創建列表元素的多個副本

Question

我有一個元素序列，每個元素都與一個元組中的權重w（一個正整數）相關聯。創建包含每個元素e_i的w_i實例的序列或列表有什麼好方法（最好是基於迭代器的）？

weighted = [ ("a", 3), ("b", 1), ("c", 4) ] 

unrolled = [ "a", "a", "a", "b", "c", "c", "c", "c" ] 

Answer 1

使用列表中理解嵌套循環：

unrolled = [c for c, count in weighted for _ in range(count)] 

如果您正在使用Python 2，你可以使用xrange()代替。

如果你喜歡itertools，您可以使用itertools.chain.from_iterable()，使之成爲一個懶惰的迭代這樣的：

from itertools import chain 

chain.from_iterable([c] * count for c, count in weighted) 

演示：

>>> weighted = [ ("a", 3), ("b", 1), ("c", 4) ] 
>>> [c for c, count in weighted for _ in range(count)] 
['a', 'a', 'a', 'b', 'c', 'c', 'c', 'c'] 
>>> from itertools import chain 
>>> list(chain.from_iterable([c] * count for c, count in weighted)) 
['a', 'a', 'a', 'b', 'c', 'c', 'c', 'c'] 

我用list()打開chain迭代器放回序列。

Answer 2

該方法基本上使用生成器表達式和join創建單個字符串，然後將該字符串轉換爲list。

weighted = [ ("a", 3), ("b", 1), ("c", 4) ] 
unrolled = list(''.join([letter*count for letter,count in weighted])) 

>>> unrolled 
['a', 'a', 'a', 'b', 'c', 'c', 'c', 'c'] 

Answer 3

Counter有這種邏輯建立在假設的順序並不重要

>>> weighted = [ ("a", 3), ("b", 1), ("c", 4) ] 
>>> from collections import Counter 
>>> c = Counter(dict(weighted)) 
>>> list(c.elements()) 
['a', 'a', 'a', 'c', 'c', 'c', 'c', 'b'] 

與itertools（很多人）

>>> from itertools import chain, repeat, starmap 
>>> list(chain.from_iterable(starmap(repeat, weighted))) 
['a', 'a', 'a', 'b', 'c', 'c', 'c', 'c']