[Numpy-discussion] simple problem with arange / roundoff

[Timothy Hochberg]

On 7/31/07, Eric Emsellem <emsellem at obs.univ-lyon1.fr> wrote:
>
> Hi,
>
> I discovered a bug in one of my program probably due to a round-off
> problem in a "arange" statement.
> I use something like:
>
> step = (end - start) / (npix - 1.)
> gridX = num.arange(start-step/2., end+step/2., step)
>
> where I wish to get a simple 1D array with npix+1 numbers going from
> (start-step/2.) to (end+step/2.).
>
> But then, "arange" often gets me an array only going from
> "start-step/2." to "end - step/2." instead, due very probably to
> round-off problems (I guess it does not reach the last value because
> <<(start-step/2.) + npix * step >> is found to be larger than
> (end+step/2.).
>
> Here is an example:
>
> start = -30.
> end = 30.
> npix = 31
> step = (end - start) / (npix - 1.)
> gridX = num.arange(start-step/2., end+step/2., step)
> array([-31., -29., -27., -25., -23., -21., -19., -17., -15., -13., -11.,
>         -9.,  -7.,  -5.,  -3.,  -1.,   1.,   3.,   5.,   7.,   9.,  11.,
>         13.,  15.,  17.,  19.,  21.,  23.,  25.,  27.,  29.])
>
> As you can see, it does not go up to 31., but only to 29, although step
> is = 2.0
>
> Is there is a way out of this ?
> (except by doing the silly:   gridX = num.arange(start-step/2.,
> end+1.001*step/2., step)   )


Yes. Don't use arange with floating point numbers steps. Either write this
as something equivalent to:

gridX = num.arange(npix) * step + start

or use linspace.

gridX = num.linspace(start, stop, npix)

Thanks for any input there (and sorry for the silly question)
>
> Eric
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>



-- 
.  __
.   |-\
.
.  tim.hochberg at ieee.org
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