How to get element not included in combinations list in python?

Question

I have generator with 3 pretty big chunks. I am creating combinations of 2 elements from it, but then in some part of code I need third part (the one not included in combinations). How should I do it? I don't need the simplest or the prettiest solution, I need the fastest solution possible.

Example:

a = ['a','b','c']
gen = chunks(a, 2) # this is not important
for x in combinations(gen, 2):
    # let's say we have x = ['a','b'] and I want to get 'c'
    # I know it is possible to put all generator elements in list and get 'c' 
    # with for loop or set, but I don't if this is the fastest way to get it

I still don't get it. What is b? Can you be more explicit about what you have and what you want to get? — cd98, Nov 15 '14 at 15:16
See [Remove all the elements that occur in 1 list from another](http://stackoverflow.com/questions/4211209/remove-all-the-elements-that-occur-in-1-list-from-another). Is ```gen``` an actual generator or a ```list```? If it is a generator, there isn't a way to *use* the sequence that the combinations are made from to extract the missing element. — wwii, Nov 15 '14 at 15:21
Maybe ```set(chunks(a,2)).difference(x)``` - but you would have to create a new set each time. — wwii, Nov 15 '14 at 15:52

score 0 · Answer 1 · answered Nov 15 '14 at 16:00

As an efficient way i suggest define a function that you could use it multitime and get the difference with set because its a pythonic and fast way :

>>> def diff(test_array=[],main_array=['a','b','c']):
...  return set(main_array).difference(set(test_array)).pop()
... 
>>> diff(['a','b'])
'c'

How to get element not included in combinations list in python?

1 Answers1