Python list排序

jopen 9年前

一、 python提供的对list进行排序的方法

1、方法:

    (1)list的内建函数list.sort进行排序, 

    (2)用序列类型函数sorted(list)进行排序。

2、示例:

    >>> a_list = [2,5,4,3,1]
    >>> a_list
    [2, 5, 4, 3, 1]
    >>> sorted(a_list)
    [1, 2, 3, 4, 5]
    >>> a_list
    [2, 5, 4, 3, 1]
    >>> a_list.sort()
    >>> a_list
    [1, 2, 3, 4, 5]
    >>> 

3、说明:

    (1)sorted(list)返回一个对象,可以用作表达式。原来的list不变,生成一个新的排好序的list对象

    (2)list.sort()不会返回对象,改变原有的list

二、list.sort()方法的讲解

1、

从Python2.4开始, sort方法有了三个可选的参数:cmp,key, reverse

Python Library Reference里这样描述:

cmp:cmp specifies a custom comparison function of two arguments (iterable elements) which should return a negative, zero or positive number depending on whether the first argument is considered smaller than, equal to, or larger than the second argument:
"cmp=lambda x,y: cmp(x.lower(), y.lower())"
key:key specifies a function of one argument that is used to extract a comparison key from each list element: "key=str.lower"
reverse:reverse is a boolean value. If set to True, then the list elements are sorted as if each comparison were reversed.In general, the key and reverse conversion processes are much faster than specifying an
equivalent cmp function. This is because cmp is called multiple times for each list element while key and reverse touch each element only once.

2、示例


实例1:
    >>>L = [2,3,1,4]
    >>>L.sort()
    >>>L
    >>>[1,2,3,4]
实例2:
    >>>L = [2,3,1,4]
    >>>L.sort(reverse=True)
    >>>L
    >>>[4,3,2,1]
实例3:对第二个关键字排序
    >>>L = [('b',6),('a',1),('c',3),('d',4)]
    >>>L.sort(lambda x,y:cmp(x[1],y[1]))
    >>>L
    >>>[('a', 1), ('c', 3), ('d', 4), ('b', 6)]
实例4: 对第二个关键字排序
    >>>L = [('b',6),('a',1),('c',3),('d',4)]
    >>>L.sort(key=lambda x:x[1])
    >>>L
    >>>[('a', 1), ('c', 3), ('d', 4), ('b', 6)]
实例5: 对第二个关键字排序
    >>>L = [('b',2),('a',1),('c',3),('d',4)]
    >>>import operator
    >>>L.sort(key=operator.itemgetter(1))
    >>>L
    >>>[('a', 1), ('b', 2), ('c', 3), ('d', 4)]
实例6:(DSU方法:Decorate-Sort-Undercorate)
    >>>L = [('b',2),('a',1),('c',3),('d',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
    >>>[('a', 1), ('b', 2), ('c', 3), ('d', 4)]
以上给出了6中对List排序的方法,其中实例3.4.5.6能起到对以List item中的某一项
为比较关键字进行排序.
效率比较:
cmp < DSU < key
通过实验比较,方法3比方法6要慢,方法6比方法4要慢,方法4和方法5基本相当

多关键字比较排序:
实例7:
>>>L = [('d',2),('a',4),('b',3),('c',2)]
>>> L.sort(key=lambda x:x[1])
>>> L
>>>[('d', 2), ('c', 2), ('b', 3), ('a', 4)]
我们看到,此时排序过的L是仅仅按照第二个关键字来排的,

如果我们想用第二个关键字排过序后再用第一个关键字进行排序呢?有两种方法
实例8:
    >>> L = [('d',2),('a',4),('b',3),('c',2)]
    >>> L.sort(key=lambda x:(x[1],x[0]))
    >>> L
    >>>[('c', 2), ('d', 2), ('b', 3), ('a', 4)]
实例9:

    >>>import operator
    >>> L = [('d',2),('a',4),('b',3),('c',2)]
    >>> L.sort(key=operator.itemgetter(1,0))
    >>> L
    >>>[('c', 2), ('d', 2), ('b', 3), ('a', 4)]
为什么实例8能够工作呢?原因在于tuple是的比较从左到右比较的,比较完第一个,如果
相等,比较第二个