checking for close enough floating point values.

[pereges]

Let's say I have two doubles:

double a, b;

a = 9.3567891003459 2
b = 9.3567892560233 4

Obviously, a < b but lets say I just want to check up to 6 places
after the decimal. I want to check if the condition a >= b is
satisfied. I have a tolerance value:

#define EPSILON 0.000001

Is this a good way to check for a >= b

if( fabs(a-b) <= EPSILON)
....

[vippstar@gmail.com]

Re: checking for close enough floating point values.

On May 29, 8:30 pm, pereges <Brol...@gmail. comwrote:

Let's say I have two doubles:
>
double a, b;
>
a = 9.3567891003459 2
b = 9.3567892560233 4
>
Obviously, a < b but lets say I just want to check up to 6 places
after the decimal. I want to check if the condition a >= b is
satisfied. I have a tolerance value:
>
#define EPSILON 0.000001
>
Is this a good way to check for a >= b
>
if( fabs(a-b) <= EPSILON)
...

Question 14.5 of the C-FAQ.
<http://c-faq.com/>

[Barry Schwarz]

Re: checking for close enough floating point values.

On Thu, 29 May 2008 10:30:49 -0700 (PDT), pereges <Broli00@gmail. com>
wrote:

>Let's say I have two doubles:
>
>double a, b;
>
>a = 9.3567891003459 2
>b = 9.3567892560233 4
>
>Obviously, a < b but lets say I just want to check up to 6 places
>after the decimal. I want to check if the condition a >= b is
>satisfied. I have a tolerance value:
>
>#define EPSILON 0.000001
>
>Is this a good way to check for a >= b
>
>if( fabs(a-b) <= EPSILON)
>...

This checks for a == b, not a >= b.

if (a + EPSILON >= b) would be the corresponding check for >=.

However, you should read 14.5 in the c-faq (www.c-faq.com) to see why
an unscaled EPSILON is a bad idea.


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[pereges]

Re: checking for close enough floating point values.

On May 30, 9:52 am, Barry Schwarz <schwa...@dqel. comwrote:

This checks for a == b, not a >= b.
>
if (a + EPSILON >= b) would be the corresponding check for >=.
>
However, you should read 14.5 in the c-faq (www.c-faq.com) to see why
an unscaled EPSILON is a bad idea.

So do you think it will be better to use the relative difference
method as suggested in the C faq 14.5 ?

#define Abs(x) ((x) < 0 ? -(x) : (x))
#define Max(a, b) ((a) (b) ? (a) : (b))

double RelDif(double a, double b)
{
double c = Abs(a);
double d = Abs(b);

d = Max(c, d);

return d == 0.0 ? 0.0 : Abs(a - b) / d;
}


....
to check for a >= b,

if(RelDif(a, b) <= TOLERANCE || a b)
...

Btw the floating point data that I'm reading has 6 places after the
decimal point so I though I would take a tolerance value of 0.000001.

[pereges]

Re: checking for close enough floating point values.

I don't know why a seperate macro has been written for finding
absolute value. Was it not possible to use fabs() ?

#define Abs(x) ((x) < 0 ? -(x) : (x))

[Bartc]

Re: checking for close enough floating point values.


"Barry Schwarz" <schwarzb@dqel. comwrote in message
news:inku34935b b3u8akoohq73ffc 3jnvj02e7@4ax.c om...

On Thu, 29 May 2008 10:30:49 -0700 (PDT), pereges <Broli00@gmail. com>
wrote:
>

>>Let's say I have two doubles:
>>
>>double a, b;
>>
>>a = 9.3567891003459 2
>>b = 9.3567892560233 4
>>
>>Obviously, a < b but lets say I just want to check up to 6 places
>>after the decimal. I want to check if the condition a >= b is
>>satisfied. I have a tolerance value:
>>
>>#define EPSILON 0.000001
>>
>>Is this a good way to check for a >= b
>>
>>if( fabs(a-b) <= EPSILON)
>>...

>
This checks for a == b, not a >= b.
>
if (a + EPSILON >= b) would be the corresponding check for >=.

What about:

if ( a>b || (fabs(a-b)<=EPSILON) )

for a>=b ? This would be faster when a is normally expected to be greater
than b.

--
Bartc

[Barry Schwarz]

Re: checking for close enough floating point values.

On Fri, 30 May 2008 02:07:53 -0700 (PDT), pereges <Broli00@gmail. com>
wrote:

>On May 30, 9:52 am, Barry Schwarz <schwa...@dqel. comwrote:
>

>This checks for a == b, not a >= b.
>>
>if (a + EPSILON >= b) would be the corresponding check for >=.
>>
>However, you should read 14.5 in the c-faq (www.c-faq.com) to see why
>an unscaled EPSILON is a bad idea.

>
>So do you think it will be better to use the relative difference
>method as suggested in the C faq 14.5 ?
>

snip

>Btw the floating point data that I'm reading has 6 places after the
>decimal point so I though I would take a tolerance value of 0.000001.

If you can guarantee that all the data will be within a certain range
then you don't need to scale based on the current value. But what
happens when one set of values is near 1e-6 and another is near 1e10?
That is the point I thought the faq was trying to make.


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