Re: Floating Point comparison problem

From:

red floyd <no.spam@here.dude>

Newsgroups:

comp.lang.c++

Date:

Thu, 14 Feb 2008 22:07:38 -0800

Message-ID:

<1_9tj.520$tW.486@nlpi070.nbdc.sbc.com>

arnuld wrote:

On Fri, 15 Feb 2008 14:13:32 +0500, arnuld wrote:

As FAQ explains Floating-Point is inaccurate but my project is heavily
oriented towards number-crunching, so I was thinking of stop developing
that sofwtare in C++ and simply use FORTRAN for this specific
application in a special domain.

searching the archives at Google Groups gave me a Template, created
by Jerry Coffin. I just added this to my program but my program is not
compiling:

Did you remember to #include <limits>?

That's where std::numeric_limits lives.

#include <iostream>
#include <cassert>

template<class T> bool float_equality(T const& a, T const& b, T const& factor)
{
  assert(!std::numeric_limits<T>::is_integer);

  T x = std::numeric_limits<T>::epsilon * factor * a;
  T y = std::numeric_limits<T>::epsilon * factor * b;

  T diff = std::abs(a-b);

  return (diff < x) || (diff < y);
}


int main()
{
  double d1 = 0.1;
  const double d2 = 0.9;
  d1 *= 9;
  const double my_epsilon = 1E-10;
  if( float_equality( d1, d2, my_epsilon ) )
    {
      std::cout << " == \n";
    }
  else
    {
      std::cout << " != \n";
    }

  return 0;
}

=============== OUTPUT ========================
/home/arnuld/programs $ g++ -ansi -pedantic -Wall -Wextra float-compare.cpp
float-compare.cpp: In function ???bool
float_equality(const T&, const T&, const T&) [with T = double]???:
float-compare.cpp:26: instantiated from here float-compare.cpp:11:

error: invalid operands of types ???double ()()throw ()??? and ???const
double??? to binary ???operator*???

float-compare.cpp:12: error: invalid operands of types ???double ()()throw ()??? and
???const double??? to binary ???operator*???

float-compare.cpp:14: error: call of overloaded ???abs(double)??? is ambiguous
/usr/include/stdlib.h:691: note: candidates are: int abs(int)
/usr/lib/gcc/x86_64-unknown-linux-gnu/4.2.2/../../../../include/c++/4.2.2/cstdlib:143:
note: long int std::abs(long int)
/usr/lib/gcc/x86_64-unknown-linux-gnu/4.2.2/../../../../include/c++/4.2.2/cstdlib:174:
note: long long int __gnu_cxx::abs(long\long int)

/home/arnuld/programs $