Re: parser needed

From: Malcolm (malcolm_at_55bank.freeserve.co.uk)
Date: 01/30/04 

Date: Fri, 30 Jan 2004 20:17:03 -0000

"Mahdiarnt" <mahdiarnt@yahoo.com> wrote in message
> Is there any out there that I can copy in my source file? Or any other
> solution?
>

Feel free to modify this as you wish.

#include <assert.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#include <math.h>

#include <stdio.h>

#include "eval.h"

/*
  evaluates an arithmetical expression.

  + - * / % (modulus) ()

  also adds keywords
  sin,
  cos,
  tan,
  ln,
  pow,
  sqrt,
  e
  PI

  returns the value of the expression

  error - return for error code
  0 = OK
  1 = an error
*/

/* tokens defined */
#define EOL 0
#define VALUE 1
#define PI 2
#define E 3
#define MOD 4
#define SIN 5
#define COS 6
#define TAN 7
#define LN 8
#define POW 9
#define DIV 10
#define MULT 11
#define OPAREN 12
#define CPAREN 13
#define PLUS 14
#define MINUS 15
#define SHRIEK 16
#define COMMA 17
#define SQRT 18
#define SPACE 19
#define ERROR 20

static double getvalue(const char *str, int *nread);
static double expr(void);
static double term(void);
static double factor(void);
static void match(int tok);

static int gettoken(const char *str);
static int tokenlen(const char *str, int token);

static double factorial(double x);
static double getvalue(const char *str, int *len);

static const char *string; /* string we are parsing */
static int token; /* current token (lookahead) */
static int errorflag; /* set when error in input encountered */

/*
  evaluate an arithetical expression.
  Params: str - string containing expression
          error - return pointer, 0 on OK, 1 if error
  Returns: the value of the expression

*/
double eval(const char *str, int *error)
{
  double answer;

  errorflag = 0;
  string = str;
  token = gettoken(str);

  answer = expr();

  match(EOL);

  if(errorflag)
  {
    if(error)
   *error = 1;
 answer = 0.0;
  }
  else
 if(error)
   *error = 0;

  return answer;
}

/*
  parses an expression
*/
static double expr(void)
{
  double left;
  double right;

  left = term();

  while(1)
  {
    switch(token)
 {
 case PLUS:
   match(PLUS);
   right = term();
   left += right;
   break;
 case MINUS:
   match(MINUS);
      right = term();
   left -= right;
   break;
 default:
   return left;
 }
  }
}

/*
  parses a term
*/
static double term(void)
{
  double left;
  double right;

  left = factor();

  while(1)
  {
    switch(token)
 {
 case MULT:
   match(MULT);
   right = factor();
   left *= right;
   break;
 case DIV:
   match(DIV);
   right = factor();
   if(right != 0.0)
     left /= right;
   else
  errorflag = 1;
   break;
 case MOD:
   match(MOD);
   right = factor();
   left = fmod(left, right);
   break;
 default:
   return left;
 }
  }

}

/*
  parses a factor
*/
static double factor(void)
{
  double answer = 0;
  int len;

  switch(token)
  {
    case OPAREN:
   match(OPAREN);
   answer = expr();
   match(CPAREN);
   break;
 case VALUE:
   answer = getvalue(string, &len);
   match(VALUE);
   break;
 case MINUS:
   match(MINUS);
   answer = -factor();
   break;
 case E:
   answer = exp(1.0);
   match(E);
   break;
 case PI:
   answer = acos(0.0) * 2.0;
   match(PI);
   break;
 case SIN:
   match(SIN);
   match(OPAREN);
   answer = expr();
   match(CPAREN);
   answer = sin(answer);
   break;
 case COS:
   match(COS);
   match(OPAREN);
   answer = expr();
   match(CPAREN);
   answer = cos(answer);
   break;
 case TAN:
   match(TAN);
   match(OPAREN);
   answer = expr();
   match(CPAREN);
   answer = tan(answer);
   break;
 case LN:
   match(LN);
   match(OPAREN);
   answer = expr();
   match(CPAREN);
   if(answer > 0)
     answer = log(answer);
   else
  errorflag = 1;
   break;
 case POW:
      match(POW);
   match(OPAREN);
   answer = expr();
   match(COMMA);
      answer = pow(answer, expr());
   match(CPAREN);
   break;
 case SQRT:
   match(SQRT);
   match(OPAREN);
   answer = expr();
   match(CPAREN);
   if(answer >= 0.0)
  answer = sqrt(answer);
   else
  errorflag = 1;
   break;
 default:
   errorflag = 1;
   break;
  }

  while(token == SHRIEK)
  {
    match(SHRIEK);
 answer = factorial(answer);
  }

  return answer;
}

/*
  check that we have a token of the passed type
  (if not set the errorflag)
  Move parser on to next token. Sets token and string.
*/
static void match(int tok)
{
  if(token != tok)
  {
 errorflag = 1;
  }

  while(isspace(*string))
 string++;

  string += tokenlen(string, token);
  token = gettoken(string);
  if(token == ERROR)
 errorflag = 1;
}

/*
  get a token from the string
  Params: str - string to read token from
  Notes: ignores white space between tokens
*/
static int gettoken(const char *str)
{
  while(isspace(*str))
    str++;

  if(isdigit(*str))
    return VALUE;

  switch(*str)
  {
    case 0:
   return EOL;
 case '/':
   return DIV;
 case '*':
   return MULT;
 case '(':
   return OPAREN;
 case ')':
   return CPAREN;
 case '+':
   return PLUS;
 case '-':
   return MINUS;
 case '!':
   return SHRIEK;
 case '%':
   return MOD;
 case 'e':
   return E;
 case ',':
   return COMMA;
 default:
   if(!strncmp(str, "sin", 3))
     return SIN;
      if(!strncmp(str, "cos", 3))
     return COS;
   if(!strncmp(str, "tan", 3))
     return TAN;
   if(!strncmp(str, "ln", 2))
     return LN;
   if(!strncmp(str, "pow", 3))
     return POW;
   if(!strncmp(str, "PI", 2))
     return PI;
   if(!strncmp(str, "sqrt", 4))
  return SQRT;

   return ERROR;
  }
}

/*
  get the length of a token.
  Params: str - pointer to the string containing the token
          token - the type of the token read
  Returns: length of the token, or 0 for EOL to prevent
           it being read past.
*/
static int tokenlen(const char *str, int token)
{
  int len = 0;

  switch(token)
  {
    case EOL:
   return 0;
    case VALUE:
   getvalue(str, &len);
   return len;
    case PI:
   return 2;
    case E:
   return 1;
    case MOD:
   return 1;
    case SIN:
   return 3;
    case COS:
   return 3;
    case TAN:
   return 3;
    case LN:
   return 2;
    case POW:
   return 3;
 case SQRT:
   return 4;
    case DIV:
   return 1;
    case MULT:
   return 1;
 case OPAREN:
   return 1;
    case CPAREN:
   return 1;
    case PLUS:
   return 1;
    case MINUS:
   return 1;
    case SHRIEK:
   return 1;
    case COMMA:
   return 1;
    case SPACE:
   return 1;
    case ERROR:
   return 0;
 default:
   assert(0);
   return 0;
  }
}

/*
  compute x!
*/
static double factorial(double x)
{
  double answer = 1.0;
  double t;

  for(t=1;t<=x;t+=1.0)
 answer *= t;
  return answer;
}

/*
  strtod implementation:
  Params: str - pointer to string
          nread - return pointer for no bytes read
  Returns: value of the string
*/
static double getvalue(const char *str, int *len)
{
  char *end;
  double answer;

  while(isspace(*str))
 str++;
  answer = strtod(str, &end);
  *len = end - str;

  return answer;
}

static double oldgetvalue(const char *str, int *nread)
{
  double answer = 0.0;
  int ndecimal = 1;

  while(isspace(*str))
 str++;

  assert(isdigit(*str));

  *nread = 0;

  while(isdigit(*str))
  {
    answer *= 10;
    answer += *str - '0';
 *nread += 1;
 str++;
  }
  if(*str == '.')
  {
    str++;
 *nread += 1;

 while(isdigit(*str))
 {
   answer += (*str - '0') / (10.0 * ndecimal);
   ndecimal++;
   str++;
   *nread += 1;
 }
  }

  return answer;
}

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