How to write a recursive descent parser

This is especially effective for languages which are as "simple" as Basic.

How to write a recursive descent parser

Tokenization The right-recursive grammar is well-suited to predictive parsing because each input token indicates unambiguously which production should be applied. Before parsing an expression with this grammar, the expression's constituent elements must be identified.

This process is called tokenization: A lexeme is an instance of a token in the input. Regular expressions are a relatively compact and flexible way of denoting tokens; they are a standard part of lexical analysis tools such as Lex.

Even though regular expressions may be overkill for tokenizing mathematical expressions, I didn't want to hard code lexemes into Tokenizer. I wanted to make it easy to represent tokens such as floating point numbers, which would otherwise require a separate grammar definition, placing their extraction in the parsing instead of the tokenization process.

Therefore, the lexemes corresponding to each TokenType are encoded in a regular expression stored in each TokenType instance. Tokenizer Class package example; import java. If no token is found, throws an exception. First, the method skips over any token separators—by default, whitespace.

Then it attempts to match each token type pattern, starting from the current input offset, until it finds a match. It is possible to implement the method more efficiently, so that only one match attempt needs to be attempted per method call. Doing so, however, would make the method harder to understand as an example.

how to write a recursive descent parser

MathTokenType Class package example; import java. MathOperator Class package example; import java. Even though it is possible to compute the value of an expression as part of the parsing process, I wanted to demonstrate the separation between parsing and evaluation present in systems such as scripting languages, which compile a program into an intermediate byte code that is interpreted later.

MathParser Class package example; import java. MathEvaluator does not attempt to be efficient; instead, it demonstrates how postfix expressions can be evaluated easily.

how to write a recursive descent parser

Operators are popped off the stack and their operands are evaluated before applying the operator and pushing the result back onto the stack.

MathEvaluator Class package example; import java. Top-down parsing applies productions to its input, starting with the start symbol and working its way down the chain of productions, creating a parse tree defined by the sequence of recursive non-terminal expansions.

The tree is such that the parsing starts at the top and works its way down to the leaves, where terminal symbols in our example, numbers reside, terminating recursion.

Recursive descent parsers are straightforward to implement once you have defined a right-recursive grammar. All you have to do is write a function for each production. The functions mirror exactly the productions; they decide how to expand a non-terminal based on a lookahead token.

The lookahead token is provided by Tokenizer. For example, parseExpression directly calls parseTerm because term is the first non-terminal in the production.Dec 08,  · So far it only supports single-digit numbers, addition, multiplication and parentheses. You decide what comes next! Wanna try skorbut yourself?

https://githu. There are two main parts to a recursive descent parser. First it is recursive.

Writing a Simple Recursive Descent Parser

This may be obvious, but unlike most recursion it uses a technique called “mutual recursion”. for a recursive. Jul 16,  · Privacy & Cookies: This site uses cookies. By continuing to use this website, you agree to their use. To find out more, including how to control cookies, see here.

Jan 30,  · It is the procedure oriented algorithm to parse the string. Some good examples have been explained in C programming language.

Eliminating Ambiguity

link to my channel-. Nov 09,  · Is pyparsing really a recursive descent parser? I ask this because there are grammars it can't parse that my recursive descent parser would parse, should I have written one. You are to write a recursive descent parser for this language.

To keep things simple, you can initially make all the tokens single characters, so that the scanner (lexical analyser) can be very simple, as shown in the earlier examples.

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