程序不总是顺序执行的,所以需要条件控制语句来支持分支处理,循环来处理反复多次调用。下面将减少python中的条件判断和循环语法。

条件判断

条件判断就是当某个条件为真时执行某个程序片段，否则执行另一个。

>>> x = int(input("Please enter an integer: "))
Please enter an integer: 2
>>> if x < 0:
		x = 0
		print('Negative changed to zero')
	elif x == 0:
		print('Zero')
	elif x == 1:
		print('Single')
	else:
		print('More')
More

循环

while

如果你需要在条件满足时反复的做一件事情，则可以使用while语句，下面是Fibonacci序列的例子。

>>> a, b = 0, 1
>>> while b < 100:
		print(b, end = ', ')
		a, b = b, a+b
1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 

name = ''
while not name:
    name = input('Please enter your name : ')
print('Hello.%s!' ,name)

for

Python中for语句和其它语言中的有些不同，Python中的for语句总是在序列（list或者String）上迭代，按照元素出现在序列中的顺序。例如：

>>> words = ['cat', 'window', 'defenestrate']
>>> for w in words:
	print(w, len(w))

cat 3
window 6
defenestrate 12

>>> for w in words[:]:
	if len(w) > 6:
		words.insert(0, w)
		
>>> words
['defenestrate', 'cat', 'window', 'defenestrate']

range()

如果你需要在一个数字序列上做迭代，则可以使用range()，它用于产生一个数字序列。

>>> for i in range(5):
	print(i)

0
1
2
3
4

>>> list(range(5, 10))
[5, 6, 7, 8, 9]
>>> list(range(0, 10, 3))
[0, 3, 6, 9]
>>> list(range(-10, -100, -30))
[-10, -40, -70]

>>> a = ['Mary', 'had', 'a', 'little', 'lamb']
>>> for i in range(len(a)):
	print(i, a[i])

0 Mary
1 had
2 a
3 little
4 lamb

>>> print(range(10))
range(0, 10)

break、continue和else

break用于中断最近的那个循环（for或者while）。
循环可以有一个else子句，用于当循环完成后或者循环条件为false后执行，但不会在循环遭遇break后执行，看下面的例子：

>>> for n in range(2, 10):
		for x in range(2, n):
			if n % x == 0:
			   print(n, 'equals', x, '*', n // x)
			   break
		else:
			# loop fell through without finding a factor
			print(n, 'is a prime number')
  
2 is a prime number
3 is a prime number
4 equals 2 * 2
5 is a prime number
6 equals 2 * 3
7 is a prime number
8 equals 2 * 4
9 equals 3 * 3

>>> for num in range(2, 10):
		if num % 2 == 0:
			print("Found an even number", num)
			continue
		print("Found a number", num)

Found an even number 2
Found a number 3
Found an even number 4
Found a number 5
Found an even number 6
Found a number 7
Found an even number 8
Found a number 9

pass

pass什么也不做，当语法上要求需要一个子句，但你不想进行任何行为时，可以使用。例如：

>>> class MyEmptyClass:
		pass

>>> def initlog(*args):
		pass

循环技巧

循环获取字典的key和value

对字典进行循环时，使用字典的items()方法，可以同时获取key和value。

>>> knights = {'gallahad': 'the pure', 'robin': 'the brave'}
>>> for k, v in knights.items():
		print(k, v)

gallahad the pure
robin the brave

循环序列同时获取索引和值

使用那个enumerate()函数可以同时获取序列的索引和值：

>>> for i, v in enumerate(['tic', 'tac', 'toe']):
		print(i, v)

0 tic
1 tac
2 toe

循环多个序列

只用zip()函数可以同时循环多个序列。

>>> questions = ['name', 'quest', 'favorite color']
>>> answers = ['lancelot', 'the holy grail', 'blue']
>>> for q, a in zip(questions, answers):
	print('What is your {0}?  It is {1}.'.format(q, a))

What is your name?  It is lancelot.
What is your quest?  It is the holy grail.
What is your favorite color?  It is blue.

反向遍历序列

使用reversed()函数可以反向遍历序列。

>>> for i in reversed(range(1, 10, 2)):
	print(i)

9
7
5
3
1

遍历排序后的序列

sorted()函数返回一个新的排序后的序列，用于遍历，并且避免源序列被改变。

>>> basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
>>> for f in sorted(set(basket)):
	print(f)

apple
banana
orange
pear

循环期间改变序列

如果需要在循环期间改变序列，你需要在循环前先将序列拷贝一份，使用切片可以很容易做到：

>>> words = ['cat', 'window', 'defenestrate']
>>> for w in words[:]:    #循环前拷贝
	if len(w) > 6:
		words.insert(0, w)

>>> words
['defenestrate', 'cat', 'window', 'defenestrate']