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Bug #74534 Floating point precision issue
Submitted: 2017-05-03 10:56 UTC Modified: 2017-05-03 11:00 UTC
From: uchihasv at yandex dot ru Assigned:
Status: Not a bug Package: *General Issues
PHP Version: 7.1.4 OS: Windows 7 x64
Private report: No CVE-ID: None
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 [2017-05-03 10:56 UTC] uchihasv at yandex dot ru 
Description:
------------
I have array with numbers and i do simple math in loop and in results i see -6.9388939039072E-17 instead 0

Test script:
---------------
<?php
$h = 0.05;
$x = [-1,-0.5,0,0.5,1];

for($i=0;$i<4;$i++)
{
	for($j=$x[$i]+$h;$j<$x[$i+1];$j+=$h)
	{
		$x2[$i][] = $j;
	}
}

echo "<pre>".print_r($x2,1)."</pre>";

Expected result:
----------------
Array
(
    [0] => Array
        (
            [0] => -0.95
            [1] => -0.9
            [2] => -0.85
            [3] => -0.8
            [4] => -0.75
            [5] => -0.7
            [6] => -0.65
            [7] => -0.6
            [8] => -0.55
        )

    [1] => Array
        (
            [0] => -0.45
            [1] => -0.4
            [2] => -0.35
            [3] => -0.3
            [4] => -0.25
            [5] => -0.2
            [6] => -0.15
            [7] => -0.1
            [8] => -0.05
            [9] => 0
        )

    [2] => Array
        (
            [0] => 0.05
            [1] => 0.1
            [2] => 0.15
            [3] => 0.2
            [4] => 0.25
            [5] => 0.3
            [6] => 0.35
            [7] => 0.4
            [8] => 0.45
            [9] => 0.5
        )

    [3] => Array
        (
            [0] => 0.55
            [1] => 0.6
            [2] => 0.65
            [3] => 0.7
            [4] => 0.75
            [5] => 0.8
            [6] => 0.85
            [7] => 0.9
            [8] => 0.95
        )

)

Actual result:
--------------
Array
(
    [0] => Array
        (
            [0] => -0.95
            [1] => -0.9
            [2] => -0.85
            [3] => -0.8
            [4] => -0.75
            [5] => -0.7
            [6] => -0.65
            [7] => -0.6
            [8] => -0.55
        )

    [1] => Array
        (
            [0] => -0.45
            [1] => -0.4
            [2] => -0.35
            [3] => -0.3
            [4] => -0.25
            [5] => -0.2
            [6] => -0.15
            [7] => -0.1
            [8] => -0.05
            [9] => -6.9388939039072E-17
        )

    [2] => Array
        (
            [0] => 0.05
            [1] => 0.1
            [2] => 0.15
            [3] => 0.2
            [4] => 0.25
            [5] => 0.3
            [6] => 0.35
            [7] => 0.4
            [8] => 0.45
            [9] => 0.5
        )

    [3] => Array
        (
            [0] => 0.55
            [1] => 0.6
            [2] => 0.65
            [3] => 0.7
            [4] => 0.75
            [5] => 0.8
            [6] => 0.85
            [7] => 0.9
            [8] => 0.95
        )

)

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 [2017-05-03 10:58 UTC] requinix@php.net
-Status: Open +Status: Not a bug
 [2017-05-03 10:58 UTC] requinix@php.net 
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/

Thank you for your interest in PHP.
 [2017-05-03 11:00 UTC] kelunik@php.net
-Summary: 123 +Summary: Floating point precision issue
 
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