www.delorie.com/gnu/docs/bison/bison_28.html search
Bison 1.875

 [ < ] [ > ] [ << ] [ Up ] [ >> ] [Top] [Contents] [Index] [ ? ]

2.2 Infix Notation Calculator: `calc`

We now modify rpcalc to handle infix operators instead of postfix. Infix notation involves the concept of operator precedence and the need for parentheses nested to arbitrary depth. Here is the Bison code for `calc.y', an infix desk-top calculator.

 ```/* Infix notation calculator. */ %{ #define YYSTYPE double #include #include int yylex (void); void yyerror (char const *); %} /* Bison declarations. */ %token NUM %left '-' '+' %left '*' '/' %left NEG /* negation--unary minus */ %right '^' /* exponentiation */ %% /* The grammar follows. */ input: /* empty */ | input line ; line: '\n' | exp '\n' { printf ("\t%.10g\n", \$1); } ; exp: NUM { \$\$ = \$1; } | exp '+' exp { \$\$ = \$1 + \$3; } | exp '-' exp { \$\$ = \$1 - \$3; } | exp '*' exp { \$\$ = \$1 * \$3; } | exp '/' exp { \$\$ = \$1 / \$3; } | '-' exp %prec NEG { \$\$ = -\$2; } | exp '^' exp { \$\$ = pow (\$1, \$3); } | '(' exp ')' { \$\$ = \$2; } ; %% ```

The functions `yylex`, `yyerror` and `main` can be the same as before.

There are two important new features shown in this code.

In the second section (Bison declarations), `%left` declares token types and says they are left-associative operators. The declarations `%left` and `%right` (right associativity) take the place of `%token` which is used to declare a token type name without associativity. (These tokens are single-character literals, which ordinarily don't need to be declared. We declare them here to specify the associativity.)

Operator precedence is determined by the line ordering of the declarations; the higher the line number of the declaration (lower on the page or screen), the higher the precedence. Hence, exponentiation has the highest precedence, unary minus (`NEG`) is next, followed by `*' and `/', and so on. See section Operator Precedence.

The other important new feature is the `%prec` in the grammar section for the unary minus operator. The `%prec` simply instructs Bison that the rule `| '-' exp' has the same precedence as `NEG`---in this case the next-to-highest. See section Context-Dependent Precedence.

Here is a sample run of `calc.y':

 ```\$ calc 4 + 4.5 - (34/(8*3+-3)) 6.880952381 -56 + 2 -54 3 ^ 2 9 ```

 [ < ] [ > ] [ << ] [ Up ] [ >> ] [Top] [Contents] [Index] [ ? ]

 webmaster   donations   bookstore delorie software   privacy Copyright © 2003   by The Free Software Foundation Updated Jun 2003