Separating elements from a list according to preceding element

[Gerard Flanagan]

Rob Cowie wrote:
> I'm having a bit of trouble with this so any help would be gratefully
> recieved...
>
> After splitting up a url I have a string of the form
> 'tag1+tag2+tag3-tag4', or '-tag1-tag2' etc. The first tag will only be
> preceeded by an operator if it is a '-', if it is preceded by nothing,
> '+' is to be assumed.
>
> Using re.split, I can generate a list that looks thus:
> ['tag1', '+', 'tag2', '+', 'tag3', '-', 'tag4']
>
> I wish to derive two lists - each containing either tags to be
> included, or tags to be excluded. My idea was to take an element,
> examine what element precedes it and accordingly, insert it into the
> relevant list. However, I have not been successful.
>
> Is there a better way that I have not considered? If this method is
> suitable, how might I implement it?
>
> Thanks all,
>
> Rob Cowie

a = [ '+', 'tag1', '+', 'tag2', '-', 'tag3', '+', 'tag4' ]

import itertools

b = list(itertools.islice(a,0,8,2))
c = list(itertools.islice(a,1,8,2))

result1 = [x[1] for x in itertools.izip(b,c) if x[0] == '+']
result2 = [x[1] for x in itertools.izip(b,c) if x[0] == '-']

print
print result1
print result2


Gerard