Edit report at https://bugs.php.net/bug.php?id=63136&edit=1
ID: 63136
Updated by: ni...@php.net
Reported by: tstobbe at sacbee dot com
Summary: pow() returns NaN where base is negative and
exponent is an odd number root
-Status: Open
+Status: Not a bug
Type: Bug
Package: Math related
Operating System: Linux (fc17, 32-bit)
PHP Version: 5.4.7
Block user comment: N
Private report: N
New Comment:
Real-exponent powers are not defined for negative numbers, see
http://en.wikipedia.org/wiki/Exponentiation#Real_exponents_with_negative_bases.
You can see this very easily if you remember that a^b is defined as
exp(b*ln(a)) and the logarithm is only defined in the range ]0,inf[.
Previous Comments:
------------------------------------------------------------------------
[2012-09-22 02:47:21] tstobbe at sacbee dot com
It should be noted that all subsequent fractional pow() issues are related to
this. For example: pow(-[any number], 1/5) vs pow(-[any number], 2/5). Likewise
I suspect that most other exponent based functions (such as log with an
arbitrary base) are probably suspect, but this hasn't been verified.
------------------------------------------------------------------------
[2012-09-21 22:38:15] tstobbe at sacbee dot com
Description:
------------
Odd roots (3, 5, etc) of negative numbers passed to pow() result in NaN rather
than the actual result. While this makes sense for even numbered roots (sqrt
and
what-not), it wrecks havoc on any system that actually uses arbitrary roots of
negative numbers.
Example: -2*-2*-2 = -8; pow(-8, 1/3) should return -2
Test script:
---------------
var_dump(pow(-8, 1/3));
Expected result:
----------------
float(-2)
Actual result:
--------------
float(NAN)
------------------------------------------------------------------------
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Edit this bug report at https://bugs.php.net/bug.php?id=63136&edit=1
