87 lines
2.7 KiB
Markdown
87 lines
2.7 KiB
Markdown
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---
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title: The Python Range
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localeTitle: Python范围
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---
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## Python范围
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范围实际上是[不可变的序列类型](https://docs.python.org/3/library/stdtypes.html#immutable-sequence-types) ，而不是函数，通常用于在for循环中循环特定次数。
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**创建：**
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`ranges`是使用`range`构造函数创建的。构造函数的参数是：
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* `start` ：包含范围的第一个值（可选整数，默认为0）。
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* `stop` ：独占停止值，当提供此值或更大值时，范围停止（必需整数）。
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* `step` ：添加到当前值以获取下一个值的量（可选整数，默认为1）。
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```python
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>>> range(10) # Only the stop parameter is required.
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range(0, 10)
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>>> range(0, 10) # Default for start parameter is 0.
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range(0, 10)
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>>> range(0, 10, 1) # Default for step is 1\. Start parameter is required if
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step is needed.
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range(0, 10)
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```
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**例子：**
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由于`ranges`是可迭代的，因此可以将它们传递给`list`和`tuple`构造函数以创建这些类型的序列。使用这个事实，我们可以想象一些例子：
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```python
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>>> list(range(10)) # range as argument for list constructor.
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[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
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>>> tuple(range(10)) # range as argument for tuple constructor.
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(0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
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```
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零长度`ranges` ：
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```python
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>>> list(range(10, 0)) # start greater than stop with postive step.
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[]
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>>> list(range(10, 10)) # start equal to stop with postive step.
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[]
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>>> list(range(10, 10, -1)) # start equal to stop with negative step.
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[]
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>>> list(range(0, 10, -1)) # start less than stop with negative step.
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[]
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```
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带步长参数的`ranges` ：
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```python
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>>> list(range(0, 10, 2)) # next value would be 10, stops at 8.
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[0, 2, 4, 6, 8]
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>>> list(range(0, 10, 3)) # next value would be 12, stops at 9.
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[0, 3, 6, 9]
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>>> list(range(0, 10, 4)) # next value would be 12, stops at 8.
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[0, 4, 8]
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>>> list(range(10, 0, -1)) # negative step makes decreasing ranges.
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[10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
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>>> list(range(-5, -30, -3)) # negative integers are valid arguments.
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[-5, -8, -11, -14, -17, -20, -23, -26, -29]
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```
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**优点：**
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使用`range`的好处是，无论指定的范围有多大，只需要少量内存来存储`range` ，即start，stop和step的值。在迭代时计算`ranges`的各个值。
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```python
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>>> import sys
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>>> a_range = range(1000000)
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>>> a_list = list(a_range)
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>>> a_tuple = tuple(a_range)
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>>> sys.getsizeof(a_range)
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48
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>>> sys.getsizeof(a_list)
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9000112
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>>> sys.getsizeof(a_tuple)
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8000048
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```
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### 更多信息：
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[Python Doc - Ranges](https://docs.python.org/3/library/stdtypes.html#ranges)
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**TODO：方法`ranges`是否实现**
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