Understanding output of slicing operation

Question

I have a list object which looks as below:

lst = [50,34,98,8,10]

The output for the statement print(lst[:5:-2]) is coming as blank []

My understanding is that print(lst[:5:-2]) translates to print(lst[-1:5:-2])

This means:

• start from index -1 # this corresponds to value 10
• stop at index 4 # this corresponds to value 10
• perform increment of -2

As start and stop are pointing to 10 so I am expecting the output to be 10 here.

Answer 1

An easy way to understand python slicing syntax is to think about it as such: slice[start:stop:step].

What this means is that your starting point is start, here that's index -1 since the starting position for a negative step is -1. It stops at stop, here that's the 4th element. When using a step of -2 the first iteration will be the 4th element. Thus nothing is returned since you've already arrived at your stop condition.

Answer 2

Your understanding is not wrong, the request print(lst[:5:-2]) is equal to print(lst[-1:5:-2]). The question here is, why the result is empty not [10].

Because python also can use negative indexing, and last item is indexed as -1... till the first item indexed as -len(list).

Your list is lst = [50,34,98,8,10] having length of 5 (5 items) so then the last item index is 4. That's why, when you use slice print(lst[-1:5:-2])

Start index: start at -1, the -1 is equal to 4. End index: is 5 Step: -2

because the step is -2 so the movement should be backward, but then the start index already <= end index(start <= stop), so no item is returned because it won't move any where.

For example to get [10] you should do it like print(lst[:3:-1])

you can read more documentation here: python 3 slice, start stop step from eyehunt toutorial, geeksforgeeks python slicing