Re: why tostring for number use LUA_NUMBER_FMT "%.14g", not "%.16g" ?

[pulleyzzz_gmail <pulleyzzz@...>]

Yes, I use tostring/tonumber for the serializing of number (include float and 52bit integer). 

so, don't care the float compare, just see the big integer:

a=4222124661844650
b=tostring(a)
c=string.format('%.16g',a)
print(a==tonumber(b),a==tonumber ( c) ) --- out: false, true

for the integer, value is error.


在 2013-9-8，15:09，Coda Highland <chighland@gmail.com> 写道：

> On Sat, Sep 7, 2013 at 1:51 PM, Leo Razoumov <slonik.az@gmail.com> wrote:
>> On 9/7/13, pulleyzzz_gmail <pulleyzzz@gmail.com> wrote:
>>> for this code:
>>> 
>>> a=tostring(1/3)
>>> print(1/3==tonumber(a))  -- false
>>> 
>>> --if use .16g
>>> 
>>> a=string.format('%.16g',1/3)
>>> print(1/3==tonumber(a))  -- true
>>> 
>> 
>> Changing format to "%.16g" would not solve your problem of comparing
>> floats with == operator.
>> 
>> 1/3 is computed in Lua natively in binary representation.
>> 
>> a=string.format('%.16g',1/3) is a binary representation converted to a
>> finite length
>> base-10 number. Very often it cannot be done without loss of accuracy.
>> Next, you are converting it to binary again with tonumber(a) which
>> causes another loss of accuracy.
>> 
>> As a rule of thumb do not compare floats with == operator. It will
>> lead to subtle and
>> difficult to catch errors.
>> 
>> --Leo--
>> 
> 
> This answer, while technically correct, is actually misguided.
> 
> 1/3 == 1/3 is guaranteed to be true. It's the same expression. (1/6 +
> 1/6 == 1/3 is a different question.)
> 
> But when you're discussing SERIALIZING floats, %.14g is not
> sufficient. %.16g is sufficient to uniquely identify every possible
> double-precision IEEE floating point number with a clean round trip.
> 
> So there IS a valid question to be asked here: Why SHOULDN'T
> tostring() use %.16g?
> 
> /s/ Adam
>