Re: comparing doubles for equality

nag wrote:
On Apr 29, 10:33 pm, Richard Weeks <rwe...@xxxxxxxxxx> wrote:

After some hair pulling over the issue of comparing doubles for
equality, I came up with this solution based on the notion of
relative difference. I'd appreciate your comments.

#define MAXRELDIFF .000000000001

/* compare doubles for equality. First compare
integer parts; if they are unequal, return
false. If they are equal, calculate relative
difference of fractional parts and compare
it to a tolerance. */
int dbl_isequal(double a, double b)
{
double fraca, fracb, reldiff;

if((int)a == (int)b)
{
fraca = a - (int)a;
fracb = b - (int)b;
reldiff = rel_diff(fraca, fracb);
if(reldiff > MAXRELDIFF) return 0;
else return 1;
}
return 0;

}

double rel_diff(double a, double b)
{
double c, d;
c = fabs(a);
d = fabs(b);
c > d ? c : d; //****
return d == 0.0 ? 0.0 : fabs(a - b) / d;

}

I broke it down into two functions because I thought it might be
useful to calculate the relative difference separately. And yes,
I know that C99 introduced a boolean type.

Richard


Hi Richard,

Is something missing in the line commented ending with //****.
I didn't get the usefulness of the expression.

Thanks,
-nag.


Sorry about that. It should be part of the return statement:

return d==0.0 ? 0.0 : fabs(a-b) / (c>d ? c : d);

i.e. divide the absolute difference of a,b by whichever has the largest absolute value.
.

Relevant Pages

  • Re: comparing doubles for equality
    ... equality, I came up with this solution based on the notion of relative difference. ... /* compare doubles for equality. ... double fraca, fracb, reldiff; ...
    (comp.programming)
  • Re: comparing doubles for equality
    ... /* compare doubles for equality. ... if(reldiff> MAXRELDIFF) return 0; ... useful to calculate the relative difference separately. ...
    (comp.programming)
  • Re: comparing doubles for equality
    ... After some hair pulling over the issue of comparing doubles for ... equality, I came up with this solution based on the notion of ... relative difference. ...
    (comp.programming)