Bottom-up parsing is a vital
technique used in compiler design to analyze and parse the syntactic structure
of programming languages. Unlike top-down parsing, which starts from the top
(the start symbol) and works its way down, bottom-up parsing begins with the
input tokens and progressively builds a parse tree until the start symbol is
reached.
This article provides a comprehensive overview of bottom-up parsing, discussing its principles, advantages, and limitations.
Table of Contents
·
Definition of Bottom-Up
Parsing· Working Principle
· Advantages of Bottom-Up Parsing
· Limitations of Bottom-Up Parsing
· Examples of Bottom-Up Parsing
· Comparison of Top-Down and Bottom-Up Parsing
· Definition of Shift-Reduce Parsing
· Working Principle
· Shift Operation
· Reduce Operation
· Shift-Reduce Conflicts
· Example of Shift-Reduce Parsing
· Advantages and Limitations
· Conclusion
Bottom-Up Parsing
Bottom-up parsing is a parsing technique that starts from
the input tokens and constructs a parse tree by repeatedly reducing a sequence
of input tokens to non-terminals until the start symbol is reached. It follows
a shift-reduce approach, where terminals are shifted onto a stack, and when a
reduce action is applicable, the right-hand side of a production rule is
reduced to its corresponding non-terminal.
Working Principle of Bottom-up parsing
The working principle of bottom-up parsing involves the
following steps:
1. Begin with the input tokens and an empty stack.
2. Shift the next input token onto the stack.
3. If a reduce action is applicable, pop the appropriate number of
tokens from the stack and replace them with the non-terminal on the left-hand
side of the reduced production rule.
4. Repeat steps 2-3 until the start symbol is reduced, or an error
is encountered.
Advantages of Bottom-Up Parsing
Bottom-up parsing offers several advantages:
i.
Handling of Complex Grammars:
Bottom-up parsers can handle a broader class of grammars,
including left-recursive and ambiguous grammars. This flexibility makes
bottom-up parsing suitable for a wide range of programming languages.
ii.
Error Recovery:
Bottom-up parsers are generally more robust when it comes to
error recovery. They can often handle syntax errors and continue parsing the
input, identifying multiple errors in a single run.
iii.
Efficient Parsing:
Bottom-up parsing can be more efficient in terms of memory
usage and parsing time for large grammars or inputs. It allows for incremental
parsing, reducing the need to backtrack.
Limitations of
Bottom-Up Parsing
i.
Complexity of Implementation:
Bottom-up parsers can be more complex to implement compared
to top-down parsers, especially for complex grammars.
ii.
Lack of Human-Readable Structure:
The parse trees generated by bottom-up parsing can be more
challenging to interpret and analyze than those generated by top-down parsing.
The structure may not align with the natural flow of the input program.
Shift-Reduce Parsing: Understanding the Process Step-by-Step
Shift-reduce parsing is a fundamental technique used in
compiler design to analyze the syntactic structure of programming languages. It
follows a bottom-up parsing approach, where the parser shifts input tokens onto
a stack and then reduces the stack using production rules until the start
symbol is reached. This article provides a detailed explanation of shift-reduce
parsing, outlining the step-by-step process and highlighting its significance
in compiler construction.
Definition of
Shift-Reduce Parsing:
Shift-reduce parsing is a bottom-up parsing technique that
uses a stack-based approach to construct a parse tree. It involves two main
operations: shift and reduce. In the shift operation, the next input token is
shifted onto the stack. In the reduce operation, the corresponding non-terminal symbol replaces a sequence of symbols on the stack that matches a production rule.
The working Principle of shift reduce parsing
The working principle of shift-reduce parsing can be
summarized as follows:
2. Repeat the following steps until the start symbol is reduced or an error is encountered:
- If the next input token matches the top of the stack, perform a shift operation by moving the token from the input to the stack.
- If a sequence of symbols on the top of the stack matches a production rule, perform a reduce operation by replacing the sequence with the corresponding non-terminal symbol.
Shift Operation
In the shift operation, the next input token is shifted from
the input to the stack. This operation reflects the recognition of a terminal
symbol in the input. The stack grows by adding the shifted token, and the input
moves to the next token.
Reduce Operation
In the reduce operation, a sequence of symbols on the top of
the stack that matches a production rule is replaced by the corresponding
non-terminal symbol. This operation reflects the recognition of a production
rule in the input. The stack is reduced by popping the symbols that match the
right-hand side of the production rule and pushing the non-terminal symbol onto
the stack.
Shift-Reduce Conflicts
Shift-reduce parsing may encounter conflicts, known as
shift-reduce conflicts when a parser faces a choice between shifting the next
input token or reducing the symbols on the stack. Shift-reduce conflicts can
arise due to ambiguities or lack of sufficient information to determine the
appropriate action. Resolving these conflicts requires additional techniques
such as precedence rules or explicit disambiguation rules. Consider the
following grammar:
S -> E
E -> E + T | T
T -> int
Shift-reduce parsing table of the above grammar.
|
Stack |
Input |
Action |
|
int + int $ |
Initial Configuration |
|
|
int + int $ |
Shift 'int' |
|
|
int |
+ int $ |
Shift '+' |
|
int + |
int $ |
Shift 'int' |
|
int + int |
$ |
Reduce T -> int |
|
int + T |
$ |
Reduce E -> T |
|
E |
$ |
Reduce E -> E + T |
|
S |
$ |
Accept |
Comparison of Top-Down and Bottom-Up Parsing
While both top-down and bottom-up parsing techniques serve the same purpose of syntactic analysis, they differ in their approach and characteristics. Top-down parsing starts from the top (start symbol) and expands non-terminals, while bottom-up parsing starts from the input tokens and reduces to non-terminals.
Top-down parsing is generally easier to implement but
may have limitations with left recursion and ambiguity. Bottom-up parsing, on
the other hand, can handle a broader range of grammars and offers a more robust
error recovery.
Bottom-up parsing is a powerful technique in compiler design
that allows for efficient and robust analysis of the syntactic structure of
programming languages. By understanding its principles, advantages, and limitations,
developers can make informed decisions on selecting the appropriate parsing
technique for their compiler construction projects.
In this article, we discussed bottom-up parsers and Shift-reduce parsing is a powerful technique used in
compiler design to analyze the syntactic structure of programming languages. By
understanding the step-by-step process of shift and reduce operations, as well
as handling conflicts, developers can construct efficient parsers that provide
meaningful error detection and recovery. Shift-reduce parsing plays a crucial
role in compiler construction, ensuring the syntactic correctness of programs.
