How to split a list based on tuple content in Python-CodePudding

I have a list of 3-tuples, and I want to split them on the last coordinate. So for example if the input were:

[(1,2,x), (2,3,x), (1,2,z), (2,20,z)]

I'd want the output to be

[(1,2,x),(2,3,x)],[(1,2,z),(2,20,z)]

My current approach involves getting all N distinct 3rd indices and looping over the list N times to build the list of lists. I am wondering if there is a pythonic way to do this without looping multiple times.

CodePudding user response：

here is one way with complexity of O(n):

lt = [(1,2,'x'), (2,3,'x'), (1,2,'z'), (2,20,'z')]

res = {}
for i in lt:
  res.setdefault(i[2], []).append(i)

print(list(res.values()))

output:

>>>
[[(1, 2, 'x'), (2, 3, 'x')], [(1, 2, 'z'), (2, 20, 'z')]]

CodePudding user response：

nested list comprehension if you dont know third_indicesa priori:

my_list = [(1,2,'x'), (2,3,'x'), (1,2,'z'), (2,20,'z')]
third_indices = set([ix[-1] for ix in my_list])
[[item for item in my_list if item[-1]==ix] for ix in third_indices]

CodePudding user response：

Similar to @eshirvana, but using defaultdict, which is very performant and common for this type of problem. I would make a defaultdict and save lists of tuples based the key determined by the last element. Then all your groups will be in the values()

from collections import defaultdict

l = [(1,2,'x'), (2,3,'x'), (1,2,'z'), (2,20,'z'), (3,4,'x')]

groups = defaultdict(list)

for t in l:
    groups[t[-1]].append(t)
    
list(groups.values())
# [[(1, 2, 'x'), (2, 3, 'x'), (3, 4, 'x')], [(1, 2, 'z'), (2, 20, 'z')]]

CodePudding user response：

You can do something like this:

my_list = [(1,2,x), (2,3,x), (1,2,z), (2,20,z)]

list_x = [item for item in my_list if item[2] == x]
list_y = [item for item in my_list if item[2] == y]
list_z = [item for item in my_list if item[2] == z]

Edit: If x,y and z are not numbers then this should work:

my_list = [(1,2,'x'), (2,3,'x'), (1,2,'z'), (2,20,'z')]

list_x = [item for item in my_list if item[2] == 'x']
list_y = [item for item in my_list if item[2] == 'y']
list_z = [item for item in my_list if item[2] == 'z']

If you want them in the same list then do this

final_list = list((list_x,list_z))