我有一个问题：
有一个元素列表，其中的元素属于 CAnswer 类（无需描述该类），我需要对它进行乱序操作。但是有一个限制，列表中有一些元素的 CAnswer.freeze 属性被设置为 True，这些元素不应该被打乱，而是保持在原来的位置上。例如，给定一个列表：
[a, b, c, d, e, f]
如果所有元素都是CAnswer的实例，但是c.freeze == True，而其他元素freeze == False，则可能的结果如下：
[e, a, c, f, b, d]
因此，索引为2的元素仍然位于其位置。
实现这个功能的最佳算法是什么？
先行致谢 :)
另一种解决方案：
# memorize position of fixed elements
fixed = [(pos, item) for (pos,item) in enumerate(items) if item.freeze]
# shuffle list
random.shuffle(items)
# swap fixed elements back to their original position
for pos, item in fixed:
index = items.index(item)
items[pos], items[index] = items[index], items[pos]
一个解决方案：
def fixed_shuffle(lst):
unfrozen_indices, unfrozen_subset = zip(*[(i, e) for i, e in enumerate(lst)
if not e.freeze])
unfrozen_indices = list(unfrozen_indices)
random.shuffle(unfrozen_indices)
for i, e in zip(unfrozen_indices, unfrozen_subset):
lst[i] = e
注意：如果lst是一个 NumPy数组 而不是一个常规列表，则可以更简单一些：
def fixed_shuffle_numpy(lst):
unfrozen_indices = [i for i, e in enumerate(lst) if not e.freeze]
unfrozen_set = lst[unfrozen_indices]
random.shuffle(unfrozen_set)
lst[unfrozen_indices] = unfrozen_set
它的使用示例：
class CAnswer:
def __init__(self, x, freeze=False):
self.x = x
self.freeze = freeze
def __cmp__(self, other):
return self.x.__cmp__(other.x)
def __repr__(self):
return "<CAnswer: %s>" % self.x
lst = [CAnswer(3), CAnswer(2), CAnswer(0, True), CAnswer(1), CAnswer(5),
CAnswer(9, True), CAnswer(4)]
fixed_shuffle(lst)
使用random.shuffle()源代码,在O(n)的时间复杂度和常数空间复杂度内完成：
from random import random
def shuffle_with_freeze(x):
for i in reversed(xrange(1, len(x))):
if x[i].freeze: continue # fixed
# pick an element in x[:i+1] with which to exchange x[i]
j = int(random() * (i+1))
if x[j].freeze: continue #NOTE: it might make it less random
x[i], x[j] = x[j], x[i] # swap
过度设计的解决方案：创建一个包含未冻结元素索引并模拟列表的包装类，并确保setter将写入原始列表：
class IndexedFilterList:
def __init__(self, originalList, filterFunc):
self.originalList = originalList
self.indexes = [i for i, x in enumerate(originalList) if filterFunc(x)]
def __len__(self):
return len(self.indexes)
def __getitem__(self, i):
return self.originalList[self.indexes[i]]
def __setitem__(self, i, value):
self.originalList[self.indexes[i]] = value
请拨打电话：
random.shuffle(IndexedFilterList(mylist, lambda c: not c.freeze))
shuffle_subset的一个可能的实现：#!/usr/bin/env python
"""Shuffle elements in a list, except for a sub-set of the elments.
The sub-set are those elements that should retain their position in
the list. Some example usage:
>>> from collections import namedtuple
>>> class CAnswer(namedtuple("CAnswer","x fixed")):
... def __bool__(self):
... return self.fixed is True
... __nonzero__ = __bool__ # For Python 2. Called by bool in Py2.
... def __repr__(self):
... return "<CA: {}>".format(self.x)
...
>>> val = [3, 2, 0, 1, 5, 9, 4]
>>> fix = [2, 5]
>>> lst = [ CAnswer(v, i in fix) for i, v in enumerate(val)]
>>> print("Start ", 0, ": ", lst)
Start 0 : [<CA: 3>, <CA: 2>, <CA: 0>, <CA: 1>, <CA: 5>, <CA: 9>, <CA: 4>]
Using a predicate to filter.
>>> for i in range(4): # doctest: +NORMALIZE_WHITESPACE
... shuffle_subset(lst, lambda x : x.fixed)
... print([lst[i] for i in fix], end=" ")
...
[<CA: 0>, <CA: 9>] [<CA: 0>, <CA: 9>] [<CA: 0>, <CA: 9>] [<CA: 0>, <CA: 9>]
>>> for i in range(4): # doctest: +NORMALIZE_WHITESPACE
... shuffle_subset(lst) # predicate = bool()
... print([lst[i] for i in fix], end=" ")
...
[<CA: 0>, <CA: 9>] [<CA: 0>, <CA: 9>] [<CA: 0>, <CA: 9>] [<CA: 0>, <CA: 9>]
"""
from __future__ import print_function
import random
def shuffle_subset(lst, predicate=None):
"""All elements in lst, except a sub-set, are shuffled.
The predicate defines the sub-set of elements in lst that should
not be shuffled:
+ The predicate is a callable that returns True for fixed
elements, predicate(element) --> True or False.
+ If the predicate is None extract those elements where
bool(element) == True.
"""
predicate = bool if predicate is None else predicate
fixed_subset = [(i, e) for i, e in enumerate(lst) if predicate(e)]
fixed_subset.reverse() # Delete fixed elements from high index to low.
for i, _ in fixed_subset:
del lst[i]
random.shuffle(lst)
fixed_subset.reverse() # Insert fixed elements from low index to high.
for i, e in fixed_subset:
lst.insert(i, e)
if __name__ == "__main__":
import doctest
doctest.testmod()