[Numpy-discussion] Bug in Numpy FFT reference?

Gary Bishop gb at cs.unc.edu
Tue Jul 23 14:24:03 CDT 2002


The example given for real_fft in the FFT section of the Sept 7, 2001 
Numpy manual makes no sense to me. The text says

>>> x = cos(arange(30.0)/30.0*2*pi)
>>> print real_fft(x)
[ -1. +0.j 13.69406641+2.91076367j
-0.91354546-0.40673664j -0.80901699-0.58778525j
-0.66913061-0.74314483j -0.5 -0.8660254j
-0.30901699-0.95105652j -0.10452846-0.9945219j
0.10452846-0.9945219j 0.30901699-0.95105652j
0.5 -0.8660254j 0.66913061-0.74314483j
0.80901699-0.58778525j 0.91354546-0.40673664j
0.9781476 -0.20791169j 1. +0.j ]

But surely x is a single cycle of a cosine wave and should have a very 
sensible and simple FT. Namely [0, 1, 0, 0, 0, ...]

Indeed, running the example using Numeric and FFT produces, within 
rounding error, exactly what I would expect.

Why the non-intuitive (and wrong) result in the example text?

gb




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