code: def flattop(M, sym=True):
      """Return a flat top window.
  
      Parameters
      ----------
      M : int
          Number of points in the output window. If zero or less, an empty
          array is returned.
      sym : bool, optional
          When True (default), generates a symmetric window, for use in filter
          design.
          When False, generates a periodic window, for use in spectral analysis.
  
      Returns
      -------
      w : ndarray
          The window, with the maximum value normalized to 1 (though the value 1
          does not appear if `M` is even and `sym` is True).
  
      Notes
      -----
      Flat top windows are used for taking accurate measurements of signal
      amplitude in the frequency domain, with minimal scalloping error from the
      center of a frequency bin to its edges, compared to others.  This is a
      5th-order cosine window, with the 5 terms optimized to make the main lobe
      maximally flat. [1]_
  
      References
      ----------
      .. [1] D'Antona, Gabriele, and A. Ferrero, "Digital Signal Processing for
             Measurement Systems", Springer Media, 2006, p. 70
             :doi:`10.1007/0-387-28666-7`.
  
      Examples
      --------
      Plot the window and its frequency response:
  
      >>> from scipy import signal
      >>> from scipy.fftpack import fft, fftshift
      >>> import matplotlib.pyplot as plt
  
      >>> window = signal.flattop(51)
      >>> plt.plot(window)
      >>> plt.title("Flat top window")
      >>> plt.ylabel("Amplitude")
      >>> plt.xlabel("Sample")
  
      >>> plt.figure()
      >>> A = fft(window, 2048) / (len(window)/2.0)
      >>> freq = np.linspace(-0.5, 0.5, len(A))
      >>> response = 20 * np.log10(np.abs(fftshift(A / abs(A).max())))
      >>> plt.plot(freq, response)
      >>> plt.axis([-0.5, 0.5, -120, 0])
      >>> plt.title("Frequency response of the flat top window")
      >>> plt.ylabel("Normalized magnitude [dB]")
      >>> plt.xlabel("Normalized frequency [cycles per sample]")
  
      """
      a = [
          0.21557895,
          0.41663158,
          0.277263158,
          0.083578947,
          0.006947368,
      ]
      return general_cosine(M, a, sym)
  
imports: from __future__ import division, print_function, absolute_import
  import operator
  import warnings
  import numpy
  from scipy import fftpack, linalg, special
  from scipy._lib.six import string_types
label_desc: Use image node_id 12 for calling a global function with example usage: flattop(M, sym) and returns: general_cosine