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252 lines
3.8 KiB
Markdown
252 lines
3.8 KiB
Markdown
# 循环
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循环的作用在于将一段代码重复执行多次。
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## while 循环
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```py
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while <condition>:
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<statesments>
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```
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**Python**会循环执行`<statesments>`，直到`<condition>`不满足为止。
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例如，计算数字`0`到`1000000`的和：
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In [1]:
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```py
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i = 0
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total = 0
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while i < 1000000:
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total += i
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i += 1
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print total
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```
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```py
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499999500000
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```
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之前提到，空容器会被当成 `False` ，因此可以用 `while` 循环来读取容器中的所有元素：
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In [2]:
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```py
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plays = set(['Hamlet', 'Macbeth', 'King Lear'])
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while plays:
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play = plays.pop()
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print 'Perform', play
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```
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```py
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Perform King Lear
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Perform Macbeth
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Perform Hamlet
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```
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循环每次从 `plays` 中弹出一个元素，一直到 `plays` 为空为止。
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## for 循环
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```py
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for <variable> in <sequence>:
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<indented block of code>
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```
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`for` 循环会遍历完`<sequence>`中所有元素为止
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上一个例子可以改写成如下形式：
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In [3]:
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```py
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plays = set(['Hamlet', 'Macbeth', 'King Lear'])
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for play in plays:
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print 'Perform', play
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```
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```py
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Perform King Lear
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Perform Macbeth
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Perform Hamlet
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```
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使用 `for` 循环时，注意尽量不要改变 `plays` 的值，否则可能会产生意想不到的结果。
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之前的求和也可以通过 `for` 循环来实现：
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In [4]:
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```py
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total = 0
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for i in range(100000):
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total += i
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print total
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```
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```py
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4999950000
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```
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然而这种写法有一个缺点：在循环前，它会生成一个长度为 `100000` 的临时列表。
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生成列表的问题在于，会有一定的时间和内存消耗，当数字从 `100000` 变得更大时，时间和内存的消耗会更加明显。
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为了解决这个问题，我们可以使用 `xrange` 来代替 `range` 函数，其效果与`range`函数相同，但是 `xrange` 并不会一次性的产生所有的数据：
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In [5]:
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```py
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total = 0
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for i in xrange(100000):
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total += i
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print total
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```
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```py
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4999950000
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```
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比较一下两者的运行时间：
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In [6]:
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```py
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%timeit for i in xrange(1000000): i = i
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```
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```py
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10 loops, best of 3: 40.7 ms per loop
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```
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In [7]:
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```py
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%timeit for i in range(1000000): i = i
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```
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```py
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10 loops, best of 3: 96.6 ms per loop
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```
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可以看出，`xrange` 用时要比 `range` 少。
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## continue 语句
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遇到 `continue` 的时候，程序会返回到循环的最开始重新执行。
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例如在循环中忽略一些特定的值：
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In [8]:
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```py
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values = [7, 6, 4, 7, 19, 2, 1]
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for i in values:
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if i % 2 != 0:
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# 忽略奇数
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continue
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print i/2
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```
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```py
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3
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2
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1
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```
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## break 语句
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遇到 `break` 的时候，程序会跳出循环，不管循环条件是不是满足：
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In [9]:
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```py
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command_list = ['start',
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'process',
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'process',
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'process',
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'stop',
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'start',
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'process',
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'stop']
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while command_list:
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command = command_list.pop(0)
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if command == 'stop':
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break
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print(command)
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```
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```py
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start
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process
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process
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process
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```
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在遇到第一个 `'stop'` 之后，程序跳出循环。
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## else语句
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与 `if` 一样， `while` 和 `for` 循环后面也可以跟着 `else` 语句，不过要和`break`一起连用。
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* 当循环正常结束时，循环条件不满足， `else` 被执行；
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* 当循环被 `break` 结束时，循环条件仍然满足， `else` 不执行。
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不执行：
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In [10]:
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```py
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values = [7, 6, 4, 7, 19, 2, 1]
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for x in values:
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if x <= 10:
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print 'Found:', x
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break
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else:
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print 'All values greater than 10'
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```
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```py
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Found: 7
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```
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执行：
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In [11]:
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```py
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values = [11, 12, 13, 100]
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for x in values:
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if x <= 10:
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print 'Found:', x
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break
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else:
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print 'All values greater than 10'
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```
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```py
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All values greater than 10
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``` |