Merging lists has made my brain hurt.

Mike C. Fletcher mcfletch at rogers.com
Wed Oct 2 18:31:44 CEST 2002


This is just off the top of my head, there are probably better ways...

PythonWin 2.2.1 (#34, Apr  9 2002, 19:34:33) [MSC 32 bit (Intel)] on win32.
Portions Copyright 1994-2001 Mark Hammond (mhammond at skippinet.com.au) - 
see 'Help/About PythonWin' for further copyright information.
 >>> lol = [['aaa', 'bbb', 'ccc'],['bbb', 'ccc', 'ddd'],['ccc', 'ddd', 
'eee']]
 >>> def common( source ):
... 	if not source:
... 		return []
... 	set = {}
... 	for item in source[0]:
... 		set[item] = 1
... 	for other in source[1:]:
... 		for item in other:
... 			if set.has_key( item ):
... 				set[item] = set[item]+1
... 	return [ key for (key,value) in set.items() if value == len(source)]
...
 >>> common(lol)
['ccc']
 >>>

I think Python 2.3 is going to have a sets module (hopefully back ported 
to Python 2.2.x eventually) which would probably be a more obvious approach.

HTH,
Mike

Huw Lynes wrote:
> Hi All,
> 
> I have a list containing an arbitrary number of other lists. The 
> internal lists contain strings. For example
> lol = [
> ['aaa', 'bbb', 'ccc'],
> ['bbb', 'ccc', 'ddd'],
> ['ccc', 'ddd', 'eee']
> ]
> 
> I want to merge the three lists into a single list that only contains 
> the strings present in all three lists. In the above case I want to end 
> up with
> ['ccc']
> 
> This has me utterly stumped. Any help is appreciated.The only articles 
> about merging strings that I've managed to find so far have been about 
> merging strings so that you don't get repeats. Not quite what I'm 
> looking for.
> 
> Thanks,
> Huw
> 
_______________________________________
   Mike C. Fletcher
   Designer, VR Plumber, Coder
   http://members.rogers.com/mcfletch/






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