遍历集合的类

Question

class Ordered:
    def __init__(self,aset):
        self.aset = aset
    def __iter__(self):
        for v in sorted(self.aset): # iterate over list of values returned by sorted
            yield v

该函数接受一个集合并返回一个列表

集合始终是

S= {1，2，4，8，16}

例如：

s = {1, 2, 4, 8, 16}
i = iter(Ordered(s))
print(next(i))
print(next(i))
s.remove(8)
print(next(i))
s.add(32)
print(next(i))
print(next(i))

it should prints 1 2 4 16 32

但是当我的函数

[next(i), next(i), s.remove(8), next(i), next(i), s.add(32), next(i)]

它应该会打印出来

[1, 2, None, 4, 16, None, 32]

但是，它输出的是：

[1, 2, None, 4, 8, None, 16]

有人能告诉我怎么修吗？谢谢

我发布了下面的错误，以帮助理解：

39 *Error: Failed [next(i), next(i), s.remove(8), next(i), next(i), s.add(32), next(i)] == [1, 2, None, 4, 16, None, 32]
      evaluated: [1, 2, None, 4, 8, None, 16] == [1, 2, None, 4, 16, None, 32]
42 *Error: [next(i), next(i), next(i), s.add(3), next(i), s.add(10), s.add(32), next(i), next(i), next(i)] raised exception; unevaluated: [1, 2, 4, None, 8, None, None, 10, 16, 32]
46 *Error: Failed [next(i), s.remove(2), s.remove(4), s.remove(8), next(i)] == [1, None, None, None, 16]
      evaluated: [1, None, None, None, 2] == [1, None, None, None, 16]
49 *Error: Failed [next(i), s.remove(2), next(i), s.remove(4), s.remove(8), next(i)] == [1, None, 4, None, None, 16]
      evaluated: [1, None, 2, None, None, 4] == [1, None, 4, None, None, 16]

Answer 1

sorted()函数对参数进行排序并返回一个列表。修改输入集不会影响排序列表。

如果您希望迭代器反映对原始集的更改，迭代器将需要在每次迭代时检查集的状态并做出适当的响应。

Answer 2

使用sorted时，您将从集合中创建一个排序列表。该列表与创建它的原始集合没有任何连接，并且不会反映对该集合的任何更改。在按排序的顺序迭代元素时，您必须自己跟踪更改。

跟踪删除的元素很简单:只需检查当前元素是否仍在原始集合中。跟踪新元素要复杂一些。您可以使用heapq模块并从集合创建一个heap，而不是一个排序列表，然后您可以在迭代堆的同时将新元素添加到堆中。要查找新元素，请创建原始集的副本，并在每次迭代中比较这两个元素。此外，根据您的测试用例，您将必须检查当前元素是否比前一个元素小，并在这种情况下跳过它。

import heapq

class Ordered:

    def __init__(self,aset):
        self.aset = aset

    def __iter__(self):
        # memorize original elements
        known = set(self.aset)
        last = None

        # create heap
        heap = list(self.aset)
        heapq.heapify(heap)

        # yield from heap
        while heap:
            v = heapq.heappop(heap)
            if (v in self.aset and  # not removed
                    (last is None or last < v)): # not smaller than last
                yield v
            last = v

            # in case of new elements, update the heap
            diff = self.aset - known
            if diff:
                for x in self.aset - known:
                    heapq.heappush(heap, x)
                known = set(self.aset)

这适用于您的所有测试用例(未显示s和i的重新初始化)：

>>> s = {1, 2, 4, 8, 16}
>>> i = iter(Ordered(s))
>>> [next(i), next(i), s.remove(8), next(i), next(i), s.add(32), next(i)]
[1, 2, None, 4, 16, None, 32]
>>> [next(i), next(i), next(i), s.add(3), next(i), s.add(10), s.add(32), next(i), next(i), next(i)]
[1, 2, 4, None, 8, None, None, 10, 16, 32])
>>> [next(i), s.remove(2), s.remove(4), s.remove(8), next(i)]
[1, None, None, None, 16]
>>> [next(i), s.remove(2), next(i), s.remove(4), s.remove(8), next(i)]
[1, None, 4, None, None, 16]