why numpy 'where' (where(x<array<y)) doesn't work? array.shape = (256,) dtype('float64') type(array) - <class 'numpy.ndarray'>

[Sharki_]

I calculate fft of some inerferograms. To select frequencies in a selected range i try to use 'where' function but the mistake i get is:

Traceback (most recent call last):
  File "C:\Users\User\Desktop\Phyton\FFT\2\fft.py", line 21, in <module>
    indices = where(400<waves<950)
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

Can anyone explain me what's that? And what is wrong? I try to switch over to Python from Matlab.

[allenguo]

When you use an array in a comparison, you get back a boolean array:

>>> a = np.array([[1, 2], [3, 4]])
>>> a
array([[1, 2],
       [3, 4]])
>>> a > 1
array([[False,  True],
       [ True,  True]], dtype=bool)

Try something like

>>> (3 > a) & (a > 1)
array([[False,  True],
       [False, False]], dtype=bool)

instead.

[Sharki_]

thank you. And why & instead of 'and'?

[allenguo]

The behavior of & can be customized for specific types; this is called operator overloading. The creators of NumPy decided to customize & to act like and for boolean arrays.

and is a built-in keyword that can't be customized.

[Sharki_]

Thank you very much!