is there a better way?

[Carl Friedrich Bolz]

Hi!

markscala at gmail.com wrote:
> Problem:
> 
> You have a list of unknown length, such as this: list =
> [X,X,X,O,O,O,O].  You want to extract all and only the X's.  You know
> the X's are all up front and you know that the item after the last X is
> an O, or that the list ends with an X.  There are never O's between
> X's.
> 
> I have been using something like this:
> _____________________
> 
> while list[0] != O:
>     storage.append(list[0])
>     list.pop(0)
>     if len(list) == 0:
>         break
> _____________________
> 
> But this seems ugly to me, and using "while" give me the heebies.  Is
> there a better approach?

Depends on what you mean with "better". I (heavily inspired by the 
bisect module) came up with:

low = 0
high = len(L)
while low < high:
     mid = (low + high) // 2
     if L[mid] == 0:
         high = mid
     else:
         low = mid + 1
storage = L[:low]

This has the advantage to be more efficient compared to other 
approaches, which of course only matters if your list is big. It still 
features a "while" loop, though.

> hope this is clear.

It is not entirely clear what the X is supposed to be. I assumed that it 
can be anything except 0.

Cheers,

Carl Friedrich