What is a Predictive Parser?



Predictive Parser is also another method that implements the technique of Top- Down parsing without Backtracking. A predictive parser is an effective technique of executing recursive-descent parsing by managing the stack of activation records, particularly.

Predictive Parsers has the following components −

  • Input Buffer − The input buffer includes the string to be parsed followed by an end marker $ to denote the end of the string.

Here a, +, b are terminal symbols.

  • Stack − It contains a combination of grammar symbols with $ on the bottom of the stack. At the start of Parsing, the stack contains the start symbol of Grammar followed by $.
  • Parsing Table − It is a two-dimensional array or Matrix M [A, a] where A is nonterminal and 'a' is a terminal symbol.

All the terminals are written column-wise, and all the Non-terminals are written rowwise.

  • Parsing Program − The parsing program performs some action by comparing the symbol on top of the stack and the current input symbol to be read on the input buffer.
  • Actions − Parsing program takes various actions depending upon the symbol on the top of the stack and the current input symbol. Various Actions taken are given below −

Algorithm to construct Predictive Parsing Table

Input − Context-Free Grammar G

Output − Predictive Parsing Table M

Method − For the production A → α of Grammar G.

  • For each terminal, a in FIRST (𝛼) add A → α to M [A, a].
  • If ε is in FIRST (α), and b is in FOLLOW (A), then add A → α to M[A, b].
  • If ε is in FIRST (α), and $ is in FOLLOW (A), then add A → α to M[A, $].
  • All remaining entries in Table M are errors.

Following are the steps to perform Predictive Parsing

  • Elimination of Left Recursion
  • Left Factoring
  • Computation of FIRST & FOLLOW
  • Construction of Predictive Parsing Table
  • Parse the Input String

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