如何以不规则的间隔切片列表？

Question

I the following list I want to do slicing such that my final result is [1,1.5,2,2.5,4.5,5] . I tried to access by indices but it gives me error.

`list = [1,1.5,2,2.5,3,3.5,4,4.5,5]
print(list)
print(list[0,1,2,3,7,8])`

Answer 1

You can't use a list of indexes to slice or index a list. You have to use a loop—eg, via a comprehension , or a function with an implicit loop in it like the one returned by itemgetter :

>>> lst = [1,1.5,2,2.5,3,3.5,4,4.5,5]
>>> print([lst[i] for i in [0, 1, 2, 3, 7, 8]])
[1, 1.5, 2, 2.5, 4.5, 5]
>>> print(operator.itemgetter(0, 1, 2, 3, 7, 8)(lst))
[1, 1.5, 2, 2.5, 4.5, 5]

---

But you can do something like this with numpy arrays. You can't directly use the six indices (because that would mean multidimensional indexing), but you can use any array-like, such as a list, containing those six indices:

>>> arr = np.array(lst)
>>> print(arr[[0, 1, 2, 3, 7, 8]])
[1.  1.5 2.  2.5 4.5 5. ]

This is explained in Index arrays in the basic indexing tutorial.

Answer 2

Python does not support irregular intervals. You must use a list comprehension.

lst = [1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5]
indices = (0, 1, 2, 3, 7, 8)
slice = [lst[i] for i in indices]

print(slice) # [1, 1.5, 2, 2.5, 4.5, 5]

Also, do not use names such as list for your variables as this overshadows builtin functions.

Answer 3

Work the other way around; instead of selecting a non-contiguous range, remove the contiguous range you don't want. You just want to get the original list minus indices 4-6 (inclusive), so copy your list , then del the indices you don't need:

mylist = [1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5]
trimmed = mylist[:]
del trimmed[4:7]
print(trimmed)

Alternatively, you could use operator.itemgetter to retrieve the specific indices you listed, eg:

from operator import itemgetter

mylist = [1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5]
print(itemgetter(0, 1, 2, 3, 7, 8)(mylist))

itemgetter of multiple values will output a tuple , so if that bothers you, you'd have to convert back to list , eg print(list(itemgetter(0, 1, 2, 3, 7, 8)(mylist))) (and note, the choice to change your variable name to mylist made it possible to access the list constructor, which would have been impossible if you named your variable list ).