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

Question

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     I wish to extract only the second elements of each tuple into a list   seconds    1  3  5    I know that it could be done with a for loop but I wanted to know if there s another way since I have thousands of tuples

User · Answer

Found this as I was searching for which way is fastest to pull the second element of a 2-tuple list. Not what I wanted but ran same test as shown with a 3rd method plus test the zip method

setup = 'elements = [(1,1) for _ in range(100000)];from operator import itemgetter'
method1 = '[x[1] for x in elements]'
method2 = 'map(itemgetter(1), elements)'
method3 = 'dict(elements).values()'
method4 = 'zip(*elements)[1]'

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)))
t = timeit.Timer(method3, setup)
print('Method 3: ' + str(t.timeit(100)))
t = timeit.Timer(method4, setup)
print('Method 4: ' + str(t.timeit(100)))

Method 1: 0.618785858154
Method 2: 0.711684942245
Method 3: 0.298138141632
Method 4: 1.32586884499

So over twice as fast if you have a 2 tuple pair to just convert to a dict and take the values.

User · Answer

Using islice and chain from iterable    gt  gt  gt  from itertools import chain  islice  gt  gt  gt  elements     1 1 1   2 3 7   3 5 10    gt  gt  gt  list chain from iterable islice item  1  2  for item in elements    1  3  5    This can be useful when you need more than one element    gt  gt  gt  elements     0  1  2  3  4  5                     10  11  12  13  14  15                     20  21  22  23  24  25    gt  gt  gt  list chain from iterable islice tuple   2  5  for tuple  in elements    2  3  4  12  13  14  22  23  24

User · Answer

Timings for Python 3 6 for extracting the second element from a 2-tuple list    Also  added numpy array method  which is simpler to read  but arguably simpler than the list comprehension    from operator import itemgetter elements     1 1  for   in range 100000     timeit second    x 1  for x in elements   timeit second   list map itemgetter 1   elements    timeit second   dict elements  values    timeit second   list zip  elements   1   timeit second   np array elements    1    and the timings   list comprehension   4 73 ms    206   s per loop list map             5 3 ms    167   s per loop dict                 2 25 ms    103   s per loop list zip             5 2 ms    252   s per loop numpy array         28 7 ms    1 88 ms per loop   Note that map   and zip   do not return a list anymore  hence the explicit conversion

User · Answer

n   1   N       x n  for x in elements

User · Answer

This also works   zip  elements  1     I am mainly posting this  to prove to myself that I have groked zip      See it in action    gt  gt  gt  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     gt  gt  gt  elements     1 1 1   2 3 7   3 5 10    gt  gt  gt  zip  elements    1  2  3    1  3  5    1  7  10    gt  gt  gt  zip  elements  1   1  3  5   gt  gt  gt    Neat thing I learned today  Use  list in arguments to create a parameter list for a function     Note  In Python3  zip returns an iterator  so instead use list zip  elements   to return a list of tuples

User · Answer

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    gt  gt  gt  from operator import itemgetter  gt  gt  gt  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

User · Answer

map  lambda x  x 1   elements

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

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