# Recursive Descent Parser

Prerequisite – Construction of LL(1) Parsing Table, Classification of top down parsers
Parsing is the process to determine whether the start symbol can derive the program or not. If the Parsing is successful then the program is a valid program otherwise the program is invalid.
There are generally two types of Parsers:

1. Top-Down Parsers:

• In this Parsing technique we expand the start symbol to the whole program.
• Recursive Descent and LL parsers are the Top-Down parsers.
2. Bottom-Up Parsers:

• In this Parsing technique we reduce the whole program to start symbol.
• Operator Precedence Parser, LR(0) Parser, SLR Parser, LALR Parser and CLR Parser are the Bottom-Up parsers.

Recursive Descent Parser:
It is a kind of Top-Down Parser. A top-down parser builds the parse tree from the top to down, starting with the start non-terminal. A Predictive Parser is a special case of Recursive Descent Parser, where no Back Tracking is required.
By carefully writing a grammar means eliminating left recursion and left factoring from it, the resulting grammar will be a grammar that can be parsed by a recursive descent parser.

Example:

Before removing left recursion After removing left recursion
E –> E + T | T
T –> T * F | F
F –> ( E ) | id
E –> T E’
E’ –> + T E’ | e
T –> F T’
T’ –> * F T’ | e
F –> ( E ) | id

**Here e is Epsilon

For Recursive Descent Parser, we are going to write one program for every variable.

```Example:
Grammar: E --> i E'
E' --> + i E' | e
```

 `int` `main() ` `{ ` `    ``// E is a start symbol. ` `    ``E(); ` ` `  `    ``// if lookahead = \$, it represents the end of the string ` `    ``// Here l is lookahead. ` `    ``if` `(l == ``'\$'``) ` `        ``printf``(``"Parsing Successful"``); ` `} ` ` `  `// Definition of E, as per the given production ` `E() ` `{ ` `    ``if` `(l == ``'i'``) { ` `        ``match(``'i'``); ` `        ``E'(); ` `    ``} ` `} ` ` `  `// Definition of E' as per the given production ` `E'()    ` `{ ` `    ``if` `(l == ``'+'``) { ` `        ``match(``'+'``); ` `        ``match(``'i'``); ` `        ``E'(); ` `    ``} ` `    ``else` `        ``return` `(); ` `} ` ` `  `// Match function ` `match(``char` `t) ` `{ ` `    ``if` `(l == t) { ` `        ``l = ``getchar``(); ` `    ``} ` `    ``else` `        ``printf``(``"Error"``); ` `} `

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