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Accessing elements in a list has many types and variations. These are an essential part of Python programming and one must have the knowledge to
perform the same. This article discusses ways to fetch the last N elements of list. Let’s discuss certain solution to perform this task.
Method #1 : Using list slicing
This problem can be performed in 1 line rather than using a loop using the list slicing functionality provided by Python. Minus operator specifies slicing to be done from rear end.
Python3
test_list =
[4, 5, 2, 6, 7, 8, 10]
print["The original list : " + str[test_list]]
N = 5
res = test_list[-N:]
print["The last N elements of list are : " + str[res]]
Output :
The original list : [4, 5, 2, 6, 7, 8, 10] The last N elements of list are : [2, 6, 7, 8, 10]
Method #2 : Using islice[] + reversed[]
The
inbuilt functions can also be used to perform this particular task. The islice function can be used to get the sliced list and reversed function is used to get the elements from rear end.
Python3
from itertools import islice
test_list = [4, 5, 2, 6, 7, 8, 10]
print["The original list : " + str[test_list]]
N =
5
res = list[islice[reversed[test_list], 0, N]]
res.reverse[]
print["The last N elements of list are : " + str[res]]
Output :
The original list : [4, 5, 2, 6, 7, 8, 10] The last N elements of list are : [2, 6, 7, 8, 10]
Slicing
Python slicing is an incredibly fast operation, and it's a handy way to quickly access parts of your data.
Slice notation to get the last nine elements from a list [or any other sequence that supports it, like a string] would look like this:
num_list[-9:]
When I see this, I read the part in the brackets as "9th from the end, to the end." [Actually, I abbreviate it mentally as "-9, on"]
Explanation:
The full notation is
sequence[start:stop:step]
But the colon is what tells Python you're giving it a slice and not a regular index. That's why the idiomatic way of copying lists in Python 2 is
list_copy = sequence[:]
And clearing them is with:
del my_list[:]
[Lists get list.copy and list.clear in Python 3.]
Give your slices a descriptive name!
You may find it useful to separate forming the slice from passing it to the list.__getitem__ method
[that's what the square brackets do]. Even if you're not new to it, it keeps your code more readable so that others that may have to read your code can more readily understand what you're doing.
However, you can't just assign some integers separated by colons to a variable. You need to use the slice object:
last_nine_slice = slice[-9, None]
The second argument, None, is required, so that the
first argument is interpreted as the start argument otherwise it would be the stop argument.
You can then pass the slice object to your sequence:
>>> list[range[100]][last_nine_slice]
[91, 92, 93, 94, 95, 96, 97, 98, 99]
islice
islice from the itertools module is another possibly performant way to get this. islice doesn't take negative arguments, so ideally your iterable has a __reversed__ special method - which list does
have - so you must first pass your list [or iterable with __reversed__] to reversed.
>>> from itertools import islice
>>> islice[reversed[range[100]], 0, 9]
islice allows for lazy evaluation of the data pipeline, so to materialize the data, pass it to a constructor [like list]:
>>> list[islice[reversed[range[100]], 0, 9]]
[99, 98, 97, 96, 95, 94, 93, 92, 91]