How to swap even lesser elements with odd greater elements in a python list?

Question

I have a list of elements from 1 to n, (range(1, n + 1))

How should I swap odd greater elements with the even elements?

for example, if I have a list with elements [1,2,3] the desired output will be [1,3,2] because odd 3 is greater than even 2 .

example 2:

if list = [1,2,3,4,5]

the desired output will be

[1,3,2,5,4]

Here 2 will be swapped with 3 and 4 will be swapped with 5 but not with 3 because 3 is smaller than 4.

what is the desired in case that the closest greater odd element is ident for two elements like this case? `[2,4,5]` ? — Yossi Levi, Oct 04 '20 at 09:36
This case is not possible because we will have a array always in a sequence of 1....n — Akash nitter, Oct 04 '20 at 09:39
So are you actually asking how to create such array for a given `n`? — Tomerikoo, Oct 04 '20 at 09:41
Also please decide whether you're using Python 2 or 3. If your question is not version specific, remove both — Tomerikoo, Oct 04 '20 at 09:46

Wups · Answer 1 · 2020-10-04T10:00:22.137

4

A simple for loop, to modify the list in place:

l = list(range(1,10))

for i, n in enumerate(l):
    if i % 2 != 0 and i < len(l) - 1:
        l[i] = l[i+1]
        l[i+1] = n

At every odd index, the element swaps places with its successor.

edited Oct 04 '20 at 10:00

answered Oct 04 '20 at 09:51

Wups

2,489
1
6
17

You can convert the `i < len(l) - 1` condition to be inside `enumerate(l[:-1])` – Tomerikoo Oct 04 '20 at 10:14

Patrick Artner · Answer 2 · 2020-10-04T10:00:55.990

3

You can use list slicing of even/odd numnbers, zip them and create a solution list from that:

def interleave(n):
    k = list(range(1, n))

    k[:] = k[:1]+[y for x in zip(k[2::2], k[1::2]) 
            for y in x] + (k[-1:] if len(k)%2 == 0 else [])

    return k

print(interleave(20))
print(interleave(21))

Output:

[1, 3, 2, 5, 4, 7, 6, 9, 8, 11, 10, 13, 12, 15, 14, 17, 16, 19, 18]
[1, 3, 2, 5, 4, 7, 6, 9, 8, 11, 10, 13, 12, 15, 14, 17, 16, 19, 18, 20]

Explanation:

you keep the 1
you get all odd ones from position 2 to end, stepsize 2
you get all even ones from position 1 to end, stepsize 2
you interleave them using zip
you recombine them using a nested list comprehension of [x for y in sequence for y in x] where y are the tuples resulting by zipping two slices
you compensate for even/odd sized input lists

How to swap even lesser elements with odd greater elements in a python list?

2 Answers2