"Collapsing" a list into a list of changes

[Steven Bethard]

Alan McIntyre wrote:
> Hi all,
> 
> I have a list of items that has contiguous repetitions of values, but 
> the number and location of the repetitions is not important, so I just 
> need to strip them out.  For example, if my original list is 
> [0,0,1,1,1,2,2,3,3,3,2,2,2,4,4,4,5], I want to end up with [0,1,2,3,2,4,5].
> 
> Here is the way I'm doing this now:
> 
> def straightforward_collapse(myList):
>     collapsed = [myList[0]]
>     for n in myList[1:]:
>         if n != collapsed[-1]:
>             collapsed.append(n)
> 
>     return collapsed
> 
> Is there an elegant way to do this, or should I just stick with the code 
> above?

Here's a solution that works for iterables other than lists:

py> def collapse(iterable):
...     enumeration = enumerate(iterable)
...     _, lastitem = enumeration.next()
...     yield lastitem
...     for i, item in enumeration:
...         if item != lastitem:
...             yield item
...             lastitem = item
...
py> lst = [0,0,1,1,1,2,2,3,3,3,2,2,2,4,4,4,5]
py> list(collapse(lst))
[0, 1, 2, 3, 2, 4, 5]

Again, I'm still not sure I'd call this more elegant...

STeVe