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138 lines
1.8 KiB
Markdown
138 lines
1.8 KiB
Markdown
# 列表推导式
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循环可以用来生成列表：
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In [1]:
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```py
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values = [10, 21, 4, 7, 12]
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squares = []
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for x in values:
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squares.append(x**2)
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print squares
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```
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```py
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[100, 441, 16, 49, 144]
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```
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列表推导式可以使用更简单的方法来创建这个列表：
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In [2]:
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```py
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values = [10, 21, 4, 7, 12]
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squares = [x**2 for x in values]
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print squares
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```
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```py
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[100, 441, 16, 49, 144]
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```
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还可以在列表推导式中加入条件进行筛选。
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例如在上面的例子中，假如只想保留列表中不大于`10`的数的平方：
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In [3]:
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```py
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values = [10, 21, 4, 7, 12]
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squares = [x**2 for x in values if x <= 10]
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print squares
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```
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```py
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[100, 16, 49]
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```
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也可以使用推导式生成集合和字典：
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In [4]:
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```py
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square_set = {x**2 for x in values if x <= 10}
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print(square_set)
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square_dict = {x: x**2 for x in values if x <= 10}
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print(square_dict)
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```
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```py
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set([16, 49, 100])
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{10: 100, 4: 16, 7: 49}
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```
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再如，计算上面例子中生成的列表中所有元素的和：
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In [5]:
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```py
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total = sum([x**2 for x in values if x <= 10])
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print(total)
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```
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```py
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165
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```
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但是，**Python**会生成这个列表，然后在将它放到垃圾回收机制中（因为没有变量指向它），这毫无疑问是种浪费。
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为了解决这种问题，与xrange()类似，**Python**使用产生式表达式来解决这个问题：
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In [6]:
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```py
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total = sum(x**2 for x in values if x <= 10)
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print(total)
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```
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```py
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165
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```
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与上面相比，只是去掉了括号，但这里并不会一次性的生成这个列表。
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比较一下两者的用时：
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In [7]:
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```py
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x = range(1000000)
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```
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In [8]:
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```py
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%timeit total = sum([i**2 for i in x])
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```
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```py
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1 loops, best of 3: 3.86 s per loop
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```
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In [9]:
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```py
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%timeit total = sum(i**2 for i in x)
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```
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```py
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1 loops, best of 3: 2.58 s per loop
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``` |