5
\$\begingroup\$

I am a newbie trying to insert elements after the n:th element in the list. Any shorter/smarter ways?

def InsertElements(listName, var1, var2, n):

    listName.insert(n, var1)
    listName.insert(n+1, var2)
    return listName

myList = ["banana", "orange", "apple", "cherry", "grape"]

result = InsertElements(myList, "mango", "papaya", 3)
\$\endgroup\$

migrated from stackoverflow.com Mar 5 '15 at 21:18

This question came from our site for professional and enthusiast programmers.

  • \$\begingroup\$ doesn't have .add(element) method ? \$\endgroup\$ – Erfan Mowlaei Feb 25 '15 at 20:31
  • \$\begingroup\$ If you have a list of things to insert, you could do newlist = list[:n] + added_elements + list[n:]. \$\endgroup\$ – Tom Hunt Feb 25 '15 at 20:32
  • \$\begingroup\$ For some reason there is a "one line frenzy" going on in SO. I highly doubt that short code means good code. Anyway.. your code seems fine, excluding the return part which is somewhat redundant. insert() modifies your list so you can simply call your list after running your function without the need of a return inside it. \$\endgroup\$ – Fermi paradox Feb 25 '15 at 20:56
10
\$\begingroup\$

Many languages (e.g. JavaScript, Perl) have a splice function of some sort to modify list contents. They allow you to insert list items or change existing ones to new sublists.

Python uses a "slice" approach, allowing you to specify many list edits without a specific splice function.

>>> myList = ["banana", "orange", "apple", "cherry", "grape"]
>>> myList[3:3] = ["mango", "papaya"]
>>> myList
['banana', 'orange', 'apple', 'mango', 'papaya', 'cherry', 'grape']

By the way, if this is hard to understand, this cheat sheet may help:

 ["banana", "orange", "apple", "cherry", "grape"]

  0_______  1_______  2______  3_______  4______ 5    # item index
  0:1_____  1:2_____  2:3____  3:4_____  4:5____      # equiv slice notation

  ^         ^         ^        ^         ^       ^    # slice notation for
  0:0       1:1       2:2      3:3       4:4     5:5  # 0_length positions
                                                      # between items

  0:2________________ 2:5_______________________      # misc longer slice 
            1:4_________________________              # examples
            1:5_________________________________
            1:__________________________________
  0:____________________________________________
  :_____________________________________________
  :3__________________________
  :1______

There are also negative indices (counting from the end of the list). Let's not get into those right now!

\$\endgroup\$
  • \$\begingroup\$ One thing I wish I'd explained a little better: When you ask a list for a single value (e.g. mylist[3]) you get back a single item. If you use the slice notation, you get back a list. Slices are also indexing, but they do not reduce the dimensionality of the result like integer indices do. \$\endgroup\$ – Jonathan Eunice Nov 30 '17 at 15:45
3
\$\begingroup\$

Note that .insert will modify the list in-place, so after the call result and myList will refer to the same list.

Your code is OK. If you wanted a bit of a shorter option, you could use slicing:

listName[n:n] = [var1, var2]

This inserts both var1 and var2 after the nth item in the list.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.