php.net |  support |  documentation |  report a bug |  advanced search |  search howto |  statistics |  random bug |  login

go to bug id or search bugs for

Bug #75800 json_encode produces unexpected results with summed floats
Submitted: 2018-01-11 11:50 UTC Modified: 2018-01-12 11:08 UTC
Votes:1
Avg. Score:5.0 ± 0.0
Reproduced:1 of 1 (100.0%)
Same Version:1 (100.0%)
Same OS:1 (100.0%)
From: yoch at yoch dot org Assigned:
Status: Not a bug Package: JSON related
PHP Version: 7.2.1 OS: Ubuntu 16.04.3 LTS
Private report: No CVE-ID: None
View Add Comment Developer Edit
Welcome back! If you're the original bug submitter, here's where you can edit the bug or add additional notes.
If this is not your bug, you can add a comment by following this link.
If this is your bug, but you forgot your password, you can retrieve your password here.
Password:
Status:
Package:
Bug Type:
Summary:
From: yoch at yoch dot org
New email:
PHP Version: OS:

 

 [2018-01-11 11:50 UTC] yoch at yoch dot org 
Description:
------------
With default php.ini settings (serialize_precision = "-1" and precision = "14"), json_encode() produces inconsistent results.
After setting serialize_precision to "14" serialization working as expected.

Test script:
---------------
php -r 'echo 2.09 + 0.4;'
php -r 'echo json_encode(2.49);'
php -r 'echo json_encode(2.09 + 0.4);'

Expected result:
----------------
2.49
2.49
2.49

Actual result:
--------------
2.49
2.49
2.4899999999999998

Patches

Add a Patch

Pull Requests

Add a Pull Request

History

AllCommentsChangesGit/SVN commitsRelated reports
 [2018-01-11 12:27 UTC] requinix@php.net
-Status: Open +Status: Not a bug
 [2018-01-11 12:27 UTC] requinix@php.net 
Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

http://php.net/manual/en/function.json-encode.php
> Changelog
> 7.1.0  serialize_precision is used instead of precision when encoding double values.
 [2018-01-11 12:45 UTC] yoch at yoch dot org 
Does that means that setting both "precision" and "serialize_precision" to "-1" must produce the expected results? It does not right now.
 [2018-01-11 15:05 UTC] requinix@php.net 
We have a template response for that one too, actually:


Floating point values have a limited precision. Hence a value might 
not have the same string representation after any processing. That also
includes writing a floating point value in your script and directly 
printing it without any mathematical operations.

If you would like to know more about "floats" and what IEEE
754 is, read this:
http://www.floating-point-gui.de/


-1 is "unlimited" precision, meaning that you can see the full float value - including possible error introduced during math. The two json_encode() tests produce different results because of the inconsis-- I mean, the magic of floating-point math.

To get the expected result in all cases you need a precision and serialize_precision of some value between 3 and 16, such as 14.

2.4899999999999998
   ^            ^
   3            16
 [2018-01-11 15:21 UTC] spam2 at rhsoft dot net 
and why "precision" defaults to 14 while serialize_precision defaults to -1
 [2018-01-12 10:41 UTC] yoch at yoch dot org 
Damian, thanks for the explanation.

In this case i believe "serialize_precision" should default to the "precision" value to produce predictable results out of the box.
 [2018-01-12 11:08 UTC] requinix@php.net 
> In this case i believe "serialize_precision" should default to the "precision" value to produce predictable results
> out of the box.
They serve two different purposes. serialize_precision is meant to be used in situations where the complete value is needed, such as serialization (obviously). precision is more for general-purpose conversions and output where some rounding is desirable.

What changed in PHP 7.1 was that json_encode() with floats should try to be as precise as possible because it acts more like serialization than as a float-to-string conversion.
https://wiki.php.net/rfc/precise_float_value
 
PHP Copyright © 2001-2024 The PHP Group
All rights reserved. 		Last updated: Fri Apr 19 00:01:29 2024 UTC