[Numpy-discussion] numpy.fft.irfftn fails apparently unexpectedly

Henry Gomersall heng@cantab....
Tue Feb 7 03:15:36 CST 2012

On Tue, 2012-02-07 at 01:04 +0100, Torgil Svensson wrote:
> irfftn is an optimization for real input and does not take complex
> input. You have to use numpy.fft.ifftn instead:
hmmm, that doesn't sound right to me (though there could be some non
obvious DFT magic that I'm missing). Indeed,

np.irfftn(np.rfftn(a)) ~= a  # The interim array is complex

Though the documentation is a bit vague as to what inputs are expected!

Actually, reading the fftpack docs, it *does* seem that this is the
correct behaviour (assuming when it says "Fourier coefficients" it means
complex), though I've not read any of the Python code.

> >>>> import numpy
> >>>> a_shape = (63, 4, 98)
> >>>> a = numpy.complex128(numpy.random.rand(*a_shape)+\
> > ...     1j*numpy.random.rand(*a_shape))
> >>>>
> >>>> axes = [0, 2]
> >>>>
> >>>> numpy.fft.ifftn(a, axes=axes)
> Or do you mean if the error message is expected? 

Yeah, the question was regarding the error message. Specifically, the
problem it seems to have with an axes argument like that.



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