将列表转换为集合会更改元素顺序 [英] Converting a list to a set changes element order

问题描述

最近我注意到，当我将 list 转换为 set 时，元素的顺序会改变并按字符排序.

考虑这个例子:

x=[1,2,20,6,210]打印 x# [1, 2, 20, 6, 210] # 顺序与初始顺序相同集合(x)# set([1, 2, 20, 210, 6]) # 在set(x) 输出顺序中排序

我的问题是 -

1. 为什么会这样?
2. 如何在不丢失初始顺序的情况下进行集合操作(​​尤其是集合差)?

解决方案

1. A set是无序的数据结构，所以不保留插入顺序.

2. 这取决于您的要求.如果您有一个普通列表，并且想要在保留列表顺序的同时删除一些元素，您可以使用列表推导式来实现:

   <预><代码>>>>a = [1, 2, 20, 6, 210]>>>b = 设置([6, 20, 1])>>>[x 代表 x 在 a 如果 x 不在 b][2, 210]

   如果您需要一个同时支持快速成员资格测试和插入顺序保留的数据结构，您可以使用 Python 字典的键，从 Python 3.7 开始保证保留插入顺序:

   <预><代码>>>>a = dict.fromkeys([1, 2, 20, 6, 210])>>>b = dict.fromkeys([6, 20, 1])>>>dict.fromkeys(x for x in a if x not in b){2:无，210:无}

   b 在这里真的不需要订购——您也可以使用 set.请注意，a.keys() - b.keys() 将集合差值作为 set 返回，因此它不会保留插入顺序.

   在旧版本的 Python 中，您可以使用 collections.OrderedDict 代替:

   <预><代码>>>>a = collections.OrderedDict.fromkeys([1, 2, 20, 6, 210])>>>b = collections.OrderedDict.fromkeys([6, 20, 1])>>>collections.OrderedDict.fromkeys(x for x in a if x not in b)OrderedDict([(2, None), (210, None)])

Recently I noticed that when I am converting a list to set the order of elements is changed and is sorted by character.

Consider this example:

x=[1,2,20,6,210]
print x 
# [1, 2, 20, 6, 210] # the order is same as initial order

set(x)
# set([1, 2, 20, 210, 6]) # in the set(x) output order is sorted

My questions are -

1. Why is this happening?
2. How can I do set operations (especially Set Difference) without losing the initial order?

解决方案

1. A set is an unordered data structure, so it does not preserve the insertion order.

2. This depends on your requirements. If you have an normal list, and want to remove some set of elements while preserving the order of the list, you can do this with a list comprehension:

   >>> a = [1, 2, 20, 6, 210]
   >>> b = set([6, 20, 1])
   >>> [x for x in a if x not in b]
   [2, 210]
   

   If you need a data structure that supports both fast membership tests and preservation of insertion order, you can use the keys of a Python dictionary, which starting from Python 3.7 is guaranteed to preserve the insertion order:

   >>> a = dict.fromkeys([1, 2, 20, 6, 210])
   >>> b = dict.fromkeys([6, 20, 1])
   >>> dict.fromkeys(x for x in a if x not in b)
   {2: None, 210: None}
   

   b doesn't really need to be ordered here – you could use a set as well. Note that a.keys() - b.keys() returns the set difference as a set, so it won't preserve the insertion order.

   In older versions of Python, you can use collections.OrderedDict instead:

   >>> a = collections.OrderedDict.fromkeys([1, 2, 20, 6, 210])
   >>> b = collections.OrderedDict.fromkeys([6, 20, 1])
   >>> collections.OrderedDict.fromkeys(x for x in a if x not in b)
   OrderedDict([(2, None), (210, None)])
   

这篇关于将列表转换为集合会更改元素顺序的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！