Inserting elements into list (without using Python's list.insert(index, item) function)

Question

I'm writing a program to insert element into a list at a position before a particular index. Example,

mylist = [1,2,3,4,5]

enter the index = 3
enter the item = 7

Output:

[1,2,3,7,4,5]

Since the input index is 3, python will append the new item at position index-1 which is 2.

I have tried something like but the output seemed a little off:

mylist = [1,2,3,4,5]

indexInput = int(input("Enter the index: "))
itemInput = int(input("Enter the item: "))

for i in mylist:
    new_slot = indexInput -1
    new_length = len(mylist) + 1
    if i == new_slot:
         mylist[i] = item
         mylist.append(mylist[i])
print(mylist)

I know about the insert() function in python but I'm not allowed to use it so i had to code it the long way. Any help is much appreciated.

Djaouad · Accepted Answer · 2017-09-09 02:26:54Z

2

You could just use list slicing:

mylist = [1,2,3,4,5]

indexInput = int(input("Enter the index: "))
itemInput = int(input("Enter the item: "))

if indexInput<0:
  indexInput = len(mylist)+indexInput+1

mylist = mylist[:indexInput] + [itemInput] + mylist[indexInput:]
print(mylist)
# for index 3 and item 7 => [1, 2, 3, 7, 4, 5]
# for index -2 and item 6 => [1, 2, 3, 4, 6, 5]

Explanation:

list[start:end:step] => list items from index [start] to index [end-1] with a step of 1
if start is not given, 0 is assumed, if end is not given, it goes all the way to the end of the list, if step is not given, it assumes a step of 1
In my code I wrote:
mylist = mylist[:indexInput] + [itemInput] + mylist[indexInput:]
for indexInput=3 and itemInput=7
mylist = mylist[:3] + [7] + mylist[3:]
mylist[:3] => mylist from 0 (start not given), to 3-1, step of 1 (step not given)
list1 + list2 => a list that contains elements from list1 and list2
mylist[:3] + [7] => [...elements of mylist[:3]..., 7]
mylist[3:] => mylist from 3, to len(mylist)-1 (end not given), step of 1 (step not given)

If you want to do it the long way, here's a solution:

mylist = [1,2,3,4,5]

indexInput = int(input("Enter the index: "))
itemInput = int(input("Enter the item: "))
if indexInput<0:
      indexInput = len(mylist)+indexInput+1
leftSide = []
rightSide = []
for i in range(len(mylist)):
  if i<indexInput:
    leftSide.append(mylist[i])
  elif i>indexInput:
    rightSide.append(mylist[i])
  else:
    leftSide.append(itemInput)
    leftSide.append(mylist[i])

mylist = leftSide + rightSide
print(mylist)
# for index 3 and item 7 => [1, 2, 3, 7, 4, 5]
# for index -2 and item 6 => [1, 2, 3, 4, 6, 5]

edited Sep 9, 2017 at 2:26

answered Sep 9, 2017 at 1:20

Djaouad

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6 Comments

Maxxx Over a year ago

thanks a bunch for the explanation. But if my index were to be negative index, the code won't work.

Djaouad Over a year ago

@Maxxx use the abs() function to get the absolute value of the number in this case, check my updated my answer.

Maxxx Over a year ago

sorry if i wasn't clear, but if negative index were to be used, it means the program will start at the end of the list just like list[-2] = item

Djaouad Over a year ago

If you use abs() it won't start at the end, if the user enters -2, abs(-2) = 2. or do you want it to start from the end?

Djaouad Over a year ago

@Maxxx I updated my answer to support negative indexing.

|

zhongjiajie · Accepted Answer · 2017-09-09 01:35:20Z

0

Here is my code

mylist = [1, 2, 3, 4, 5]

indexInput = int(input("Enter the index: "))
itemInput = int(input("Enter the item: "))

for idx, val in enumerate(mylist):
    if idx == indexInput:
        mylist = mylist[:idx] + [itemInput] + mylist[idx:]

print(mylist)

Hope it can help you, just use enumerate function to get mylist index

answered Sep 9, 2017 at 1:35

zhongjiajie

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1 Comment

zhongjiajie Over a year ago

@Mr Geek 's answer is better than mine

Rodrigo Estrella · Accepted Answer · 2017-09-09 01:55:28Z

0

list = [1,2,3,4,5]
inputIndex = 3
inputItem = 7
temp = 0

for i in range(len(list)):#continuously swap your elements until you reach the end
    if(i == len(list) - 1): # if you're at the end, swap and add the temp to the end
        temp = list[i]
        list[i] = inputItem
        list.append(temp)
    elif(i >= inputIndex):
        temp = list[i]
        list[i] = inputItem
        inputItem = temp

print(list)

This is probably what you're looking for, if this is for an assignment where you're not allowed to use insert then I am assuming you probably aren't allowed to use slicing

answered Sep 9, 2017 at 1:55

Rodrigo Estrella

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