-1 数组中的索引？

Question

I am trying to use:

num_new = num[i,-1]

to get the last segment of the array. However, it returns [] when i = 0 , and num = [3]

Do I have to use like num[i, len(num)] ? Is there any other attention need to pay when using -1 to retrieve array elements?

Answer 1

I think this is not a matter of -1, but python list slicing.

some_list[-n] means n th element of list from end of list, so you will get 5 as a result in following example:

some_list = [1, 3, 5]
last_elem = some_list[-1]  # 5

And this is not a core issue of your question.

List slicing in python works with this args:
some_list[_start_:_end_:_step_]
And end th element is exclusive .
So if you are trying to [3][0:-1], this excludes last element and returns empty list.

If you want to get last segment of list, you should slice like this:

some_list = [1, 2, 3, 4, 5]
sliced_list = some_list[3:]   # [4, 5]
neg_1_list = some_list[3:-1]  # [4]

This will help you.