(bison.info.gz) Rpcalc Lexer

 
 The `rpcalc' Lexical Analyzer
 -----------------------------
 
    The lexical analyzer's job is low-level parsing: converting
 characters or sequences of characters into tokens.  The Bison parser
 gets its tokens by calling the lexical analyzer.   The Lexical
 Analyzer Function `yylex' Lexical.
 
    Only a simple lexical analyzer is needed for the RPN calculator.
 This lexical analyzer skips blanks and tabs, then reads in numbers as
 `double' and returns them as `NUM' tokens.  Any other character that
 isn't part of a number is a separate token.  Note that the token-code
 for such a single-character token is the character itself.
 
    The return value of the lexical analyzer function is a numeric code
 which represents a token type.  The same text used in Bison rules to
 stand for this token type is also a C expression for the numeric code
 for the type.  This works in two ways.  If the token type is a
 character literal, then its numeric code is that of the character; you
 can use the same character literal in the lexical analyzer to express
 the number.  If the token type is an identifier, that identifier is
 defined by Bison as a C macro whose definition is the appropriate
 number.  In this example, therefore, `NUM' becomes a macro for `yylex'
 to use.
 
    The semantic value of the token (if it has one) is stored into the
 global variable `yylval', which is where the Bison parser will look for
 it.  (The C data type of `yylval' is `YYSTYPE', which was defined at
 the beginning of the grammar;  Declarations for `rpcalc' Rpcalc
 Decls..)
 
    A token type code of zero is returned if the end-of-input is
 encountered.  (Bison recognizes any nonpositive value as indicating
 end-of-input.)
 
    Here is the code for the lexical analyzer:
 
      /* The lexical analyzer returns a double floating point
         number on the stack and the token NUM, or the numeric code
         of the character read if not a number.  It skips all blanks
         and tabs, and returns 0 for end-of-input.  */
      
      #include <ctype.h>
      
      int
      yylex (void)
      {
        int c;
      
        /* Skip white space.  */
        while ((c = getchar ()) == ' ' || c == '\t')
          ;
        /* Process numbers.  */
        if (c == '.' || isdigit (c))
          {
            ungetc (c, stdin);
            scanf ("%lf", &yylval);
            return NUM;
          }
        /* Return end-of-input.  */
        if (c == EOF)
          return 0;
        /* Return a single char.  */
        return c;
      }
Info Catalog
(bison.info.gz) Rpcalc Rules
(bison.info.gz) RPN Calc
(bison.info.gz) Rpcalc Main
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