calcparser.cpp

用于词法分析的词法分析器

/*
  $Id: CalcParser.cpp,v 1.2 1997/04/20 01:17:54 matt Exp $

  CalcParser class.  Implements a simple calculator parser and
  evaluator.
  
  (c) Matt Phillips, 1997.  */

#include <stdlib.h>
#include <math.h>

#include "CalcParser.h"

CalcParser::CalcParser (istream &input_) :
  input (input_), scanner (input), result (0.0)
{
  scanner.getNext ();

  result = parseExpr ();

  if (!scanner.eof ())
    scanner.error ("junk at end of expression");
}

double CalcParser::parseExpr ()
{
  double acc = parseTerm ();
  
  while (1)
  {
    if (scanner.have (CalcScanner::SymPlus))
      acc += parseTerm ();
    else if (scanner.have (CalcScanner::SymMinus))
      acc -= parseTerm ();
    else
      break;
  }

  return acc;
}

double CalcParser::parseTerm ()
{
  double acc = parseFactor ();
  
  while (1)
  {
    if (scanner.have (CalcScanner::SymStar))
      acc *= parseFactor ();
    else if (scanner.have (CalcScanner::SymSlash))
      acc /= parseFactor ();
    else if (scanner.have (CalcScanner::SymMod))
      acc = fmod (acc, parseFactor ());
    else
      break;
  }

  return acc;
}

double CalcParser::parseFactor ()
{
  double acc = 0.0;

  if (scanner.getSymbol () == CalcScanner::SymVariable)
  {
    acc = getVariableValue (scanner.getText ());
    scanner.getNext ();
  } else if (scanner.getSymbol () == CalcScanner::SymNumber)
  {
    acc = getNumberValue (scanner.getText ());
    scanner.getNext ();
  } else if (scanner.have (CalcScanner::SymLBracket))
  {
    acc = parseExpr ();
    if (!scanner.have (CalcScanner::SymRBracket))
    {
      scanner.error ("missing ')'");
    }
  } else
  {
    scanner.error ("expected a factor, but found '" + scanner.getText () + "'");
  }

  return acc;
}

double CalcParser::getVariableValue (const string &variable)
{
  struct Variable
  {
    char *name;
    double value;
  };

  static Variable variables [] =
  {
    {"pi", 3.1415927},
    {"e",  2.7182818}
  };

  for (int i = 0; i < sizeof (variables) / sizeof (Variable); i++)
  {
    if (variables [i].name == variable)
      return variables [i].value;
  }

  scanner.error ("'" + variable + "' is not a defined variable name");

  return 0.0;
}

double CalcParser::getNumberValue (const string &number)
{
  return atof (number.c_str ());
}