Insert an element at a specific index in a list and return the updated list

Question

I have this   gt  gt  gt  a    1  2  4   gt  gt  gt  print a  1  2  4    gt  gt  gt  print a insert 2  3  None   gt  gt  gt  print a  1  2  3  4    gt  gt  gt  b   a insert 3  6   gt  gt  gt  print b None   gt  gt  gt  print a  1  2  3  6  4   Is there a way I can get the updated list as the result  instead of updating the original list in place

User · Accepted Answer

The shortest I got  b   a  2     3    a 2    gt  gt  gt   gt  gt  gt  a    1  2  4   gt  gt  gt  print a  1  2  4   gt  gt  gt  b   a  2     3    a 2    gt  gt  gt  print a  1  2  4   gt  gt  gt  print b  1  2  3  4

User · Answer

l insert index  obj  doesn t actually return anything  It just updates the list  As ATO said  you can do b   a  index     obj    a index    However  another way is  a    1  2  4  b   a    b insert 2  3

User · Answer

Use the Python list insert   method  Usage    Syntax   The syntax for the insert   method -   list insert index  obj     Parameters   index - This is the Index where the object obj need to be inserted  obj - This is the Object to be inserted into the given list     Return Value This method does not return any value  but it inserts the given element at the given index   Example  a    1 2 4 5   a insert 2 3   print a   Returns  1  2  3  4  5

User · Answer

The cleanest approach is to copy the list and then insert the object into the copy  On Python 3 this can be done via list copy  new   old copy   new insert index  value   On Python 2 copying the list can be achieved via new   old     this also works on Python 3   In terms of performance there is no difference to other proposed methods    python --version Python 3 8 1   python -m timeit -s  quot a   list range 1000   quot   quot b   a copy    b insert 500  3  quot  100000 loops  best of 5  2 84   sec per loop   python -m timeit -s  quot a   list range 1000   quot   quot b   a copy    b 500 500     3   quot  100000 loops  best of 5  2 76   sec per loop

User · Answer

Most performance efficient approach You may also insert the element using the slice indexing in the list  For example   gt  gt  gt  a    1  2  4   gt  gt  gt  insert at   2    Index at which you want to insert item   gt  gt  gt  b   a        Created copy of list  quot a quot  as  quot b quot                    Skip this step if you are ok with modifying the original list   gt  gt  gt  b insert at insert at     3     Insert  quot 3 quot  within  quot b quot   gt  gt  gt  b  1  2  3  4   For inserting multiple elements together at a given index  all you need to do is to use a list of multiple elements that you want to insert  For example   gt  gt  gt  a    1  2  4   gt  gt  gt  insert at   2     Index starting from which multiple elements will be inserted    List of elements that you want to insert together at  quot index at quot   above  position  gt  gt  gt  insert elements    3  5  6    gt  gt  gt  a insert at insert at    insert elements  gt  gt  gt  a      3  5  6  are inserted together in  a  starting at index  quot 2 quot   1  2  3  5  6  4   To know more about slice indexing  you can refer  Understanding slice notation  Note  In Python 3 x  difference of performance between slice indexing and list index      is significantly reduced and both are almost equivalent  However  in Python 2 x  this difference is quite noticeable  I have shared performance comparisons later in this answer   Alternative using list comprehension  but very slow in terms of performance   As an alternative  it can be achieved using list comprehension with enumerate too   But please don t do it this way  It is just for illustration    gt  gt  gt  a    1  2  4   gt  gt  gt  insert at   2   gt  gt  gt  b    y for i  x in enumerate a  for y in   3  x  if i    insert at else  x       gt  gt  gt  b  1  2  3  4    Performance comparison of all solutions Here s the timeit comparison of all the answers with list of 1000 elements on Python 3 9 1 and Python 2 7 16  Answers are listed in the order of performance for both the Python versions  Python 3 9 1  My answer using sliced insertion - Fastest   2 25   sec per loop  python3 -m timeit -s  quot a   list range 1000   quot   quot b   a     b 500 500     3  quot  100000 loops  best of 5  2 25   sec per loop   Rushy Panchal s answer with most votes using list insert     - Second  2 33   sec per loop  python3 -m timeit -s  quot a   list range 1000   quot   quot b   a     b insert 500  3  quot  100000 loops  best of 5  2 33   sec per loop   ATOzTOA s accepted answer based on merge of sliced lists - Third  5 01   sec per loop  python3 -m timeit -s  quot a   list range 1000   quot   quot b   a  500     3    a 500   quot  50000 loops  best of 5  5 01   sec per loop   My answer with List Comprehension and enumerate - Fourth  very slow with 135   sec per loop  python3 -m timeit -s  quot a   list range 1000   quot   quot  y for i  x in enumerate a  for y in   3  x  if i    500 else  x       quot  2000 loops  best of 5  135   sec per loop    Python 2 7 16  My answer using sliced insertion - Fastest  2 09   sec per loop  python -m timeit -s  quot a   list range 1000   quot   quot b   a     b 500 500     3  quot  100000 loops  best of 3  2 09   sec per loop   Rushy Panchal s answer with most votes using list insert     - Second  2 36   sec per loop  python -m timeit -s  quot a   list range 1000   quot   quot b   a     b insert 500  3  quot  100000 loops  best of 3  2 36   sec per loop   ATOzTOA s accepted answer based on merge of sliced lists - Third  4 44   sec per loop  python -m timeit -s  quot a   list range 1000   quot   quot b   a  500     3    a 500   quot  100000 loops  best of 3  4 44   sec per loop   My answer with List Comprehension and enumerate - Fourth  very slow with 103   sec per loop  python -m timeit -s  quot a   list range 1000   quot   quot  y for i  x in enumerate a  for y in   3  x  if i    500 else  x       quot  10000 loops  best of 3  103   sec per loop

[python] Insert an element at a specific index in a list and return the updated list

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