Bizarre output.

Bizarre output.

[Srinath M]

Can someone tell me why the text, "I see it" is not displayed when this
program is run?

#include <stdio.h>
                                                                       
                                                                
int main(void)
{
  float a, b;
                                                                       
                                                                
  double d;
                                                                       
                                                                
  a = 0.;
                                                                       
                                                                
  b = 1.;
                                                                       
                                                                
  for (d  = 0.0; d < 0.011; d = d + 0.001)
    {
      printf("d = %f\n",d);
                                                                       
                                                                
      if (d == 0.009)
        {
          printf ("I see it\n");
        }
    }
  return 0;
}


		
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Re: Bizarre output.

[Eljay Love-Jensen]

Hi Antonio,

>Hmm, maybe i understood something wrong, but didn't corey mean that  his function double eps(double a, double b) calculates a magnitude based epsilon from fuzzyEpsilon and a ?

The definition of "close enough" can vary by the problem being considered.  As such, there are several strategies (algorithms) one can employ to determine if a float or double is "close enough" to a given value.

My usual suggestion is "Use integers unless you can't."

>By the way, something completely differnet: What means HTH ?

HTH - hope that helps

--Eljay

Re: Bizarre output.

[corey taylor]

Yes, I'm sorry if this was confusing, it was the only method I had and
works well.

This is the same as an evaluation within a fuzzyEpsilon error period,
except that error period changes according to the precision of the
numbers being evaluated.

corey


On Wed, 30 Mar 2005 01:33:55 +0200, Antonio Coralles
<noche.suapie@reflex.at> wrote:
> Eljay Love-Jensen wrote:
> 
> > Hi Antonio,
> >
> >>What value of fuzzyEpsilon is appropriate for float / long double ? I
> > guess there is somkind of mathematical law in calculating it's value
> > from sizeof(value_type) ... Or is 0.000001 just an educated guess ?
> >
> > The value for fuzzyEpsilon is dependent on the magnitude of the
> > numbers involved, not on whether it is a double or float.
> >
> > HTH,
> > --Eljay
> >
> Hmm, maybe i understood something wrong, but didn't corey mean that  his
> function
> double eps(double a, double b) calculates a magnitude based epsilon from
> fuzzyEpsilon and a ?
> 
> By the way, something completely differnet: What means HTH ?
> antonio
>

Re: Bizarre output.

[corey taylor]

> >     (void)b;
> 
> If never seen this kind of cast before ... Is this statndard c/c++ or is
> it an gcc extension ? What is the effect of it ?

My apologies for the extra paramter.  This function is part of a
library I help maintain, and we don't break old revisions much so it
was just left in apparantly.

(void)b; 

This removes the unused parameter warning by using the parameter by
voiding the effect.

corey

Re: Bizarre output.

[Antonio Coralles]

Eljay Love-Jensen wrote:

> Hi Antonio,
>
>>What value of fuzzyEpsilon is appropriate for float / long double ? I
> guess there is somkind of mathematical law in calculating it's value
> from sizeof(value_type) ... Or is 0.000001 just an educated guess ?
>
> The value for fuzzyEpsilon is dependent on the magnitude of the
> numbers involved, not on whether it is a double or float.
>
> HTH,
> --Eljay
>
Hmm, maybe i understood something wrong, but didn't corey mean that  his
function
double eps(double a, double b) calculates a magnitude based epsilon from
fuzzyEpsilon and a ?

By the way, something completely differnet: What means HTH ?
antonio

Re: Bizarre output.

[Eljay Love-Jensen]

Hi Antonio,

>What value of fuzzyEpsilon is appropriate for float / long double ? I guess there is somkind of mathematical law in calculating it's value from sizeof(value_type) ... Or is 0.000001 just an educated guess ?

The value for fuzzyEpsilon is dependent on the magnitude of the numbers involved, not on whether it is a double or float.

HTH,
--Eljay

Re: Bizarre output.

[Antonio Coralles]

corey taylor wrote:

> This is one method of fuzzy floating point equates using a
> magnitude-based epsilon.
>
> const double fuzzyEpsilon = 0.000001;

What value of fuzzyEpsilon is appropriate for float / long double ? I
guess there is somkind of mathematical law in calculating it's value
from sizeof(value_type) ... Or is 0.000001 just an educated guess ?

>
> /**
>  Computes an appropriate epsilon for comparing a and b.
>  */
> inline double eps(double a, double b) {
>     // For a and b to be nearly equal, they must have nearly
>     // the same magnitude.  This means that we can ignore b
>     // since it either has the same magnitude or the comparison
>     // will fail anyway.
>     (void)b;

If never seen this kind of cast before ... Is this statndard c/c++ or is
it an gcc extension ? What is the effect of it ?

>     const double aa = abs(a) + 1;
>     if (aa == inf()) {
>         return fuzzyEpsilon;
>     } else {
>         return fuzzyEpsilon * aa;
>     }
> }
>
> inline bool fuzzyEq(double a, double b) {
>     return (a == b) || (abs(a - b) <= eps(a, b));
> }
>
> corey
>
antonio

Re: Bizarre output.

[corey taylor]

This is one method of fuzzy floating point equates using a
magnitude-based epsilon.

const double fuzzyEpsilon = 0.000001;

/**
 Computes an appropriate epsilon for comparing a and b.
 */
inline double eps(double a, double b) {
    // For a and b to be nearly equal, they must have nearly
    // the same magnitude.  This means that we can ignore b
    // since it either has the same magnitude or the comparison
    // will fail anyway.
    (void)b;
    const double aa = abs(a) + 1;
    if (aa == inf()) {
        return fuzzyEpsilon;
    } else {
        return fuzzyEpsilon * aa;
    }
}

inline bool fuzzyEq(double a, double b) {
    return (a == b) || (abs(a - b) <= eps(a, b));
}

corey

On Wed, 30 Mar 2005 00:12:58 +0200, Antonio Coralles
<noche.suapie@reflex.at> wrote:
> Srinath M wrote:
> 
> > Can someone tell me why the text, "I see it" is not displayed when this
> > program is run?
> >
> > #include <stdio.h>
> >
> >
> > int main(void)
> > {
> >   float a, b;
> >
> >
> >   double d;
> >
> >
> >   a = 0.;
> >
> >
> >   b = 1.;
> >
> >
> >   for (d  = 0.0; d < 0.011; d = d + 0.001)
> >     {
> >       printf("d = %f\n",d);
> >
> >
> >       if (d == 0.009)
> >         {
> >           printf ("I see it\n");
> >         }
> >     }
> >   return 0;
> > }
> >
> Well, in my opinion you should normally not use "==" in conection with
> float or double, because floating-point arithmetic is not exact (not
> only with gcc). Besides 0.001 has not an exact representation as
> floating point number ... Try
> ...
> if(d > 0.008999 && d < 0.009001)
>     printf(...);
> ...
> antonio
> 
>

Re: Bizarre output.

[Antonio Coralles]

Srinath M wrote:

> Can someone tell me why the text, "I see it" is not displayed when this
> program is run?
>
> #include <stdio.h>
>                                                                        
>                                                                 
> int main(void)
> {
>   float a, b;
>                                                                        
>                                                                 
>   double d;
>                                                                        
>                                                                 
>   a = 0.;
>                                                                        
>                                                                 
>   b = 1.;
>                                                                        
>                                                                 
>   for (d  = 0.0; d < 0.011; d = d + 0.001)
>     {
>       printf("d = %f\n",d);
>                                                                        
>                                                                 
>       if (d == 0.009)
>         {
>           printf ("I see it\n");
>         }
>     }
>   return 0;
> }
>
Well, in my opinion you should normally not use "==" in conection with
float or double, because floating-point arithmetic is not exact (not
only with gcc). Besides 0.001 has not an exact representation as
floating point number ... Try
...
if(d > 0.008999 && d < 0.009001)
    printf(...);
...
antonio