Question

In Python, I have two lists that either have equal number of elements (e.g. 8 and 8) or one less than the other (e.g. 7 and 8; 3 and 4):

list1 = ['A', 'B', 'C', 'D']
list2 = ['E', 'F', 'G', 'H']

or

list3 = ['A', 'B', 'C']
list4 = ['D', 'E', 'F', 'G']

I’m trying to figure out the best way to build an algorithm that will switch the last half of the first list with the first half of the last list, resulting in this, when both lists have an even number of elements:

switched_list1 = ['A', 'B', 'E', 'F']
switched_list2 = ['C', 'D', 'G', 'H']

…and this when the one of the lists has an odd number:

switched_list3 = ['A', 'D', 'E']
switched_list4 = ['B', 'C', 'F', 'G']

What’s the most efficient way to build an algorithm that can switch list elements like this?

Answer 1

>>> def StrangeSwitch(list1,list2):
    return (list1[:len(list1)/2]+list2[:len(list2)/2],list1[len(list1)/2:]+list2[len(list2)/2:])

>>> list1 = ['A', 'B', 'C', 'D']
>>> list2 = ['E', 'F', 'G', 'H']
>>> (list1,list2)=StrangeSwitch(list1,list2)
>>> list1
['A', 'B', 'E', 'F']
>>> list2
['C', 'D', 'G', 'H']
>>> list3 = ['A', 'B', 'C']
>>> list4 = ['D', 'E', 'F', 'G']
>>> (list3,list4)=StrangeSwitch(list3,list4)
>>> list3
['A', 'B', 'C']
>>> list4
['B', 'C', 'F', 'G']
>>> 

Reading the Comments by OP I would take the priviledge of proposing another approach

>>> def StrangeSwitchFast(list1,list2):
    #return (list1[:len(list1)/2]+list2[:len(list2)/2],list1[len(list1)/2:]+list2[len(list2)/2:])
    return (list(itertools.chain(itertools.islice(list1,0,len(list1)/2),itertools.islice(list2,0,len(list2)/2))),
        list(itertools.chain(itertools.islice(list1,len(list1)/2,None),itertools.islice(list2,len(list2)/2,None))))

The above doesn’t create any temporary list and if OP desires to use it as an iterator rather than a list for the downstream processing, then the list can be safely dropped from the function and can be left to return as a tuple of iterators.