8000 fftshift incompatible with fft for even n (Trac #1077) · Issue #1675 · numpy/numpy · GitHub
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thouis opened this issue Oct 19, 2012 · 4 comments
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fftshift incompatible with fft for even n (Trac #1077) #1675

thouis opened this issue Oct 19, 2012 · 4 comments
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00 - Bug component: numpy.fft
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1.4.1

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@thouis
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thouis commented Oct 19, 2012

Original ticket http://projects.scipy.org/numpy/ticket/1077 on 2009-04-03 by trac user alk, assigned to unknown.

If n is even (for example n = 8, then fft is described, to return the frequencies at:

[0, 1, 2, 3, 4, -3, -2, -1].

But fftshift(array([0,1,2,3,4,-3,-2,-1]))

returns:
[ 4, -3, -2, -1, 0, 1, 2, 3]

and not:

[-3, -2, -1, 0, 1, 2, 3, 4]
@thouis thouis closed this as completed Oct 19, 2012
This was referenced Oct 19, 2012
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Attachment added by trac user alk on 2009-04-03: helper.py.patch

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@pv wrote on 2009-04-03

See comment in #1674

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Milestone changed to 1.4.0 by @cournape on 2009-06-01

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@rgommers wrote on 2010-07-26

If n is even, there is one more negative than positive value. This is a choice, and consistently done in numpy as far as I'm aware.

In [27]: np.fft.fftshift(np.array([0,1,2,3,-4,-3,-2,-1])) 
Out[27]: array([-4, -3, -2, -1,  0,  1,  2,  3])

From docstring:

    >>> freqs = np.fft.fftfreq(10, 0.1)
    >>> freqs
    array([ 0.,  1.,  2.,  3.,  4., -5., -4., -3., -2., -1.])
    >>> np.fft.fftshift(freqs)
    array([-5., -4., -3., -2., -1.,  0.,  1.,  2.,  3.,  4.])

certik pushed a commit to certik/numpy that referenced this issue Nov 14, 2012
@mwiebe @charris
ENH: ticket numpy#1675, Add scalar support for the format() function.
5fe46bd 
Backport of 88e8c15.
@mwiebe mwiebe mentioned this issue Dec 13, 2013
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