How can I remove every the first number, last number and middle number in a list?-CodePudding

So I'm trying to write a function chosen(lst:list[int]) -> list[int] that returns a list that contains all elements of lst but the front element, the middle element, and the last element. You may assume that the length of lst is greater than 3 and always odd.

For example, chosen([9,3,5,7,1]) returns [3,7] and chosen([0,2,7,0,0,5,0,0,0]) returns [2,7,0,5,0,0]. Here is my code so far....

def first(lst: list) -> list:
    if len(lst)%2 == 0:
        lst.remove(lst[0])
        lst.remove(lst[len(lst)//2])
        lst.remove(lst[-1])
    else:
        lst.remove(lst[0])
        lst.remove(lst[(len(lst) // 2)-1])
        lst.remove(lst[-1])
    return lst

What should I change to make sure this works??

CodePudding user response：

Remove from the last otherwise indices won't be correct

l = [9,3,5,7,1]
indices_to_be_poped = [0, len(l)//2, len(l)-1]

for i in indices_to_be_poped[::-1]:
    l.pop(i)
    
    
print(l) #[3, 7]

CodePudding user response：

One approach:

def chosen(lst):
    indices = [0, len(lst) // 2, len(lst) - 1]
    return [v for i, v in enumerate(lst) if i not in indices]


res = chosen([9,3,5,7,1])
print(res)

Output

[3, 7]

The idea is to first select the indices to remove and then simply filter out the elements at those indices.

Note that remove, actually removes the first item from the list whose value is equal to x. So your approach wont work with duplicated values.

CodePudding user response：

You can use tuple unpacking to remove the first and the last item. Then pop the middle element.

def chosen(lst):
    _, *out, _ = lst
    out.pop(len(out)//2)
    return out

chosen([9,3,5,7,1])
# [3, 7]

CodePudding user response：

You can use list.pop(index) function to remove element by providing index value.

NOte you need to provide index from the largest to lowest, as once you remove a element, list got reindexed, and element index changes

# your code goes here
def chosen(array: list):
    length = len(array)
    remove = [length-1, length//2, 0]
    
    for i in remove:
        array.pop(i)
    return array

assert chosen([0,2,7,0,0,5,0,0,0]) == [2,7,0,5,0,0]