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Python黑帽编程2.4 流程控制
本节要介绍的是Python编程中和流程控制有关的关键字和相关内容。
2.4.1 if …..else
先上一段代码：
#!/usr/bin/python
# -*- coding: UTF-8 -*-
x=int(input('请输入一个整数:'))
if x==0:
print '%d ==0' % x
elif x<0:
print '%d <0' % x
else:
print '%d >0' % x
这段代码使用if，elif和else三个关键字，每一个关键字后面跟一个布尔表达式，以冒号结尾。跟在条件判断语句下面的是子语句，就是我们在此条件下要做的事情。
我将上面的代码保存到ifelse.py运行结果如下：
图2
2.4.2 for 语句
Python 中的 for 语句和C中的略有不同。通常的循环可能会由用户来定义迭代步骤和中止条件，Python 的 for 语句依据任意序列（链表或字符串）中的子项，按它们在序列中的顺序来进行迭代。例如：
words = ['cat', 'window', 'defenestrate']
for word in words:
print(word,len(word))
运行结果如下：
图3
在迭代过程中修改迭代序列不安全（只有在使用链表这样的可变序列时才会有这样的情况）。如果你想要修改你迭代的序列，可以迭代它的副本。使用切割标识就可以很方便的做到这一点:
words = ['cat', 'window', 'defenestrate']
for word in words[:]:
if len(word)>6:
words.insert(0,word)
print words
上面的代码通过切片操作得到了words的一个拷贝，循环过程中可以修改words列表但是不会对循环造成影响。运行结果如下：
图4
2.4.3 while语句
while和if的区别在于，if如果表达式为true的话会一次执行内部的代码，而while会循环执行，直到表达式为false。例如：
count=0
while(count<9):
print 'the index is:',count
count +=1
运行结果如下：
图5
2.4.5 range和xrange
使用range函数可以很方便的生成一个等差系列。range函数完整的声明如下：
range(start,end,step =1)
下面我们通过一小段代码看看range的使用方法：
a=range(5)
b=range(2,5)
c=range(2,5,2)
print a
print b
for i in c:
print "value is",i
这段代码示例了三种使用方式：
1) 只有一个参数时，传入值为end，起始值为0，步长为1；
2) 传递两个参数时，传入值为start和end，步长为1；
3) 传递三个参数时，传入值为start，end和步长。
xrange和range使用方法一样，区别有以下两点：
1) xrange不生成完整的列表，效率更高；
2) xrange只有在for循环中使用才有意义。
2.4.6 break 、 continue和Pass
break 语句和 C中的类似，用于跳出最近的一级for或while循环。
循环可以有一个else子句，它在循环迭代完整个列表（对于 for ）或执行条件为 false （对于 while ）时执行，但循环被 break 中止的情况下不会执行。
例如下面的代码：
for n in range(2, 10):
for x in range(2, n):
if n % x == 0:
print(n, 'equals', x, '*', n//x)
break
else:
print(n, 'is a prime number')
上面的代码循环一个包含从2到9的一个系列，内部的循环实际是判断该数是不是素数。if语句如果为true的话证明找到了除2和本身以外的分解因子，证明这个数不是素数，接着会执行break，此时会跳出当前循环，因为有了break语句，和当前for循环对应的else语句也不会执行。在相反的情况下，如果是素数，当前循环会执行完毕，else子句会执行。结果如下：
图6
continue 语句是从 C 中借鉴来的，它表示循环继续执行下一次迭代，如下所示:
for num in range(2,10):
if(num %2 ==0):
continue
print num
这是一段输出奇数的代码，结果如下：
图7
pass 语句什么也不做，相当于汇编的nop指令。它用于那些语法上必须要有什么语句，但程序什么也不做的场合。通常我们使用pass语句来进行占位，比如规划程序功能和结构的时候，我们想好要定义哪些类，哪些方法，但是还没有具体实现的时候。比如：
def funcname(parameter_list):
pass
class classname(object):
pass
if a==0:
pass
else:
pass
2.4.7 小结
流程控制的内容，就到此为止，下一节介绍函数。
第2.5节《函数》已经在微信订阅号抢先发布，心急的同学进入订阅号（二维码在下方），从菜单“网络安全”—>”Python黑帽编程”进入即可。
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