自定义迭代器：比如输入奇数项,反向迭代等

有时需要自定义一个迭代模式，如以0.5的步长迭代，或者只输出奇数项，反向迭代等

所谓的迭代器其实也是使用了next方法，所以，只要 合理利用next，就可以达到目的：

#!/usr/bin/env python
#coding:utf-8
#@Author:Andy
#Date: 2017/6/13

def frange(start, stop, step):
	"""
	Use yield to set a new iteration pattern such as float iterate
	you can set start=0, and the step you want
	"""
	x = start
	while x < stop:
		yield x
		x += step

if __name__ == ‘__main__‘:
	for i in frange(0, 10, 0.5):
		print(i, end=‘ ‘)
		# 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5

def impar_range(start, stop):
	"""
	yield impart index of a iterable
	"""
	i = start
	while i < stop:
		yield i
		i += 2
for i in impar_range(1, 20):
	print(i, end=‘ ‘)
	# 1 3 5 7 9 11 13 15 17 19

def count_down(n):
	"""
	count down from n to 0
	"""
	i = n
	while 0 < i <= n:
		yield i
		i -= 1

for i in count_down(10):
	print(i, end = ‘ ‘)
	# 10 9 8 7 6 5 4 3 2 1