How to get first element in a list of tuples

Question

I have a list like below where the first element is the id and the other is a string     1  u abc     2  u def      I want to create a list of ids only from this list of tuples as below    1 2    I ll use this list in   in so it needs to be a list of integer values

User · Answer

when I ran  as suggested above     gt  gt  gt  a     1  u abc     2  u def     gt  gt  gt  import operator  gt  gt  gt  b   map operator itemgetter 0   a   gt  gt  gt  b   instead of returning    1  2    I received this as the return    lt map at 0xb387eb8 gt    I found I had to use list      gt  gt  gt  b   list map operator itemgetter 0   a     to successfully return a list using this suggestion  That said  I m happy with this solution  thanks   tested run using Spyder  iPython console  Python v3 6

User · Answer

You can use  quot tuple unpacking quot    gt  gt  gt  my list     1   abc     2   def     gt  gt  gt  my ids    idx for idx  val in my list   gt  gt  gt  my ids  1  2   At iteration time each tuple is unpacked and its values are set to the variables idx and val   gt  gt  gt  x    1   abc    gt  gt  gt  idx  val   x  gt  gt  gt  idx 1  gt  gt  gt  val  abc

User · Answer

gt  gt  gt  a     1  u abc     2  u def     gt  gt  gt   i 0  for i in a   1  2

User · Answer

I was thinking that it might be useful to compare the runtimes of the different approaches so I made a benchmark  using simple benchmark library   I  Benchmark having tuples with 2 elements   As you may expect to select the first element from tuples by index 0 shows to be the fastest solution very close to the unpacking solution by expecting exactly 2 values  import operator import random  from simple benchmark import BenchmarkBuilder  b   BenchmarkBuilder       b add function   def rakesh by index l       return  i 0  for i in l     b add function   def wayneSan zip l       return list list zip  l   0      b add function   def bcattle itemgetter l        return list map operator itemgetter 0   l      b add function   def ssoler upacking l       return  idx for idx  val in l    b add function   def kederrack unpacking l       return  f for f     in l      b add arguments  Number of tuples   def argument provider        for exp in range 2  21           size   2  exp         yield size    random choice range 100    random choice range 100    for   in range size     r   b run   r plot     II  Benchmark having tuples with 2 or more elements   import operator import random  from simple benchmark import BenchmarkBuilder  b   BenchmarkBuilder     b add function   def kederrack unpacking l       return  f for f     in l     b add function   def rakesh by index l       return  i 0  for i in l     b add function   def wayneSan zip l       return list list zip  l   0      b add function   def bcattle itemgetter l        return list map operator itemgetter 0   l      b add arguments  Number of tuples   def argument provider        for exp in range 2  21           size   2  exp         yield size   tuple random choice range 100   for                        in range random choice range 2  100     for   in range size    from pylab import rcParams rcParams  figure figsize     12  7  r   b run   r plot

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From a performance point of view  in python3 X    i 0  for i in a  and list zip  a   0  are equivalent they are faster than list map operator itemgetter 0   a     Code  import timeit   iterations   100000 init time   timeit timeit    a     i  u abc   for i in range 1000       number iterations  iterations print timeit timeit    a     i  u abc   for i in range 1000   nb    i 0  for i in a      number iterations  iterations - init time  print timeit timeit    a     i  u abc   for i in range 1000   nb   list zip  a   0      number iterations  iterations - init time    output  3 491014136001468e-05  3 422205176000717e-05

User · Answer

if the tuples are unique then this can work   gt  gt  gt  a     1  u abc     2  u def     gt  gt  gt  a   1  u abc     2  u def     gt  gt  gt  dict a  keys    1  2   gt  gt  gt  dict a  values    u abc   u def    gt  gt  gt

User · Answer

This is what operator itemgetter is for     gt  gt  gt  a     1  u abc     2  u def     gt  gt  gt  import operator  gt  gt  gt  b   map operator itemgetter 0   a   gt  gt  gt  b  1  2    The itemgetter statement returns a function that returns the index of the element you specify  It s exactly the same as writing   gt  gt  gt  b   map lambda x  x 0   a    But I find that itemgetter is a clearer and more explicit    This is handy for making compact sort statements  For example    gt  gt  gt  c   sorted a  key operator itemgetter 0   reverse True   gt  gt  gt  c   2  u def     1  u abc

User · Answer

you can unpack your tuples and get only the first element using a list comprehension    l     1  u abc     2  u def     f for f     in l    output    1  2    this will work no matter how many elements you have in a tuple   l     1  u abc     2  u def   2  4  5  6  7    f for f     in l    output    1  2

User · Answer

Those are tuples  not sets  You can do this   l1     1  u abc     2  u def    l2     tup 0    for tup in l1  l2  gt  gt  gt    1     2

User · Answer

I wondered why nobody suggested to use numpy  but now after checking i understand  It is maybe not the best for mixed type arrays   This would be a solution in numpy    gt  gt  gt  import numpy as np   gt  gt  gt  a   np asarray   1  u abc     2  u def      gt  gt  gt  a    0  astype int  tolist    1  2

User · Answer

Use the zip function to decouple elements    gt  gt  gt  inpt     1  u abc     2  u def     gt  gt  gt  unzipped   zip  inpt   gt  gt  gt  print unzipped   1  2    u abc   u def     gt  gt  gt  print list unzipped 0    1  2    Edit   BradSolomon   The above works for Python 2 x  where zip returns a list   In Python 3 x  zip returns an iterator and the following is equivalent to the above    gt  gt  gt  print list list zip  inpt   0     1  2

User · Answer

do you mean something like this   new list     seq 0  for seq in yourlist     What you actually have is a list of tuple objects  not a list of sets  as your original question implied    If it is actually a list of sets  then there is no first element because sets have no order   Here I ve created a flat list because generally that seems more useful than creating a list of 1 element tuples   However  you can easily create a list of 1 element tuples by just replacing seq 0  with  seq 0

[python] How to get first element in a list of tuples?

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