[Numpy-discussion] Bug in Numpy FFT reference?

Paul Dubois dubois1 at llnl.gov
Tue Jul 23 14:32:04 CDT 2002


The person who wrote the manual cut and pasted from running the code. I
think there was a bug in FFT at the time. (:->

On Tue, 2002-07-23 at 14:23, Gary Bishop wrote:
> 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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