Re: Float numbers equality.

[Leo Razoumov <slonik.az@...>]

On 8/18/13, Coda Highland <chighland@gmail.com> wrote:
> On Sun, Aug 18, 2013 at 4:09 PM, Leo Razoumov <slonik.az@gmail.com> wrote:
>> On 8/9/13, Luiz Henrique de Figueiredo <lhf@tecgraf.puc-rio.br> wrote:
>>>> function float_equal(lhs, rhs, epsilon)
>>>>     return math.abs(lhs - rhs) < epsilon
>>>> end
>>>
>>> As mentioned before, if you have to do this, you need to use *relative*
>>> error,
>>> not absolute error:
>>>       return math.abs(lhs - rhs) < epsilon*rhs
>>>
>>
>> The code above is incorrect for rhs <= 0.
>>
>> It is better to try the following (untested):
>>
>> function float_equal(lhs, rhs, epsilon)
>>     local abs= math.abs
>>     epsilon  = epsilon or 1E-12
>>     return abs(lhs - rhs) <= epsilon * (abs(lhs) + abs(rhs))
>> end
>>
>> This implementation handles correctly the cases when one or both
>> numbers are zero or negative,
>>  and, (the added bonus) the function is symmetrical w.r.t. lhs<->rhs
>> substitutions.
>>
>> --Leo--
>>
>
> Actually, the correction is simply to use "epsilon * abs(rhs)" instead
> of "epsilon * rhs".
>

No, expression
    return abs(lhs - rhs) <= epsilon * abs(rhs)
does not handle the case of rhs=0.
E.g.: epsilon=1E-12; lhs= 1E-16; rhs=0;

--Leo--