这些集合操作是什么,为什么它们会给出不同的结果? [英] What are these set operations, and why do they give different results?
问题描述
我在 Pluralsight 上看到过这个测试题:
给定这些集合:
x = {'a', 'b', 'c', 'd'}y = {'c', 'e', 'f'}z = {'a', 'g', 'h', 'i'}
x 的值是多少 |y^z
?
预期的答案是:
{'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i'}
组合集合(自动丢弃重复项),并将它们从最低到最高排序.
我的问题是:
- 这个表达式叫什么?
- 为什么我从 3 个不同的 Python 版本得到 3 个不同的结果?
Python 3.7.5 在 Ubuntu 18.04 上的结果:
{'c', 'h', 'f', 'd', 'b', 'i', 'g', 'a', 'e'}
在 Ubuntu 18.04 上的 Python 2.17.17rc1 上的结果:
set(['a', 'c', 'b', 'e', 'd', 'g', 'f', 'i', 'h'])
在 Windows 10 上的 Python 3.7.2 上的结果:
{'a', 'd', 'h', 'f', 'b', 'g', 'e', 'c', 'i'}
这是我为此使用的相同代码的副本:https://repl.it/repls/RudeMoralWorkplace
我想了解这些表达式在幕后发生了什么,以便我可以揭穿为什么我得到不同的结果.
你提到的集合操作是:
^
- 对称差异(异或):
返回一个新集合,其中包含该集合中的元素或其他元素,但不能同时包含两者.
示例: {'1', '2', '3'} ^ {'2', '3', '4'} = {'1', '4'}
|
- union(或):
返回一个包含集合中的元素和所有其他元素的新集合.
示例: {'1', '2', '3'} |{'2', '3', '4'} = {'1', '2', '3', '4'}
python中还有其他set操作:
&
- 交集(AND):
返回一个新集合,其中包含与集合和所有其他元素共有的元素.
示例: {'1', '2', '3'} &{'2', '3', '4'} = {'2', '3'}
-
-差异:
返回一个新集合,该集合中的元素不在其他集合中.
示例: {'1', '2', '3'} - {'2', '3', '4'} = {'1'}代码>
这些操作的优先顺序是-, &, ^, |
,所以在你的例子中,我们首先应用^
:
然后|
:
你描述的不同输出实际上是同一个集合,因为集合没有排序.
<预><代码>>>>{'c', 'h', 'f', 'd', 'b', 'i', 'g', 'a', 'e'} == {'a', 'd', 'h', 'f', 'b', 'g', 'e', 'c', 'i'}真的集合的字符串表示中显示的任何顺序都是实现细节,不应依赖,因为正如您所发现的,它会发生不可预测的变化.
I had seen this test question on Pluralsight:
Given these sets:
x = {'a', 'b', 'c', 'd'}
y = {'c', 'e', 'f'}
z = {'a', 'g', 'h', 'i'}
What is the value of x | y ^ z
?
The expected answer is:
{'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i'}
Combines the sets (automatically discarding duplicates), and orders them from lowest to greatest.
My questions are:
- What is this expression called?
- Why do I get 3 different results from 3 different Python versions?
Result on Python 3.7.5 on Ubuntu 18.04:
{'c', 'h', 'f', 'd', 'b', 'i', 'g', 'a', 'e'}
Result on Python 2.17.17rc1 on Ubuntu 18.04:
set(['a', 'c', 'b', 'e', 'd', 'g', 'f', 'i', 'h'])
Result on Python 3.7.2 on Windows 10:
{'a', 'd', 'h', 'f', 'b', 'g', 'e', 'c', 'i'}
Here is a repl of the same code I'm using for this: https://repl.it/repls/RudeMoralWorkplace
I'd like to understand what happens behind the scenes with these expressions so I can debunk why I get different results.
The set operations you have mentioned are:
^
- symmetric difference (XOR):
Return a new set with elements in either the set or other but not both.
Example: {'1', '2', '3'} ^ {'2', '3', '4'} = {'1', '4'}
|
- union (OR):
Return a new set with elements from the set and all others.
Example: {'1', '2', '3'} | {'2', '3', '4'} = {'1', '2', '3', '4'}
There are also other set operations in python:
&
- intersection (AND):
Return a new set with elements common to the set and all others.
Example: {'1', '2', '3'} & {'2', '3', '4'} = {'2', '3'}
-
- difference:
Return a new set with elements in the set that are not in the others.
Example: {'1', '2', '3'} - {'2', '3', '4'} = {'1'}
The order of precedence for these operations is -, &, ^, |
, so in your example, we first apply ^
:
>>> y^z
{'a', 'c', 'e', 'f', 'g', 'h', 'i'}
And then |
:
>>> x|{'a', 'c', 'e', 'f', 'g', 'h', 'i'}
{'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i'}
The different outputs you describe are actually the same set, as sets are not ordered.
>>> {'c', 'h', 'f', 'd', 'b', 'i', 'g', 'a', 'e'} == {'a', 'd', 'h', 'f', 'b', 'g', 'e', 'c', 'i'}
True
Any order shown in the string representation of a set is an implementation detail and should not be relied upon as it will vary unpredictably, as you have found.
这篇关于这些集合操作是什么,为什么它们会给出不同的结果?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!