the rewriting rules of a formal grammar. [1] LL parsers are a type of parser that uses a top-down parsing strategy.
The algorithm, named after its inventor, Jay Earley, is a chart parser that uses dynamic programming; it is mainly used for parsing in computational linguistics. It was first introduced in his dissertation [ 2 ] in 1968 (and later appeared in an abbreviated, more legible, form in a journal [ 3 ] ).
Another chart parsing algorithm is the Cocke-Younger-Kasami (CYK) algorithm. Chart parsers can also be used for parsing computer languages. Earley parsers in particular have been used in compiler-compilers where their ability to parse using arbitrary Context-free grammars eases the task of writing the grammar for a particular language.
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]
Bottom-up parsing is parsing strategy that recognizes the text's lowest-level small details first, before its mid-level structures, and leaves the highest-level overall structure to last. [3] In top-down parsing, on the other hand, one first looks at the highest level of the parse tree and works down the parse tree by using the rewriting rules ...
The end result is then a shared-forest of possible parse trees, where common trees parts are factored between the various parses. This shared forest can conveniently be read as an ambiguous grammar generating only the sentence parsed, but with the same ambiguity as the original grammar, and the same parse trees up to a very simple renaming of ...
Top-down parsers build the parts of Parse_Tree_Example.svg in the top-down steps numbered here. Used in Bottom-up Parsing. Source Manually drawn by me with Google Docs Date 2012/04/16 Author DBSand. Permission (Reusing this file) See below.
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.