[Numpy-discussion] fft output and input storage

Warren Focke focke at slac.stanford.edu
Tue Jul 26 17:11:20 EDT 2005

>>> print FFT.fft.__doc__
fft(a, n=None, axis=-1)

    Will return the n point discrete Fourier transform of a. n defaults to the
    length of a. If n is larger than a, then a will be zero-padded to make up
    the difference. If n is smaller than a, the first n items in a will be

    The packing of the result is "standard": If A = fft(a, n), then A[0]
    contains the zero-frequency term, A[1:n/2+1] contains the
    positive-frequency terms, and A[n/2+1:] contains the negative-frequency
    terms, in order of decreasingly negative frequency. So for an 8-point
    transform, the frequencies of the result are [ 0, 1, 2, 3, 4, -3, -2, -1].

    This is most efficient for n a power of two. This also stores a cache of
    working memory for different sizes of fft's, so you could theoretically
    run into memory problems if you call this too many times with too many
    different n's.

Hmph.  The style guide says I should've put a blank line on the end of
that docstring.  But I don't think there was a style guide then.


On Tue, 26 Jul 2005, Daniel Sheltraw wrote:

> Hello all
> Would someone please tell me how negative and positive
> frequencies are
> stored in the output of the Numerical Python fft?
> Cheers,
> Daniel
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