CODEWITHSUNDEEP
python
Jordan Williams
Jordan Williams

Reputation: 191

How Can I Slice the Three Middle Indexes of Any List?

I am trying to print the three middle indexes of any list in Python, but am having trouble figuring it out. I know how to find the three middle indexes on their own, but I can't seem to print all three in a list. Here's what I have so far. 

print("Three items from the middle of the list are:")
middle = (int(len(numbers))/2)
middle_two = (int(len(numbers))/2) - 1
middle_three = (int(len(numbers))/2) + 1
print(list(middle + middle_two + middle_three))

Whenever I try to put list()around any of the integers or concatenate them, I get an error: "'float' object is not interable'. I know what that means in practice, but I am stuck on how to turn all three middle indexes into a list.

Upvotes: 1

Views: 805

Answers (2)

David Culbreth
David Culbreth

Reputation: 2776

There is a much simpler method than calculating these individually. You can simply find the middle of the list with len(numbers)//2. We do the //2 to force integer division, which will return an int. This is critical because the float that comes out of 3/2, which is 1.5, is not a valid list index. This makes sense, as there wouldn't be a 'one-and-a-half-th' item in a list.

On the other hand, 3//2 will return 1, which is a valid index (remember, integer division)

Putting all these together in a function, we get this:

def middle_three(items:list):
  assert len(items) >= 3, "middle_three needs at least 3 items in the input!"
  middle = len(items)//2 # find the index of the middle of the list, rounded down for even lists
  return items[middle-1:middle+2] # +2 because we want +1 from the middle, +1 more because of indexing syntax

We can try this out on some sample lists:

for i in range(3, 20):
  print(i, ':', middle_three([*range(i)]))

3 : [0, 1, 2]
4 : [1, 2, 3]
5 : [1, 2, 3]
6 : [2, 3, 4]
7 : [2, 3, 4]
8 : [3, 4, 5]
9 : [3, 4, 5]
10 : [4, 5, 6]
11 : [4, 5, 6]
12 : [5, 6, 7]
13 : [5, 6, 7]
14 : [6, 7, 8]
15 : [6, 7, 8]
16 : [7, 8, 9]
17 : [7, 8, 9]
18 : [8, 9, 10]
19 : [8, 9, 10]

Upvotes: 1

tenhjo
tenhjo

Reputation: 4537

In you last line you are adding your indices a+b+c and get a number, than you want to convert this number to a list, which results in an error:

list(9.0) # TypeError: 'float' object is not iterable
list(9)   # TypeError: 'int' object is not iterable

You need to combine the indices to a list: with [a, b, c]:

import numpy as np

numbers = np.arange(10, 20)

print("Three items from the middle of the list are:")
middle = len(numbers) // 2  # ensure that your indices are integers
middle_two = middle - 1
middle_three = middle + 1
middle_list = [middle, middle_two, middle_three]
#             ^      ^           ^             ^
print(middle_list)           # [5, 4, 6]
print(numbers[middle_list])  # [15 14 16]

Upvotes: 0

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