How to extract the n-th elements from a list of tuples in python?

Question

Hello, I'm trying to obtain the n-th elements from a list of tuples. I have something like

elements = [(1,1,1),(2,3,7),(3,5,10)]

and I want to extract only the second elements of each tuple into an array:

seconds = [1, 3, 5]

I know that it could be done with a for but I wanted to know if there's another way since I have thousands of tuples.
any ideas? Thank you =)

Answer 1

[x[1] for x in elements]

Answer 2

I know that it could be done with a FOR but I wanted to know if there's another way

There is another way. You can also do it with map and itemgetter:

>>> from operator import itemgetter
>>> map(itemgetter(1), elements)

This still performs a loop internally though and it is slightly slower than the list comprehension:

setup = 'elements = [(1,1,1) for _ in range(100000)];from operator import itemgetter'
method1 = '[x[1] for x in elements]'
method2 = 'map(itemgetter(1), elements)'

import timeit
t = timeit.Timer(method1, setup)
print('Method 1: ' + str(t.timeit(100)))
t = timeit.Timer(method2, setup)
print('Method 2: ' + str(t.timeit(100)))

Results:

Method 1: 1.25699996948
Method 2: 1.46600008011

If you need to iterate over a list then using a for is fine.

Answer 3

This also works:

zip(*elements)[1]

(I am mainly posting this, to prove to myself that I have groked zip...)

See it in action:

>>> help(zip)

Help on built-in function zip in module builtin:

zip(...)

zip(seq1 [, seq2 [...]]) -> [(seq1[0], seq2[0] ...), (...)]

Return a list of tuples, where each tuple contains the i-th element from each of the argument sequences. The returned list is truncated in length to the length of the shortest argument sequence.

>>> elements = [(1,1,1),(2,3,7),(3,5,10)]
>>> zip(*elements)
[(1, 2, 3), (1, 3, 5), (1, 7, 10)]
>>> zip(*elements)[1]
(1, 3, 5)
>>>

Neat thing I learned today: Use *list in arguments to create a parameter list for a function...