Slicing sublists with different lengths

Question

I have a list of lists. Each sublist has a length that varies between 1 and 100. Each sublist contains a particle ID at different times in a set of data. I would like to form lists of all particle IDs at a given time. To do this I could use something like:

    list = [[1,2,3,4,5],[2,6,7,8],[1,3,6,7,8]]
    list2 = [item[0] for item in list]

list2 would contain the first elements of each sublist in list. I would like to do this operation not just for the first element, but for every element between 1 and 100. My problem is that element number 100 (or 66 or 77 or whatever) does not exists for every sublist.

Is there some way of creating a lists of lists, where each sublist is the list of all particle IDs at a given time.

I have thought about trying to use numpy arrays to solve this problem, as if the lists were all the same length this would be trivial. I have tried adding -1's to the end of each list to make them all the same length, and then masking the negative numbers, but this hasn't worked for me so far. I will use the list of IDs at a given time to slice another separate array:

    pos = pos[satIDs]

Answer 1

lst = [[1,2,3,4,5],[2,6,7,8],[1,3,6,7,8]]
func =  lambda x: [line[x] for line in lst if len(line) > x]

func(3)
[4, 8, 7]
func(4)
[5, 8]

--update--

func =  lambda x: [ (line[x],i) for i,line in enumerate(lst) if len(line) > x]
func(4)
[(5, 0), (8, 2)]

Answer 2

You could use itertools.zip_longest. This will zip the lists together and insert None when one of the lists is exhausted.

>>> lst = [[1,2,3,4,5],['A','B','C'],['a','b','c','d','e','f','g']]    
>>> list(itertools.zip_longest(*lst))
[(1, 'A', 'a'),
 (2, 'B', 'b'),
 (3, 'C', 'c'),
 (4, None, 'd'),
 (5, None, 'e'),
 (None, None, 'f'),
 (None, None, 'g')]

If you don't want the None elements, you can filter them out:

>>> [[x for x in sublist if x is not None] for sublist in itertools.zip_longest(*lst)]
[[1, 'A', 'a'], [2, 'B', 'b'], [3, 'C', 'c'], [4, 'd'], [5, 'e'], ['f'], ['g']]