def sinc_plot ( ):

  import matplotlib.pyplot as plt
  import numpy as np
#
#  Set up x,y data for the sinc function
#
  x1 = -12.0
  x2 = +12.0
  x = np.linspace ( x1, x2, 101 )
  y = np.sin ( x ) / x
#
#  Try to compute maximum value of y.
#
  ymax = np.max ( y )
  imax = np.argmax ( y )
  xmax = x[imax]
  print ( ymax, imax, xmax )
#
#  Redefine y(0.0) to be 1.
#
  y = np.sin(x) / x
  y[x==0] = 1.0
  ymax = np.max ( y )
  imax = np.argmax ( y )
  xmax = x[imax]
  print ( '' )
  print ( '  Redefine y[0]=1' )
  print ( ymax, imax, xmax )

  ymin = np.min ( y )
  imin = np.argmin ( y )
  xmin = x[imin]

  ymean = np.mean ( y )
#
#  Plot the data.
#
  plt.clf ( )
  plt.plot ( x, y, 'g-' )
  plt.grid ( True )
  plt.title ( 'The sinc function' )
  plt.savefig ( 'sinc.png' )
  plt.show ( )
#
#  Make a fancy plot with lines marking max and min y values.
#
  plt.clf ( )
  plt.plot ( x, y, 'g-', linewidth = 2 )
  plt.plot ( [x1,x2], [ymax,ymax], 'b-' )
  plt.plot ( [xmax,xmax], [0.0,ymax], 'b-' )
  plt.plot ( [x1,x2], [ymin,ymin], 'r-' )
  plt.plot ( [xmin,xmin], [0.0,ymin], 'r-' )
  plt.plot ( [x1,x2], [ymean,ymean], 'c:', linewidth = 3 )
  plt.plot ( [x1,x2], [0.0,0.0], 'k-' )
  plt.grid ( True )
  plt.title ( 'Maximum, minimum, mean values of sinc(x)' )
  plt.savefig ( 'sinc_marked.png' )
  plt.show ( )
#
#  Mark all the negative points.
#
  plt.clf ( )
  plt.plot ( x, y, 'g-', linewidth = 2 )
  plt.plot ( x[y<0], y[y<0], 'r.', markersize = 10 )
  plt.plot ( [x1,x2], [0.0,0.0], 'k-' )
  plt.grid ( True )
  plt.title ( 'Location of negative values of sinc(x)' )
  filename = 'sinc_negative.png'
  plt.savefig ( filename )
  plt.show ( )

  return

if ( __name__ == "__main__" ):
  sinc_plot ( )