Detailed explanation of two example methods of python list sorting-Python Tutorial-php.cn

Detailed explanation of two example methods of python list sorting

高洛峰

Release： 2017-03-17 16:39:37

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To sort List, Python provides two methods

Method 1. Use List’s built-in function list.sortSort

list.sort(func=None, key=None, reverse=False)

Python example:

>>> list = [2,5,8,9,3]  
>>> list  
[2,5,8,9,3]  
>>> list.sort()  
>>> list  
[2, 3, 5, 8, 9]

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Method 2. Use the sequence type function sorted(list) to sort (starting from 2.4)

Python example:

>>> list = [2,5,8,9,3]  
>>> list  
[2,5,8,9,3]  
>>> sorted(list)  
[2, 3, 5, 8, 9]

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The difference between the two methods:

sorted( list) returns an object that can be used as an expression. The original list remains unchanged and a new sorted list object is generated.

list.sort() will not return objects and change the original list.

Other examples of sort:

Example 1: Forward sorting

>>>L = [2,3,1,4]
>>>L.sort()
>>>L
>>>[1,2,3,4]

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Example 2: Reverse sorting

>>>L = [2,3,1,4]
>>>L.sort(reverse=True)
>>>L
>>>[4,3,2,1]

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Example 3: For the second Keyword sorting

>>>L = [(&#39;b&#39;,6),(&#39;a&#39;,1),(&#39;c&#39;,3),(&#39;d&#39;,4)]
>>>L.sort(lambda x,y:cmp(x[1],y[1])) 
>>>L
>>>[(&#39;a&#39;, 1), (&#39;c&#39;, 3), (&#39;d&#39;, 4), (&#39;b&#39;, 6)]

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Example 4: Sort the second keyword

>>>L = [(&#39;b&#39;,6),(&#39;a&#39;,1),(&#39;c&#39;,3),(&#39;d&#39;,4)]
>>>L.sort(key=lambda x:x[1]) 
>>>L
>>>[(&#39;a&#39;, 1), (&#39;c&#39;, 3), (&#39;d&#39;, 4), (&#39;b&#39;, 6)]

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Example 5: Sort the second keyword

>>>L = [(&#39;b&#39;,2),(&#39;a&#39;,1),(&#39;c&#39;,3),(&#39;d&#39;,4)]
>>>import operator
>>>L.sort(key=operator.itemgetter(1)) 
>>>L
>>>[(&#39;a&#39;, 1), (&#39;b&#39;, 2), (&#39;c&#39;, 3), (&#39;d&#39;, 4)]

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Example 6: (DSU method: Decorate-Sort-Undercorate)

>>>L = [(&#39;b&#39;,2),(&#39;a&#39;,1),(&#39;c&#39;,3),(&#39;d&#39;,4)]
>>>A = [(x[1],i,x) for i,x in enumerate(L)] #i can confirm the stable sort
>>>A.sort()
>>>L = [s[2] for s in A]
>>>L
>>>[(&#39;a&#39;, 1), (&#39;b&#39;, 2), (&#39;c&#39;, 3), (&#39;d&#39;, 4)]

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The above gives 6 methods for sorting List, among which Example 3.4.5.6 can play a role in sorting a certain item in the List item

Sort the comparison keywords.

Efficiency comparison:

cmp < DSU < key

Through experimental comparison, method 3 is slower than method 6. 6 is slower than method 4. Method 4 and method 5 are basically equivalent

Multiple keyword comparison sorting:

Example 7:

>>>L = [(&#39;d&#39;,2),(&#39;a&#39;,4),(&#39;b&#39;,3),(&#39;c&#39;,2)]
>>> L.sort(key=lambda x:x[1])
>>> L
>>>[(&#39;d&#39;, 2), (&#39;c&#39;, 2), (&#39;b&#39;, 3), (&#39;a&#39;, 4)]

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We see that at this time, the sorting The L is only sorted according to the second keyword.

What if we want to sort by the second keyword and then use the first keyword to sort? There are two methods

Example 8:

>>> L = [(&#39;d&#39;,2),(&#39;a&#39;,4),(&#39;b&#39;,3),(&#39;c&#39;,2)]
>>> L.sort(key=lambda x:(x[1],x[0]))
>>> L
>>>[(&#39;c&#39;, 2), (&#39;d&#39;, 2), (&#39;b&#39;, 3), (&#39;a&#39;, 4)]

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Example 9:

>>> L = [(&#39;d&#39;,2),(&#39;a&#39;,4),(&#39;b&#39;,3),(&#39;c&#39;,2)]
>>> L.sort(key=operator.itemgetter(1,0))
>>> L
>>>[(&#39;c&#39;, 2), (&#39;d&#39;, 2), (&#39;b&#39;, 3), (&#39;a&#39;, 4)]

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Why does example 8 work? The reason is that tuples are compared from left to right. After comparing the first one, if

is equal, compare the second

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