CODEWITHSUNDEEP
pythonlistindexing
Sayuj
Sayuj

Reputation: 7622

How to obtain the last index of a list?

Suppose I've the following list:

list1 = [1, 2, 33, 51]
                    ^
                    |
indices  0  1   2   3

How do I obtain the last index, which in this case would be 3, of that list?

Upvotes: 52

Views: 238490

Answers (10)

Artem Artem
Artem Artem

Reputation: 1

This might be more pythonic way:

list1.index(list1[-1])

Upvotes: -1

strawbot
strawbot

Reputation: 391

Thanks for the discussion. This made me look deeper at a few assumptions and a couple of asymmetries are worth noting:

>>> l = [1,2,3,4]
>>> l
[1, 2, 3, 4]
>>> l[0:-1]
[1, 2, 3]
>>> l[0:3]
[1, 2, 3]
>>> l[0:4]
[1, 2, 3, 4]
>>> l[0:1000]
[1, 2, 3, 4]
>>> l[-1:]
[4]
  1. In indexing a list, the first index returns the first element, it also can be used to include it in a slice. Whereas the last index cannot.
  2. The last index for slicing can be any number larger than the index of the last element but the first index can only be 0.

I was debugging a function that scanned a list of data points looking for a grouping and rejecting occasional glitches. The function used a numpy windowed median filter to find the 2/3 and 1/3 transition points as indices and then it returned them. Depending on how the indices are used after the function is called could make a slight difference of 1. If the returned indeces are used to subset the data, then the last one should be:

  len(data)
instead of:
  len(data) - 1
 or:
  -1

In my gut, returning the subset list would be better because there would be no wiggle room to miss the last element. Originally, I was fixing a condition where the data ended without dropping below the 1/3 level and the first index was used as the last index. When I looked closer at the solution, I realized that the last index was always missing the last element because it returned the index to it instead of 1 past it.

Upvotes: 0

Ali Akhtari
Ali Akhtari

Reputation: 1297

The best and fast way to obtain the content of the last index of a list is using -1 for number of index , for example:

my_list = [0, 1, 'test', 2, 'hi']
print(my_list[-1])

Output is: 'hi'.

Index -1 shows you the last index or first index of the end.

But if you want to get only the last index, you can obtain it with this function:

def last_index(input_list:list) -> int:
    return len(input_list) - 1

In this case, the input is the list, and the output will be an integer which is the last index number.

Upvotes: 43

Osi
Osi

Reputation: 448

list1[-1]

will return the last index of your list.

If you use minus before the array index it will start counting downwards from the end. list1[-2] would return the second to last index etc.

Important to mention that -0 just returns the "first" (0th) index of the list because -0 and 0 are the same number,

Upvotes: -4

Mark Anthony Libres
Mark Anthony Libres

Reputation: 1054

all above answers is correct but however

a = [];
len(list1) - 1 # where 0 - 1 = -1

to be more precisely

a = [];
index = len(a) - 1 if a else None;

if index == None : raise Exception("Empty Array")

since arrays is starting with 0

Upvotes: 2

UncleZeiv
UncleZeiv

Reputation: 18488

I guess you want

last_index = len(list1) - 1

which would store 3 in last_index.

Upvotes: 13

Diego Navarro
Diego Navarro

Reputation: 9704

You can use the list length. The last index will be the length of the list minus one.

len(list1)-1 == 3

Upvotes: 1

Gandi
Gandi

Reputation: 3652

Did you mean len(list1)-1?

If you're searching for other method, you can try list1.index(list1[-1]), but I don't recommend this one. You will have to be sure, that the list contains NO duplicates.

Upvotes: 16

Haresh Shyara
Haresh Shyara

Reputation: 1886

a = ['1', '2', '3', '4']
print len(a) - 1
3

Upvotes: 0

Austin Marshall
Austin Marshall

Reputation: 3107

len(list1)-1 is definitely the way to go, but if you absolutely need a list that has a function that returns the last index, you could create a class that inherits from list.

class MyList(list):
    def last_index(self):
        return len(self)-1


>>> l=MyList([1, 2, 33, 51])
>>> l.last_index()
3

Upvotes: 51

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