Access multiple elements of list knowing their index

Question

I need to choose some elements from the given list  knowing their index  Let say I would like to create a new list  which contains element with index 1  2  5  from given list  -2  1  5  3  8  5  6   What I did is   a    -2 1 5 3 8 5 6  b    1 2 5  c     a i  for i in b    Is there any better way to do it  something like c   a b

User · Answer

I m sure this has already been considered  If the amount of indices in b is small and constant  one could just write the result like    c    a b 0       a b 1       a b 2      Or even simpler if the indices itself are constants     c    a 1      a 2      a 5     Or if there is a consecutive range of indices     c   a 1 3     a 5

User · Answer

Alternatives    gt  gt  gt  map a   getitem    b   1  5  5       gt  gt  gt  import operator  gt  gt  gt  operator itemgetter  b  a   1  5  5

User · Answer

Static indexes and small list   Don t forget that if the list is small and the indexes don t change  as in your example  sometimes the best thing is to use sequence unpacking     a1 a2     a3     a   The performance is much better and you can also save one line of code     timeit   a1 b1     c1     a 10000000 loops  best of 3  154 ns per loop   timeit itemgetter  b  a  1000000 loops  best of 3  753 ns per loop   timeit   a i  for i in b  1000000 loops  best of 3  777 ns per loop   timeit map a   getitem    b  1000000 loops  best of 3  1 42   s per loop

User · Answer

Another solution could be via pandas Series   import pandas as pd  a   pd Series  -2  1  5  3  8  5  6   b    1  2  5  c   a b    You can then convert c back to a list if you want    c   list c

User · Answer

Here s a simpler way   a    -2 1 5 3 8 5 6  b    1 2 5  c    e for i  e in enumerate a  if i in b

User · Answer

Kind of pythonic way   c    x for x in a if a index x  in b

User · Answer

You can use operator itemgetter   from operator import itemgetter  a    -2  1  5  3  8  5  6  b    1  2  5  print itemgetter  b  a     Result   1  5  5    Or you can use numpy   import numpy as np a   np array  -2  1  5  3  8  5  6   b    1  2  5  print list a b      Result   1  5  5      But really  your current solution is fine  It s probably the neatest out of all of them

User · Answer

My answer does not use numpy or python collections   One trivial way to find elements would be as follows   a    -2  1  5  3  8  5  6  b    1  2  5  c    i for i in a if i in b    Drawback  This method may not work for larger lists  Using numpy is recommended for larger lists

User · Answer

Basic and not very extensive testing comparing the execution time of the five supplied answers   def numpyIndexValues a  b       na   np array a      nb   np array b      out   list na nb       return out  def mapIndexValues a  b       out   map a   getitem    b      return list out   def getIndexValues a  b       out   operator itemgetter  b  a      return out  def pythonLoopOverlap a  b       c     a i  for i in b      return c  multipleListItemValues   lambda searchList  ind   searchList i  for i in ind    using the following input   a   range 0  10000000  b   range 500  500000    simple python loop was the quickest with lambda operation a close second  mapIndexValues and getIndexValues were consistently pretty similar with numpy method significantly slower after converting lists to numpy arrays If data is already in numpy arrays the numpyIndexValues method with the numpy array conversion removed is quickest   numpyIndexValues - gt  time 1 38940598  when converted the lists to numpy arrays  numpyIndexValues - gt  time 0 0193445  using numpy array instead of python list as input  and conversion code removed  mapIndexValues - gt  time 0 06477512099999999 getIndexValues - gt  time 0 06391049500000001 multipleListItemValues - gt  time 0 043773591 pythonLoopOverlap - gt  time 0 043021754999999995

[python] Access multiple elements of list knowing their index

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