[Numpy-discussion] Oddity with numpy.int64 integer division
David M. Cooke
cookedm@physics.mcmaster...
Mon Apr 23 20:35:20 CDT 2007
On Apr 23, 2007, at 16:41 , Christian Marquardt wrote:
> On Mon, April 23, 2007 22:29, Christian Marquardt wrote:
>> Actually,
>>
>> it happens for normal integers as well:
>>
>>>>> n = np.array([-5, -100, -150])
>>>>> n // 100
>> array([ 0, -1, -1])
>>>>> -5//100, -100//100, -150//100
>> (-1, -1, -2)
>
> and finally:
>
>>>> n % 100
> array([95, 0, 50])
>>>> -5 % 100, -100 % 100, -150 % 100
> (95, 0, 50)
>
> So plain python / using long provides consistent results across //
> and %, but numpy doesn't...
Python defines x // y as returning the floor of the division, and x %
y has the same sign as y. However, in C89, it is implementation-
defined (i.e., portability-pain-in-the-ass) whether the floor or ceil
is used when the signs of x and y differ. In C99, the result should
be truncated. From the C99 spec, sec. 6.5.5, #6:
When integers are divided, the result of the / operator
is the algebraic quotient with any fractional part
discarded.76) If the quotient a/b is representable, the
expression (a/b)*b + a%b shall equal a.
Numpy follows whatever the C compiler decides to use, because of
speed-vs.-Python compatibility tradeoff.
>
> Christian.
>
>> On Mon, April 23, 2007 22:20, Christian Marquardt wrote:
>>> Dear all,
>>>
>>> this is odd:
>>>
>>>>>> import numpy as np
>>>>>> fact = 28250000L * 86400L
>>>>>> nn = np.array([-20905000L])
>>>>>> nn
>>> array([-20905000], dtype=int64)
>>>>>> nn[0] // fact
>>> 0
>>>
>>> But:
>>>
>>>>>> long(nn[0]) // fact
>>> -1L
>>>
>>> Is this a bug in numpy, or in python's implementation of longs? I
>>> would
>>> think both should give the same, really... (Python 2.5, numpy
>>> 1.0.3dev3725,
>>> Linux, Intel compilers...)
>>>
>>> Many thanks for any ideas / advice,
>>>
>>> Christian
--
|>|\/|<
/------------------------------------------------------------------\
|David M. Cooke http://arbutus.physics.mcmaster.ca/dmc/
|cookedm@physics.mcmaster.ca
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