text: #!/usr/bin/env/python3
  """
      Desciption:
  	    This is in chapter 1. 
  	    This code shows the vector add in python. 
  	    Using the commands below to run this program.
  	    python3 vectorsum.py n
  	args:
  	    n is size of the vector.
  """
  
  
  import sys
  from datetime import datetime
  import numpy as np
  
  def python_sum(n):
  	a = list(range(n)) # range in pthon2.x return a list,
  	b = list(range(n)) # range in python3.x return a range obj.
  	c = []
  	for i in range(n):
  		a[i] = i ** 2
  		b[i] = i ** 3
  		c.append(a[i] + b[i])
  	return c
  def numpy_sum(n):
  	a = np.arange(n, dtype=np.int64) ** 2
  	b = np.arange(n, dtype=np.int64) ** 3
  	c = a + b
  	return c
  
  size = int(sys.argv[1])
  start = datetime.now()
  c = python_sum(size)
  delta = datetime.now() - start
  print("The last 2 elements of the sum: ", c[-2:])
  print("PythonSum elapsed time in microseconds", delta.microseconds)
  start = datetime.now()
  c = numpy_sum(size)
  delta = datetime.now() - start
  print("The last 2 elements of the sum: ", c[-2:])
  print("NumpySum elapsed time in microseconds", delta.microseconds)
  
  
  
  """
  C:\software\python\python.exe C:/Users/yuyan01/Desktop/numpy/1_tutorial/1_12_vector_add.py 1000
  The last 2 elements of the sum:  [995007996, 998001000]
  PythonSum elapsed time in microseconds 997
  The last 2 elements of the sum:  [995007996 998001000]
  NumpySum elapsed time in microseconds 0
  
  C:\software\python\python.exe C:/Users/yuyan01/Desktop/numpy/1_tutorial/1_12_vector_add.py 2000
  The last 2 elements of the sum:  [7980015996, 7992002000]
  PythonSum elapsed time in microseconds 1994
  The last 2 elements of the sum:  [-609918596 -597932592]
  NumpySum elapsed time in microseconds 0
  
  C:\software\python\python.exe C:/Users/yuyan01/Desktop/numpy/1_tutorial/1_12_vector_add.py 3000
  The last 2 elements of the sum:  [26955023996, 26982003000]
  PythonSum elapsed time in microseconds 2037
  The last 2 elements of the sum:  [26955023996 26982003000]
  NumpySum elapsed time in microseconds 0
  
  """