用函数[python 3]替换列表中的两个元素

Question

I want to create a function such that when I enter a list and parameters, it will replace two items of the list with each other. This code needs to be in place.

The code needs to start like so:

def reverse_sublist(lst,start,end): 
...

An example of what it's supposed to do:

>>> lst = [1, 2, 3, 4, 5]
>>> reverse_sublist (lst,0,4)
>>> lst
[4, 3, 2, 1, 5]

I tried to write some pseudo code of how to do it:

start=i, end=n #1#
make a copy of both lst[i], lst[n] inside list, right next to the original #2#
do lst[i]=lst[n+1] and lst[n]=lst[i+1] 
delete lst[i+1], lst[n+1]

I just began learning python so I have several questions:

How can I define the parameters of the function (#1#) ?

How can I do these operations (#2#) on the list with the function ?

Thanks in advance.

Answer 1

I think what you want is:

def reverse_sublist(lst, start, end):
    lst[start:end] = reversed(lst[start:end])

Note the definition of the three arguments to the function on the first line, in parentheses after the name of the function.

Your current pseudocode appears to be for swapping the items at start and end , which is not what your example shows. If you did want to do this, you could do:

def swap_items(lst, index1, index2):
    lst[index1], lst[index2] = lst[index2], lst[index1]

---

An additional note on your first approach: if you add extra items into the list you will throw off the indexing for the rest of the list. For example, try this test implementation of your pseudocode:

def test(lst, start, end):
    lst.insert(start+1, lst[start])
    lst.insert(end+1, lst[end])
    print(lst)
    lst[start] = lst[end+1]
    lst[end] = lst[start+1]
    print(lst)
    del lst[start+1]
    del lst[end+1]
    print(lst)

The result this gives is not the [5, 2, 3, 4, 1] you are expecting:

>>> test([1, 2, 3, 4, 5], 0, 4)
[1, 1, 2, 3, 4, 4, 5] # after insert
[4, 1, 2, 3, 1, 4, 5] # after swap
[4, 2, 3, 1, 4] # after del

Instead, you would have to do the insert and swap on end+2 and end+1 to account for the extra item from the insert at start+1 :

def test(lst, start, end):
    lst.insert(start+1, lst[start])
    lst.insert(end+2, lst[end+1])
    lst[start], lst[end+1] = lst[end+2], lst[start+1]
    del lst[start+1]
    del lst[end+1]

Answer 2

I wanted to answer this with slices, but there's an edgecase that makes slices less elegant than @jonsharpe's answer, since when reverse slicing, you can't slice up to less than zero while reverse stepping, and the only way to accomplish it is to leave out the terminal element (please correct me if there's a more unified way of doing this):

def reverse_sublist(lst, i, j):
    if i == 0: 
        lst[i:j] = lst[j-1::-1]
    else:
        lst[i:j] = lst[j-1:i-1:-1]

lst = [1, 2, 3, 4, 5]
reverse_sublist(lst,0,4)

and lst now returns:

[4, 3, 2, 1, 5]

---

As Jon suggested, this can be further simplified, and I'll break the slice notation out for ease of understanding:

def reverse_sublist(lst, i, j):
    lst[i:j] = lst[j-1 :                # inclusive slice starting point
                   i-1 if i else None : # exclusive slice end point
                   -1]                  # step by negative one, i.e. reverse 

And then:

>>> lst = [1, 2, 3, 4, 5]
reverse_sublist(lst,0,4)
>>> 
>>> lst
[4, 3, 2, 1, 5]

I would still consider this less elegant than Jon's, because it's harder to read and more complex. Also, Jon's will work for negative indexes, this has problems with that.

Answer 3

I think this accomplished what you want, your startlist (slist, the indices of where you start and finish the reversing are the parameters

def reverse(slist,start,stop):
     temp = slist[start:stop]

temp is a sublist of the original list, reverse it in place

     temp.reverse()

you need to get the portion to the left (index < start) to have that available to prepend to your new list

     originalLeft = slist[0:start]
     originalLeft.extend(temp)

You also need the segment from stop to the end of your original sublist

     originalRight = slist[stop:]
     originalLeft.extend(originalRight)
     return originalLeft

>>> myList = [1,2,3,4,5,6,7,8,9,10,11,12,13]
>>> reverse(myList,4,9)
[1, 2, 3, 4, 9, 8, 7, 6, 5, 10, 11, 12, 13]
>>>