Classification of Top Down Parsers
Parsing is classified into two categories, i.e. Top-Down Parsing, and Bottom-Up Parsing. Top-Down Parsing is based on Left Most Derivation whereas Bottom-Up Parsing is dependent on Reverse Right Most Derivation.
The process of constructing the parse tree which starts from the root and goes down to the leaf is Top-Down Parsing.
- Top-Down Parsers constructs from the Grammar which is free from ambiguity and left recursion.
- Top-Down Parsers uses leftmost derivation to construct a parse tree.
- It does not allow Grammar With Common Prefixes.
Classification of Top-Down Parsing –
1. With Backtracking: Brute Force Technique
2. Without Backtracking:
- Recursive Descent Parsing
- Predictive Parsing or Non-Recursive Parsing or LL(1) Parsing or Table Driver Parsing
Recursive Descent Parsing –
- Whenever a Non-terminal spend the first time then go with the first alternative and compare it with the given I/P String
- If matching doesn’t occur then go with the second alternative and compare with the given I/P String.
- If matching is not found again then go with the alternative and so on.
- Moreover, If matching occurs for at least one alternative, then the I/P string is parsed successfully.
LL(1) or Table Driver or Predictive Parser –
- In LL1, first L stands for Left to Right and second L stands for Left-most Derivation. 1 stands for a number of Look Ahead tokens used by parser while parsing a sentence.
- LL(1) parsing is constructed from the grammar which is free from left recursion, common prefix, and ambiguity.
- LL(1) parser depends on 1 look ahead symbol to predict the production to expand the parse tree.
- This parser is Non-Recursive.
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