If I have a list of lists and just want to manipulate an individual item in that list, how would I go about doing that?
For example:
List1 = [[10,13,17],[3,5,1],[13,11,12]] What if I want to take a value (say 50) and look just at the first sublist in List1, and subtract 10 (the first value), then add 13, then subtract 17?
7 Answers
You can access the elements in a list-of-lists by first specifying which list you're interested in and then specifying which element of that list you want. For example, 17 is element 2 in list 0, which is list1[0][2]:
>>> list1 = [[10,13,17],[3,5,1],[13,11,12]] >>> list1[0][2] 17 So, your example would be
50 - list1[0][0] + list1[0][1] - list1[0][2] You can use itertools.cycle:
>>> from itertools import cycle >>> lis = [[10,13,17],[3,5,1],[13,11,12]] >>> cyc = cycle((-1, 1)) >>> 50 + sum(x*next(cyc) for x in lis[0]) # lis[0] is [10,13,17] 36 Here the generator expression inside sum would return something like this:
>>> cyc = cycle((-1, 1)) >>> [x*next(cyc) for x in lis[0]] [-10, 13, -17] You can also use zip here:
>>> cyc = cycle((-1, 1)) >>> [x*y for x, y in zip(lis[0], cyc)] [-10, 13, -17] This code will print each individual number:
for myList in [[10,13,17],[3,5,1],[13,11,12]]: for item in myList: print(item) Or for your specific use case:
((50 - List1[0][0]) + List1[0][1]) - List1[0][2] List1 = [[10,-13,17],[3,5,1],[13,11,12]] num = 50 for i in List1[0]:num -= i print num 50 - List1[0][0] + List[0][1] - List[0][2] List[0] gives you the first list in the list (try out print List[0]). Then, you index into it again to get the items of that list. Think of it this way: (List1[0])[0].
for l in list1: val = 50 - l[0] + l[1] - l[2] print "val:", val Loop through list and do operation on the sublist as you wanted.
new_list = list(zip(*old_list))) *old_list unpacks old_list into multiple lists and zip picks corresponding nth element from each list and list packs them back.
