Secondly, if a given grammar is not LL(1), then usually, it is not LL(k), for any given k. We may stick to deterministic LL(1) for parser explanation, as the size of table grows exponentially with the value of k.
#RECURSIVE DESCENT PARSING IN COMPILER DESIGN CODE#
Some of the main concepts that constitute the Compiler Design questions are Lexical Analysis, Code Generation and Optimization, Parsing and more. Generally k = 1, so LL(k) may also be written as LL(1). Compiler Design is an interesting topic covered in the GATE CSE Question Paper, and candidates are encouraged to solve and practise these Compiler Design GATE questions. The first L in LL(k) is parsing the input from left to right, the second L in LL(k) stands for left-most derivation and k itself represents the number of look aheads. LL grammar can be implemented by means of both algorithms namely, recursive-descent or table-driven. LL grammar is a subset of context-free grammar but with some restrictions to get the simplified version, in order to achieve easy implementation. LL ParserĪn LL Parser accepts LL grammar. There might be instances where there is no production matching the input string, making the parsing procedure to fail. In recursive descent parsing, the parser may have more than one production to choose from for a single instance of input, whereas in predictive parser, each step has at most one production to choose. The parser refers to the parsing table to take any decision on the input and stack element combination. Both the stack and the input contains an end symbol $ to denote that the stack is empty and the input is consumed. Predictive parsing uses a stack and a parsing table to parse the input and generate a parse tree. To make the parser back-tracking free, the predictive parser puts some constraints on the grammar and accepts only a class of grammar known as LL(k) grammar. To accomplish its tasks, the predictive parser uses a look-ahead pointer, which points to the next input symbols. The predictive parser does not suffer from backtracking. Predictive parser is a recursive descent parser, which has the capability to predict which production is to be used to replace the input string. To understand this, take the following example of CFG: Top- down parsers start from the root node (start symbol) and match the input string against the production rules to replace them (if matched). This parsing technique is regarded recursive as it uses context-free grammar which is recursive in nature. A form of recursive-descent parsing that does not require any back-tracking is known as predictive parsing. But the grammar associated with it (if not left factored) cannot avoid back-tracking. This parsing technique recursively parses the input to make a parse tree, which may or may not require back-tracking. It uses procedures for every terminal and non-terminal entity. Recursive descent is a top-down parsing technique that constructs the parse tree from the top and the input is read from left to right. The types of Compiler Design top-down parser are depicted below: Recursive Descent Parsing We have learned in the last chapter of Compiler design Top-down parser that the top-down parsing technique parses the input, and starts constructing a parse tree from the root node gradually moving down to the leaf nodes.
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