Python - Swap consecutive elements of a list?

Question

I want to swap the consecutive elements of a list such that the first element will go to the last. eg [4, 7, 3, 4, 3] should print as [7,3,4,3,4] This is the code I've written but it doesn't work right. How can I modify it to get it working?

ls = [4, 7, 3, 4, 3]
i=0
b=1
while i+1<len(ls):
    ls[i]=a
    ls[i]=ls[i+1]
    ls[i+1]=a
    i+=1
print ls

I do not want to just switch the first element with the last one. I want to modify this code further to create a bubble sort algorithm, I am just confused at how to get done what I just explained.

Updated : Thanks for the answer that I should change "ls[i]=a" with "a=ls[i]", but can someone explain to me how this differs in logic?

Can you clarify "swap consecutive elements of a list"? How is this swapping consecutive elements? — mgilson, Oct 18 '16 at 18:45
swapping in pairs. eg the first element will swap with the second then the second with the third and so on — Hammad Manzoor, Oct 18 '16 at 18:46

score 4 · Answer 1 · answered Oct 18 '16 at 18:46

4

You don't need a loop to move the first element to the last. Just pop the first element off the list, then append it.

ls.append(ls.pop(0))

answered Oct 18 '16 at 18:46

kindall

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I know I can do that but i want to swap the elements one by one in pairs from left to right – Hammad Manzoor Oct 18 '16 at 18:47
unless you pass an index, then it removes the element at that index. – Tore Eschliman Oct 18 '16 at 18:48
1

@PatrickHaugh " list.pop([i]) Remove the item at the given position in the list, and return it. If no index is specified, a.pop() removes and returns the last item in the list. " – kindall Oct 18 '16 at 18:48
@HogRider123 why? – Patrick Haugh Oct 18 '16 at 18:48
1

"i want to swap the elements one by one in pairs from left to right" ... er, no you don't. :-) – kindall Oct 18 '16 at 18:50
@PatrickHaugh because I have to work on this code further and modify it to create a bubble sort algorithm – Hammad Manzoor Oct 18 '16 at 18:50

Guillaume Jacquenot · Answer 2 · 2016-10-18T20:10:59.757

2

Python allows to work with lists by selecting one or several elements at the time and to concatenate the content of the list

ls = [4, 7, 3, 4, 3]
new_list_with_first_element_at_the_end = ls[1:] + [ls[0]]

edited Oct 18 '16 at 20:10

answered Oct 18 '16 at 18:45

Guillaume Jacquenot

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In principle yes, but... did you try this? – Antti Haapala -- Слава Україні Oct 18 '16 at 19:14
Yeah, you can only add a sequence and `ls[0]` isn't a list. Should be `... + [ls[0]]` – kindall Oct 18 '16 at 19:49

score 1 · Answer 3 · answered Oct 18 '16 at 18:54

1

If you really feel you must, make a swap function:

def swap(l, i, j):
    tmp = l[j]
    l[j] = i
    l[i] = tmp

Then you can:

for i in range(len(l)-1):
     swap(l, i, i+1)

answered Oct 18 '16 at 18:54

Patrick Haugh

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`tmp` is not needed, you can just do `l[i], l[j] = l[j], l[i]` – kindall Aug 25 '20 at 22:21

Python - Swap consecutive elements of a list?

3 Answers3