I'd like to join a list into a string by index in Python 3.3. I can do it for items the follow each other, but I would like to access it by index only.
this works:
list = ['A', 'B', 'C', 'D']
partList = "".join(list[1:3])
-> BC
But how can I achieve this (it doesn't work):
list = ['A', 'B', 'C', 'D']
partList = "".join(list[0,3])
-> AD
You can't slice lists into arbitrary chunks using slice notation. Probably your best bet for the general case is to use a list of indices to build a list comprehension.
mylist = ['A', 'B', 'C', 'D'] # the full list
indices = [0, 3] # the indices of myList that you want to extract
# Now build and join a new list comprehension using only those indices.
partList = "".join([e for i, e in enumerate(mylist) if i in indices])
print(partList) # >>> AD
As pointed out in the comments by DSM, if you're concerned with efficiency and you know your list of indices will be "friendly" (that is, it won't have any indices too big for the list you're cutting up), you can use a simpler expression without enumerate:
partList = "".join([mylist[i] for i in indices])
enumerate doesn't add much; cf. ''.join([mylist[i] for i in indices]).
enumerate to allow for indices outside the length of the list (say, if some index list is being reused for slicing a bunch of different lists that may or not be of the same length for whatever reason).indices a set, but then we lose the possibility for out-of-order access. There's always operator.itemgetter, too.. ;^)What you are intending to perform is not slicing but selecting.
The closest possibility, I can think of is to use operator.itemgetter
>>> from operator import itemgetter
>>> mylist = ['A', 'B', 'C', 'D']
>>> ''.join(itemgetter(0,3)(mylist))
'AD'
If you need to use a variable, then use a splat operator
index = (0,3)
''.join(itemgetter(*index)(mylist))
list). You'll save yourself a lot of trouble.my_list[0:4:3]works for this specific case, but is... unappealing. Any reason you specifically want a slice?