Node:Rpcalc Decls, Next:Rpcalc Rules, Up:RPN Calc

Declarations for rpcalc

Here are the C and Bison declarations for the reverse polish notation calculator. As in C, comments are placed between /*...*/.

/* Reverse polish notation calculator. */

%{
#define YYSTYPE double
#include <math.h>
%}

%token NUM

%% /* Grammar rules and actions follow */

The C declarations section (see The C Declarations Section) contains two preprocessor directives.

The #define directive defines the macro YYSTYPE, thus specifying the C data type for semantic values of both tokens and groupings (see Data Types of Semantic Values). The Bison parser will use whatever type YYSTYPE is defined as; if you don't define it, int is the default. Because we specify double, each token and each expression has an associated value, which is a floating point number.

The #include directive is used to declare the exponentiation function pow.

The second section, Bison declarations, provides information to Bison about the token types (see The Bison Declarations Section). Each terminal symbol that is not a single-character literal must be declared here. (Single-character literals normally don't need to be declared.) In this example, all the arithmetic operators are designated by single-character literals, so the only terminal symbol that needs to be declared is NUM, the token type for numeric constants.