Everything

php

Re: Proposal: change precision for output functions

php.internals

Legale Legage

7 years ago

I think your solution by changing precision is not good enough because float summation is still not working properly.

<?php
var_dump(0.1 + 0.7);

returns:

0.7999999999

expected: 0.8

On Jan 7, 2019 7:17 PM, Semen Dubina <php@sam002.net> wrote:
>
> > 07.01.2019, 20:24, "Thomas Bley" <mails@thomasbley.de>: 
> > 
> > Hello, 
> > 
> > good point, having: 
> > 
> > echo ini_get('precision') . PHP_EOL; 
> > echo ini_get('serialize_precision') . PHP_EOL; 
> > echo json_encode(array('price' => round('45.99', 2))) . PHP_EOL; 
> > echo (0.1+0.7), json_encode(0.1+0.7) . PHP_EOL; 
> > 
> > gives (https://3v4l.org/ldgo8): 
> > 
> > Output for 7.1.0 - 7.3.0 
> >     14 
> >     -1 
> >     {"price":45.99} 
> >     0.80.7999999999999999 
> > 
> > Output for 5.3.6 - 5.6.38, 7.0.0 - 7.0.33 
> >     14 
> >     17 
> >     {"price":45.99} 
> >     0.80.8 
> > 
> > what is the preferred way to upgrade from php 5.6 to 7.x in order to get the same results? 
> > 
> > Regards 
> > Thomas 
> > 
>
> Force 'serialize_precision': https://3v4l.org/coaWm 
> But remember - a float is not suitable for output. You need rounded and formatted manually. 
>
> P.S. Try with JS: `JSON.stringify(0.1+0.7);` 
>
> -- Semen V. Dubina https://sam002.net/ 
>
> -- 
> PHP Internals - PHP Runtime Development Mailing List 
> To unsubscribe, visit: http://www.php.net/unsub.php 
>

Semen Dubina

7 years ago

> 07.01.2019, 22:17, "Legale.legale" <legale.legale@gmail.com>:
> I think your solution by changing precision is not good enough because float summation is still not working properly.
>
> <?php
> var_dump(0.1 + 0.7);
>
> returns:
>
> 0.7999999999
>
> expected: 0.8
>

Hi!

If you are about the proposal, then '0.1 + 0.7 !== 0.8'.
Why you expected 0.8?
Check, plz: https://3v4l.org/Ughn2

 -- Semen V. Dubina https://sam002.net/

Lauri Kenttä

7 years ago

On 2019-01-07 21:17, Legale.legale wrote:

> I think your solution by changing precision is not good enough because
> float summation is still not working properly.
>
> <?php
> var_dump(0.1 + 0.7);
>
> returns:
>
> 0.7999999999
>
> expected: 0.8

As you may know, most computers use binary numbers. This applies to
floating-point values as well. They have a limited number of binary
digits. Many base-10 numbers cannot be expressed exactly in base-2 with
limited number of digits. When you write 0.1 + 0.7, what you get (with
double-precision floats) is closer to 0.1000000000000000055511 +
0.6999999999999999555911 = 0.7999999999999999333866. This is expected
and proper and well known.

You have to use formatting functions like number_format if you need
neatly rounded base-10 output.

There are also a lot of libraries for precise base-10 calculations. They
will be a lot slower than native binary floating-point calculations,
though.

Regards,
Lauri Kenttä