10.12.  Recursive-Descent Parsing

Recursive-descent parsing is one of the simplest parsing techniques that is used in practice. Recursive-descent parsers are also called top-down parsers, since they construct the parse tree top down (rather than bottom up).

The basic idea of recursive-descent parsing is to associate each non-terminal with a procedure. The goal of each such procedure is to read a sequence of input characters that can be generated by the corresponding non-terminal, and return a pointer to the root of the parse tree for the non-terminal. The structure of the procedure is dictated by the productions for the corresponding non-terminal.

The procedure attempts to "match" the right hand side of some production for a non-terminal.

• To match a terminal symbol, the procedure compares the terminal symbol to the input; if they agree, then the procedure is successful, and it consumes the terminal symbol in the input (that is, moves the input cursor over one symbol).

• To match a non-terminal symbol, the procedure simply calls the corresponding procedure for that non-terminal symbol (which may be a recursive call, hence the name of the technique).

One example: --- We want to write a program for the following CFG:

<Sum>:           <Sum> + <Product>
             |   <Sum> - <Product>
             |   <Product>
<Product>:       <Product> * <N>
             |   <Product> / <N>
             |   <N>
<N>:             ( <Sum> )
             |   number

Last modified: 27/July/98 (12:14)