json_real(float or double value)

[SJwith]

json_real(25.89)

-> 25.8899999999999999

json_real((double)25.89)

-> 25.8899999999999999

json_real((float)25.89)

-> 25.8899999999999999

json_t* rtmp = json_real(0.0);

json_real_set(rtmp, 25.89);

->25.890000000000001

can I express float or double value exactly (like 25.89)

plz help;

do I use wrong way?;;

json_real(0.1)

-> 0.09999999999999999

json_real(0.2)

->0.019999999999999999

json_real(55.98)

->55.9799999999999999

[Petri Lehtinen]

You do nothing wrong. What you're seeing is a "feature" of IEEE 754
floating points.

There's no exact binary representation for the decimal numbers 25.89,
0.1, 0.2 or 55.98, and many others. So when converted to double and
back to decimal again with precision 17 you get a different end
result.

Precision 17 is required, because otherwise some numbers would lose
precision. For example:

printf("%.17g", 1.0000000000000002)
-> 1.0000000000000002

(this is what e.g. json_dumps uses internally)

But:

printf("%.16g", 1.0000000000000002)
-> 1.0

So if Jansson used less precision, you would get a rounded result.

Petri

[Basile Starynkevitch]

On Wed, Apr 09, 2014 at 10:40:28PM -0700, SJwith wrote:
> json_real(25.89)
> -> 25.8899999999999999

> can I express float or double value exactly (like 25.89)
> plz help;

The short answer is no, and your question is not JSON or jansson specific....

Read
http://floating-point-gui.de/
http://en.wikipedia.org/wiki/Floating_point
http://en.wikipedia.org/wiki/IEEE_floating_point

Cheers.
--
Basile STARYNKEVITCH http://starynkevitch.net/Basile/
email: basile<at>starynkevitch<dot>net mobile: +33 6 8501 2359
8, rue de la Faiencerie, 92340 Bourg La Reine, France
*** opinions {are only mines, sont seulement les miennes} ***

[SJwith]

2014년 4월 10일 목요일 오후 4시 26분 34초 UTC+9, Basile Starynkevitch 님의 말:

thanks for interest my ask

i'm newbie programmer and feel i need much study;

I could find in strconv.c precision expression - "%.17g" (with Petri's advice, thnkyou so much;)

thanks for your advice and reference

I'll meet my team in parse part 

thanks for helps