[SciPy-user] numpy.dft.real+fft problem?

[Andrew Jaffe]

Hi All,

[This has already appeared on python-scientific-devel; apologies to
those of you seeing it twice...]

There seems to be a problem with the real_fft routine; starting with a
length n array, it should give a length n/2+1 array with real numbers
in the first and last positions (for n even). However, note that the
last entry on the first row has a real part, as opposed to the
complex routine fft, which has it correct.

The weirdness of the real part seems to imply  it's due to uninitialized
memory.

The problem persists in 2d, as well.

In [6]:numpy.__version__
Out[6]:'0.9.4.1915'

In [53]:a = numpy.randn(16)

In [54]:fa = numpy.dft.real_fft(a)

In [55]:fa
Out[55]:
array([ 0.4453+0.0000e+000j, -3.5009-3.4618e+000j, -0.1276+3.5740e-001j,
         -1.043+2.3504e+000j, -1.2028+1.6121e-001j, -7.1041-1.1182e+000j,
          0.09-1.6013e-001j,  4.2079-1.0106e-001j, -4.8295+2.3091e+251j])
                                                 **wrong** ^^^^^^^^^^^^^

In [56]:f2a = numpy.dft.fft(a)

In [57]:f2a
Out[57]:
array([  0.4453+0.j    , -3.5009-3.4618j, -0.1276+0.3574j,
          -1.043+2.3504j, -1.2028+0.1612j, -7.1041-1.1182j,
           0.09-0.1601j,   4.2079-0.1011j, -4.8295+0.j    ,
                                       **correct** ^^^
           4.2079+0.1011j,
           0.09+0.1601j, -7.1041+1.1182j, -1.2028-0.1612j,
          -1.043-2.3504j,-0.1276-0.3574j, -3.5009+3.4618j])

-Andrew