Swapping arguments inside a list of liss of tuples

Question

I have a list of lists of tuples in the following form:

a = [[( 0,  1),
      ( 2,  3),
      ( 4,  5)],

     [( 6,  7),
      ( 8,  9),
      (10, 11)],

     [(12, 13),
      (14, 15),
      (16, 17)]]

What I want to do, is to swap the two arguments inside the tuples.

I have tried the two following options, but without succes, the argumets keep their positions:

for i in range(0, len(a)-1):
    for j in range(0, len(a)/2):

        a[i][j] = a[i][j][1], a[i][j][0]

        a[i][j] = tuple(reversed(a[i][j]))

Every help would be very much appreciated

Tuples are immutable, so your first approach shouldn't work. The second one should. — N. P., Nov 14 '17 at 16:27
https://stackoverflow.com/questions/8056130/immutable-vs-mutable-types — oshaiken, Nov 14 '17 at 16:28
@MosesKoledoye, I am using two for loops to call i and j. I have edited my question accordinlgly. — Maximilian A. Graves, Nov 14 '17 at 16:31
(1) The `range` function goes from 0 to one less than the end value. So passing 0 as an argument is redundant, and subtracting 1 from `len(a)` is incorrect. Just do `range(len(a))`. (2) I don't see why you're doing `len(a)/2)` for j. You should do `len(a[i]) instead`. — Acccumulation, Nov 14 '17 at 16:43

Moses Koledoye · Answer 1 · 2017-11-14T16:35:37.037

Tuples are immutable, so you should replace the tuples with the reversed tuples, not try to modify the first tuple in-place:

for lst in a:
   for i, x in enumerate(lst):
      lst[i] = x[::-1]

This takes updates the items at the position of each tuple in the list with a new reversed tuple (with reversed slice notation [::-1]) using list subscription.

To create a new list of lists with the swapped objects, you can use a list comprehension:

new_a = [[tup[::-1] for tup in lst]for lst in a]

score 0 · Answer 2 · answered Nov 14 '17 at 16:44

Consider changing your loops to:

for i in range(0, len(a)):
    for j in range(0, len(a[i])):
        a[i][j] = tuple(reversed(a[i][j]))

In order to iterate over a list of lists (of tuples, in this case), you need to make sure you're specifying the lengths of each part correctly. In len(a)-1 in the outer loop, you don't need the -1, as range(x,y) takes you from x to y-1 already. In the inner loop, you need to specify the length of the inner list.

@Moses has a good answer on why creating a new tuple is the correct approach.

Swapping arguments inside a list of liss of tuples

2 Answers2