Re: [Dev-C++] atof 2...

[Per W. <pw...@ia...>]

Hello!

Don't use atof() for converting from text string to a double.

Use strtod() instead, since it will inform you how many characters it
consumed.

char mystring[] =3D "1.2E-3*3";
char *p =3D mystring;
char *end;
num =3D strtod(p,&end);

Now end will point to the '*' character, since '1.2E-3' is consumed and
returned as the double 0.0012.

Note also when parsing infix expressions, that you normally have to take
care of unary signs, i.e. 1+-2 which should be evaluated as 1 + (-2).

Easiest is to perform the parsing with a recursive-descent parser, i.e.
something like:

eval_expr()
  return eval_additive()

eval_additive()
  while (*p =3D=3D '+' || *p =3D=3D '-')
    if (*p =3D=3D '+') {
      p++;
      n +=3D eval_multiplicative()
    } else {
      p++;
      n -=3D eval_multiplicative()
    }
  return n;

eval_multiplicative()
  n =3D eval_factor()
  while (*p =3D=3D '*' || *p =3D=3D '/')
    if (*p =3D=3D '*') {
      p++;
      n *=3D eval_factor();
    } else {
      p++;
      n /=3D eval_factor();
    }
  return n;

eval_factor()
  if (*p =3D=3D =B4+') {p++; return eval_factor(); }
  if (*p =3D=3D '-') {p++; return -eval_factor(); }
  if (*p =3D=3D '(') {p++; n =3D eval_expr(); if (p !=3D ')') error(); retu=
rn n; }
  return number();

Good luck!

/Per W

On Sun, 19 Jan 2003, j b wrote:

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