Python 理解切片符号题

Question

I know that there are a lot of slice notation questions but I think this should work but it doesn't. Say we have:

a = [1, 2, 3, 4]

Let's say we want to reverse subarray from index 0 to 1:

a = a[1:-1:-1] + a[2:] 

I would expect to get a=[2, 1, 3, 4] but what I get is a = [3, 4]

Why is that? Why doesn't a[1:-1:-1] return [2, 1] ? Am I missing something?

Answer 1

When slicing with a negative step (Eg. list_[a, b, -n] ), a will be the inclusive index to start from, b will be the index to go up to without including it (hence exclusive) while taking steps of size n towards the left. That's how slicing works with negative steps.

Try this:

a = [1, 2, 3, 4]
a = a[1::-1] + a[2:] 
print(a)
# Out: [2, 1, 3, 4]

Note: if you slice as a[1:0:-1] , then the element at index zero is not included due to slicing rules. Hence you have to omit that index to tell python to keep including elements till you run out of elements.

If this sort of slicing confuses you, the simpler way would be to do:

a = a[:2][::-1] + a[2:]  # take normal slice first, then reverse it

Edit: Your a[1:-1:-1] logic is flawed because -1 is the index of the last element of the list (negative indexing exists,). so you can't use math and argue that -1 is one less than 0 so it should work.

Answer 2

Here is a what your code is doing:

You have a list a=[1,2,3,4]

Now when you do a[1:-1:-1] , it will return a [] . That is because list slicing work: [start index: end index: steps] . Any -1,-2 negative operator means go from the back. When you do [1:-1:-1] , you are telling it to go from lower position and higher position backwards. That is why it will give you an [] empty list.

What you really want is a[1::-1] telling go from index 1 to end of list backwards - ie 1 then 0

And a[2:] means go from index position 2 to end.

There fore a[1::-1]+a[2:] will give you [2,1,3,4]