Predictive parsing is a form of recursive descent parsing, in which no backtracking is needed, so it can predict which products to use as the replacement of the input string. There's a version called LL(1) parsing that uses a table to help make decisions, which makes it very efficient and straightforward.
Predictive parsing is a parsing technique used in compiler construction and syntax analysis. It helps compilers analyze and understand the structure of code by predicting the part that comes next depending on what's already there. This simplifies the process of translating code into machine-readable instructions.
We'll go through the basics of Predictive Parser and concentrate on the role of Predictive Parser. We will discuss everything about Predictive Parsing. Also will cover the algorithm for implementing the Predictive parser or Predictive Parsing, as well as an example involving the construction of the algorithm for precedence parsing but before that let's have a look at the basic hierarchy of parsing and their different types.
Predictive parsing is a parsing technique used in compiler design to analyze and validate the syntactic structure of a given input string based on a grammar. It predicts the production rules to apply without backtracking, making it efficient and deterministic.
Also, read - Recursive Descent Parser
Type of Predictive Parsing
1. Top-down Parsing
In top-down parsing, the parser starts with the top-level non-terminal of the grammar and attempts to construct a parse tree by repeatedly expanding non-terminals until the input string is derived. It predicts the production rules to apply from left to right, hence the name "top-down."
Eg. Consider the grammar:
S -> A B
A -> a
B -> b
Let's parse the input string “ab” using top-down predictive parsing.
1. Construct the predictive parsing table:
| a | b | |
|---|---|
| S | S -> A B |
| A | A -> a |
| B | B-> b |
2. Initialising stack and input buffer:
Stack: '$ S'
Input Buffer: 'ab $'
3. Apply top-down predictive parsing algorithm.
| Stack | Input Buffer | Action |
|---|---|---|
| $ S | a b $ | Match terminal ‘a’ |
| $ S A | b $ | Match terminal ‘b’ |
| $ S A B | $ | Accept |
2. Bottom up Parsing
In bottom-up parsing, the parser begins with the input string and attempts to reduce it to the start symbol of the grammar by applying production rules in a right-to-left manner. It builds the parse tree from the bottom-up, hence the name "bottom-up."
Eg. Consider the grammar:
S -> A B
A -> a
B -> b
Let's parse the input string “ab” using bottom-up predictive parsing.
1. Initialize the stack and input buffer.
Stack: $
Input Buffer: ab $
2. Applying bottom-up parsing algorithm:
| Stack | Input Buffer | Action |
|---|---|---|
| $ | a b $ | Shift ‘a’ onto stack |
| $ a | b $ | Shift ‘b’ onto stack |
| $ a b | $ | Reduce using production B -> b |
| $ a B | $ | Reduce using production A -> a |
| $ A | $ | Reduce using production S -> A B |
| $ S | $ | Accept |
Components of Predictive Parsing
1. Input Buffer
The input buffer is a temporary storage area that holds the input symbols yet to be processed by the parser. It provides a lookahead mechanism, allowing the parser to examine the next input symbol without consuming it.
In this diagram, the input buffer is shown as a series of unprocessed tokens (T) and non-terminals (N). The end-of-input marker is denoted by the "$" symbol. Based on the top of the parsing stack and the current input symbol, the parsing table is a data structure that directs the parser's activities. The parser's next token to process is indicated by the arrow "^" in the input buffer. The arrow moves to the right as the parser reads tokens from the input buffer, processing each token until the end-of-input marker is reached.
2. Stack
The stack, also known as the parsing stack or the pushdown stack, is a data structure used by the parser to keep track of the current state and to guide the parsing process. It stores grammar symbols, both terminals and non-terminals, during the parsing process.
The stack in predictive parsing is a vertical sequence of symbols representing terminals (T) and non-terminals (N) from the language's grammar. The top of the stack, indicated by the "^" symbol, represents the symbol currently being processed by the parser. By comparing the input symbol with the top of the stack, the parser determines the next action to take. The parser can shift new symbols onto the stack or reduce symbols using grammar productions as it processes symbols from the input stream. The stack is crucial for maintaining the parser's state and enabling parsing actions. It allows the parser to track encountered symbols, make decisions based on the grammar, and interact with the parsing table. In practical implementations, the stack may contain additional information, such as attributes or semantic actions associated with the symbols.
3. Predictive Parsing Table
The predictive parsing table is a data structure used in predictive parsing. It maps the combination of a non-terminal and a lookahead symbol to a production rule. It guides the parser's decisions, determining which production to apply based on the current non-terminal and the next input symbol.
This diagram represents the parsing table as a grid with rows and columns. The rows correspond to the non-terminals in the grammar, and the columns represent the input symbols (terminals and the end-of-input marker $).
Each cell in the table contains information about the action or the production to be applied when a specific non-terminal and input symbol combination is encountered. Here's what each entry in the table represents:
- S, A, B: Non-terminals in the grammar.
- S->, A->, B->: Indicates a production rule to be applied.
- a, b: Terminal symbols from the input alphabet.
- ε: Represents an empty or epsilon production.
-
Empty cells: Indicate that no action or production is defined for that combination of non-terminal and input symbols.
For example, if the parser is in state S (row) and the next input symbol is "a" (column), the corresponding cell entry "S->aAB" instructs the parser to apply the production rule "S -> aAB".
The parsing table is a critical component of predictive parsing as it systematically guides the parser's decisions and actions. By consulting the table, the parser can determine the appropriate production or action based on the current state and the lookahead input symbol, enabling it to construct the parse tree or detect syntax errors during the parsing process.
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