CODEWITHSUNDEEP
pythonlist
Burcu İçen
Burcu İçen

Reputation: 155

How can I pop elements in a list from a given array of indexes?

I have a list

list=[[5,7,8],[2,8,4],[1,72,7],[8,79,80],[4,39,84],[6,78,89],[80,78,81],[12,39,50]]

and I have a list for indexes:

indexes=[1,3,7]

In the end, I want to remove 1st, 3rd and 7th element from the list

and the output should be like:

[[5,7,8],[1,72,7],[4,39,84],[6,78,89],[80,78,81]]

I used this code:

for i in indexes:
    list.pop(i)

but it does not work, how can I fix that?

Upvotes: 1

Views: 1595

Answers (5)

Ranjith Reddy Karre
Ranjith Reddy Karre

Reputation: 16

list=[[5,7,8],[2,8,4],[1,72,7],[8,79,80],[4,39,84],[6,78,89],[80,78,81],[12,39,50]]
indexes=[1,3,7]
for i in indexes[::-1]:
    list.pop(i)

print(list)
  1. When you run this code you will get an error Showing IndexError: pop index out of range.
  2. This error occurs because, in the for loop for the first iteration the 2nd element i.e list[1] is poped and length of the list i.e len(list) becomes to 6, in the same way in second iteration after poping the list. The length of the list becomes 5. But in the third iteration from indexes you are passing 7 as index to the pop function but the length of the list after 2nd iteration is 5, so your passing the index which is greater then the length of the list this leads to IndexError: pop index out of range
  3. To over come this error you have to iterate the list indexes from right to left i.e in reverse order.

Upvotes: 0

Pedro Dias
Pedro Dias

Reputation: 1

the code that you're looking for is this

    my_list=[[5,7,8],[2,8,4],[1,72,7],[8,79,80],[4,39,84],[6,78,89],[80,78,81],[12,39,50]]
indexes=[1,3,7]
for i in indexes[::-1]:
    my_list.pop(i)

print(my_list)

  1. Rename the variable from "list" to something else since list is a reserved word. you'll also have to do this in the for loop.

  2. because you want to pop items out of a list it's important you think about the order you do it in. So if you pop from position 1 then that means position 2 will now become position 1. this probably isn't what you want.

so it's important to pop from the end of the list first and work your way to the front if possible which is why I used the

indexes[::-1]

this is called list slicing you can learn more about it here. https://www.youtube.com/watch?v=ajrtAuDg3yw

Upvotes: 0

SCL
SCL

Reputation: 86

Your way of doing this won't work since you will mess up the original list indexes by working with a non-reversed list for the loop.

Given your values, you should sort the indexes list and reverse the order to not mess with the list index, I'm also using del to avoid useless returns from the pop method.

list=[ [5,7,8],[2,8,4],[1,72,7],[8,79,80],[4,39,84],[6,78,89],[80,78,81],[12,39,50] ]
indexes=[1,3,7]

for index in sorted(indexes, reverse=True):
    del list[index]

Upvotes: 1

Elisha
Elisha

Reputation: 23770

Assuming the problem is caused by the mutation of the list during items popping:

for i in sorted(indexes, reverse=True):
    list.pop(i)

The change in the code pops the items from the end first, so the indices of the other items is not affected.

Upvotes: 2

Mureinik
Mureinik

Reputation: 311393

I'd create a new list that doesn't include these indexes. Using a list comprehension over the enumerate of the original list (so you can get the list and the index easily) can even make this a one liner:

result = [e[1] for e in enumerate(lst) if e[0] not in indexes]

Upvotes: 3

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