Swap indexes using slices?

Question

I know that you can swap 2 single indexes in Python

r = ['1', '2', '3', '4', '5', '6', '7', '8'] 
r[2], r[4] = r[4], r[2]

output:

['1', '2', '5', '4', '3', '6', '7', '8']

But why can't you swap 2 slices of indexes in python?

r = ['1', '2', '3', '4', '5', '6', '7', '8'] 

I want to swap the numbers 3 + 4 with 5 + 6 + 7 in r:

r[2:4], r[4:7] = r[4:7], r[2:4]

output:

['1', '2', '5', '6', '3', '4', '7', '8']

expected output:

['1', '2', '5', '6', '7', '3', '4', '8']

What did I wrong? output:

Answer 1

The slicing is working as it should. You are replacing slices of different lengths. r[2:4] is two items, and r[4:7] is three items.

>>> r = ['1', '2', '3', '4', '5', '6', '7', '8']
>>> r[2:4]
['3', '4']
>>> r[4:7]
['5', '6', '7']

So when ['3', '4'] is replaced, it can only fit ['5', '6'], and when ['5', '6', '7'] is replaced, it only gets ['3', '4']. So you have ['1', '2',, then the next two elements are the first two elements from ['5', '6', '7'] which is just ['5', '6', then the two elements from ['3', '4' go next, then the remaining '7', '8'].

If you want to replace the slices, you have to start slices at the right places and allocate an appropriate size in the array for each slice:

>>> r = ['1', '2', '3', '4', '5', '6', '7', '8']
>>> r[2:5], r[5:7] = r[4:7], r[2:4]
>>> r
['1', '2', '5', '6', '7', '3', '4', '8']
 old index: 4    5    6    2    3
 new index: 2    3    4    5    6

Answer 2

Think of this:

r[2:4], r[4:7] = r[4:7], r[2:4]

as similar to this:

original_r = list(r)
r[2:4] = original_r[4:7]
r[4:7] = original_r[2:4]

So, by the time it gets to the third line of that, the 4th element isn't what you think it is anymore... You replaced '3', '4' with '5', '6', '7', and now the [4:7] slice starts with that '7'.

Answer 3

>>> r = ['1', '2', '3', '4', '5', '6', '7', '8']
>>> r[2:5], r[5:7] = r[4:7], r[2:4]
>>> r
['1', '2', '5', '6', '7', '3', '4', '8']

In your code:

>>> r[2:4], r[4:7] = r[4:7], r[2:4]

You are assigning r[4:7] which have 3 elements to r[2:4] which have only 2.

In the code I posted:

>>> >>> r[2:5], r[5:7] = r[4:7], r[2:4]

r[4:7] which is ['5', '6', '7'], replaces r[2:5] which is ['3', '4', '5']

r resulting in ['1', '2', '5', '6', '7', '6', '7', '8']

and then:

r[2:4] which was ['3', '4'], replaces r[5:7] which is ['6', '7']

So final result being:

['1', '2', '5', '6', '7', '3', '4', '8']