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Two methods and examples of python list sorting

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Release： 2018-04-18 11:18:59

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This article mainly introduces two methods and examples of python list sorting. It has a very good reference value. Let’s take a look at it.

To sort the List, Python provides two methods

Method 1 .Use List's built-in function list.sort to sort

##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 sort

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

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Example 2: Reverse sort

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

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Example 3: Sort the second keyword

>>>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 be used to sort a certain item in the List item

Sort for comparison keywords.

Efficiency comparison:

cmp < DSU < keyThrough experimental comparison, method 3 is slower than method 6, method 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 the sorted L at this time is only based on the second keyword Ranked,

What if we want to use the second keyword to sort 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 instance 8 work? The reason is that tuples are compared from left to right. After comparing the first one, if they are equal, compare the second one Related recommendations:

The sorting method of nested tuples in the python list object using sort

The sorting method when the python list element is a tuple

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