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Bug #50095 Unexpected behavior of precision of float numbers
Submitted: 2009-11-05 20:12 UTC Modified: 2009-11-05 21:22 UTC
Votes:1
Avg. Score:4.0 ± 0.0
Reproduced:1 of 1 (100.0%)
Same Version:1 (100.0%)
Same OS:1 (100.0%)
From: majkl578 at gmail dot com Assigned:
Status: Not a bug Package: Scripting Engine problem
PHP Version: 5.2.11 OS: Linux Debian
Private report: No CVE-ID: None
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 [2009-11-05 20:12 UTC] majkl578 at gmail dot com 
Description:
------------
When dumping a float number, it is represented incorrectly. Generally, in some cases, PHP handles float numbers in invalid format.

It is related to a setting of php.ini directive "precision". When a float number with one decimal place is dumped and high precision is set, it is not dumped in an original format, but in damaged one.

Additional info:
32-bit system
CPU: AMD Athlon64

Reproduce code:
---------------
foreach (range(0, 30) as $precision) {
	ini_set('precision', $precision);
	echo $precision . ': ';
	var_dump(20.2);
}

Expected result:
----------------
0: float(20)
1: float(20.2)
2: float(20.2)
3: float(20.2)
4: float(20.2)
5: float(20.2)
6: float(20.2)
7: float(20.2)
8: float(20.2)
9: float(20.2)
10: float(20.2)
11: float(20.2)
12: float(20.2)
13: float(20.2)
14: float(20.2)
15: float(20.2)
16: float(20.2)
17: float(20.2)
18: float(20.2)
19: float(20.2)
20: float(20.2)
21: float(20.2)
22: float(20.2)
23: float(20.2)
24: float(20.2)
25: float(20.2)
26: float(20.2)
27: float(20.2)
28: float(20.2)
29: float(20.2)
30: float(20.2)

Actual result:
--------------
0: float(2.0E+1)
1: float(2.0E+1)
2: float(20)
3: float(20.2)
4: float(20.2)
5: float(20.2)
6: float(20.2)
7: float(20.2)
8: float(20.2)
9: float(20.2)
10: float(20.2)
11: float(20.2)
12: float(20.2)
13: float(20.2)
14: float(20.2)
15: float(20.2)
16: float(20.2)
17: float(20.199999999999999)
18: float(20.1999999999999993)
19: float(20.19999999999999929)
20: float(20.199999999999999289)
21: float(20.1999999999999992895)
22: float(20.19999999999999928946)
23: float(20.199999999999999289457)
24: float(20.1999999999999992894573)
25: float(20.19999999999999928945726)
26: float(20.199999999999999289457264)
27: float(20.1999999999999992894572642)
28: float(20.19999999999999928945726424)
29: float(20.19999999999999928945726424)
30: float(20.1999999999999992894572642399)

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 [2009-11-05 21:22 UTC] rasmus@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://docs.sun.com/source/806-3568/ncg_goldberg.html
 
Thank you for your interest in PHP.

The output is exactly as it should be.  20.2 at a high precision shows you the actual value that the computer is using to try to represent 20.2.  There is no such thing as precisely 20.2 on a computer.  It cannot be represented using the floating point mechanism computers use.
 
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