Lecture 07 Syntax Analysis and Parsing

What is the role of a Syntax Analyser?

[1] read a sequence of tokens returned from the lexical analyser [2] group these tokens into phrases of certain phrase types according to the syntax of the programming language.

What is the output of the Syntax Analysis phase

The output of this phase is an abstract syntax tree—a data structure that represents the syntactic structure of the source program.

Given an input program, the goals of parsing is to?

[1] find all syntax errors and for each, produce an appropriate diagnostic message and recover quickly [2] produce the parse tree for the program for code generation.

What are the 2 categories of parsers?

Top down and bottom up

describe a top down parser

[1] Beginning with the root (the start symbol); [2] Each node is visited before its branches are followed; [3] Branch from a particular node are visited in left-to-right order - a leftmost derivation;

Describe a bottom up parser

[1] Beginning at the leaves of parse tree (terminal symbols) and progressing toward the root. [2] Order is that of the reverse of a rightmost derivation

Consider the following grammar: S → AB A → aA | ε B → b | bB What language does the grammar define?

The grammar defines strings consisting of any number (zero included) of a’s followed by at least one (and possibly more) b’s

What is Recursive-Descent Parsers (RDP)

[1] A RDP consists of a collection of subprograms, many of which are recursive (therefore its name) and produces a parse tree in top-down order. [2] There is a subprogram for each nonterminal in the grammar.

A nonterminal that has more than one RHS requires an initial process to determine which RHS it is to choose, give an example method to find The correct RHS

[1] The next token of input is compared with the first token that can be generated by each RHS until a match is found (e.g., the left parenthesis of the <factor> rule) [2] If no match is found, it is a syntax error.

If a grammar has left recursion, either direct or indirect can it be used?

not directly as it leads to non-terminating recursion

how can you transform left recursive grammar to one that is not?

For each nonterminal, A, Group the A-rules as A → Aα1 | ... | Aαm | β1 | β2 | ... | βn where none of the β’s begins with A Replace the original A-rules with A → β1A’ | β2A’ | ... | βnA’ A’ → α1A’ | α2A’ | ... | αmA’ | ε