从给定列表和给定索引中提取列表

Question

I have a list containinig, more or less, random values. The list always has a fixed length. I have another list containing integer values. These values are always smaller than the length of the first list.

I want to calculate a list containing all values from the first list whose indices are described by the values in the second list. I came up with the following:

>>> values = ['000', '111', '222', '333', '444', '555', '666', '777']
>>> indices = [2, 4, 7]
>>> [v for i, v in enumerate(values) if i in indices]
['222', '444', '777']

As my lists are rather small (24 elements) this is OK for me. Anyway, I wonder if there is some more elegant solution which does not calculate a temporary list (with enumerate() ).

Answer 1

>>> values = ['000', '111', '222', '333', '444', '555', '666', '777']
>>> indices = [2, 4, 7]

1. You can use a simple list comprehension

    >>> [values[index] for index in indices] ['222', '444', '777'] 

2. You can use operator.itemgetter , like this

    >>> from operator import itemgetter >>> itemgetter(*indices)(values) ('222', '444', '777') >>> list(itemgetter(*indices)(values)) ['222', '444', '777'] 

3. Or you can invoke the magic method, __getitem__ with map , like this

    >>> map(values.__getitem__, indices) ['222', '444', '777'] 

   If you are using Python 3.x, then you might want to use list with map

    >>> list(map(values.__getitem__, indices)) ['222', '444', '777'] 

4. If you don't want to create the entire list, then you can create a generator expression and use next with that to get the values whenever you want.

    >>> filtered = (values[index] for index in indices) >>> next(filtered) '222' >>> next(filtered) '444' >>> next(filtered) '777' >>> next(filtered) Traceback (most recent call last): File "<input>", line 1, in <module> StopIteration 

   If you are going to just iterate the result, then I would recommend using the generator expression approach.

    >>> for item in (values[index] for index in indices): ... print(item + ' ' + item) ... 222 222 444 444 777 777