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.

CodePudding user response：

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.

CodePudding user response：

So the documentation is here, and additional explanation here. And another additional explanation you can read here. On your case that syntax is same as

print(lst[-1:5:-2])

OR

print(lst[-1:-1:-2])

• Note this is because python also can use negative indexing, and last item is indexed as -1... till the first item indexed as -len(list). So index 5 is same as -1
• Also the negative step indicates that the starting point is the last item of list or index -1

So what it's done:

• Start from index -1 # this corresponds to value 10
• Stop at index -1 # this correspond to value 10
• Using -2 as step

because the step is -2 and the start is already smaller than or same with stop (start <= stop) the slice result to empty because nothing taken from the list.

for example how to use negative step, you can run

print(lst[5:0:-2])

same as

print(lst[-1:-5:-2])

that's will return [10, 98]

print(lst[5::-2])

same as

print(lst[-1:-6:-2])

that's will return [10, 98, 50]

You can still use -len(list)-1 for indexing to inclusive the first item that contains in index -len(list).