Shift particular range of elements in list

Question

I want to do something like this:

myList[n]. Shift myList[k:l] to the left by s.

Example: Let n be 5, s = 1 and k and l be the 3rd and 4th element in the list.

Before:

[a,a,b,b,a]

After:

[a,b,b,a,a]

To be clear: I don't want to loose 'b' items in my list, 'a' can be overwritten.

What would be the most time and space efficient way to achieve this in Python?

Edit: NO ROTATION!

Your example is not very clear. You could just as well have used "before: `[a,a,a,a,a]`, after: `[a,a,a,a,a]`"... — tobias_k, Mar 24 '14 at 13:02
The problem with your example is that it is not clear which a/b in 'before' maps to which a/b in 'after'. The same 'after' result could have been achieved with a shift, a rotation, or even by mirroring the list. My example was not meant to be a better example, but for exemplifying the problem with your example. — tobias_k, Mar 24 '14 at 13:22

falsetru · Accepted Answer · 2014-03-24T14:05:24.587

2

Using list slice:

>>> lst = [1, 2, 3, 4, 5]
>>> s, k, l = 1, 2, 4 # 4 = 3 + 1
>>> x = lst[k:l]
>>> del lst[k:l]
>>> lst[k-s:k-s] = x
>>> lst
[1, 3, 4, 2, 5]

edited Mar 24 '14 at 14:05

answered Mar 24 '14 at 12:57

falsetru

Did I spot a typo? :) – Stephan Kristyn Mar 24 '14 at 13:26
@AchmedDurangi, Sorry, I don't understand what do you mean. – falsetru Mar 24 '14 at 13:29
@AchmedDurangi, That is not a typo. `lst[k-s:k-s] = x` was used to insert items at the position `k-s` (1). – falsetru Mar 24 '14 at 13:37
I'm implementing it now. – Stephan Kristyn Mar 24 '14 at 13:38
Wouldn't have [k-s] sufficed? – Stephan Kristyn Mar 24 '14 at 13:42
@AchmedDurangi, Try `lst[k-s] = x`. It will not give you what you want; it will insert a list. – falsetru Mar 24 '14 at 13:43
You are right, it would create a tuple if using only one delimiter. I'm giving you the answer in case my vote count goes up otherwise I will kill this flame thread. – Stephan Kristyn Mar 24 '14 at 13:54
@AchmedDurangi, a tuple (`(1, 2, 3)`) is different from a list (`[1, 2, 3]`). BTW, I updated the answer to make it clear that you want shift. How about change the example in the question? – falsetru Mar 24 '14 at 14:06

1 Answers1