[Numpy-discussion] Oddity with numpy.int64 integer division

Warren Focke focke@slac.stanford....
Mon Apr 23 21:04:10 CDT 2007


But even C89 required that x == (x/y)*y + (x%y), and that's not the case
here.

w

On Mon, 23 Apr 2007, David M. Cooke wrote:

> 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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