diff lists

[Ken Seehof]

It's often a good idea to use dictionaries instead of lists for
this kind of thing if performance is important.  A dictionary
is a better model for a set than a list is, although this may seem
intuitively strange at first.  People should use dictionaries more;
they are very powerful, and not just for lookup tables.

I am assuming your lists won't have duplicates (e.g. ['a','b','b'])
since you seem to be interested in set operations.

Try this:

>>> def andnot(a, b):
...  d = {}
...  for x in a:
...   d[x]=0 # value doesn't matter, we're just using the key
...  for x in b:
...   if d.has_key(x):
...    del d[x]
...  return d.keys()
... 
>>> a = ['a', 'b', 'c', 'd', 'e', 'f']
>>> b = ['e', 'c', 'f']
>>> andnot(a,b)
['d', 'b', 'a']

This will be super fast (linear time) even for really big lists.

Note that if b is small compared to a, you are significantly
better off using dictionaries in the first place rather than
translating as in the above example.

This one uses dictionaries instead of lists:
>>> def andnot2(a, b):
...  d = a.copy()
...  for x in b.keys():
...   if d.has_key(x):
...    del d[x]
...  return d
... 
>>> a = {'a':0, 'b':0, 'c':0}
>>> b = {'b':0}
>>> andnot2(a,b)
{'c': 0, 'a': 0}

----- Original Message ----- 
From: "Oliver Vecernik" <vecernik at aon.at>
Newsgroups: comp.lang.python
To: <python-list at python.org>
Sent: Wednesday, March 28, 2001 4:47 AM
Subject: diff lists


> Hi,
> 
> I've got following two lists:
> 
> ['a', 'b', 'c', 'd', 'e', 'f']
> ['e', 'c', 'f']
> 
> I'd like to have the result:
> 
> ['a', 'b', 'd']
> 
> The list need not to be ordered. ['d', 'a', 'b'] will also be ok. What
> is the most effective way to achive that result?
> 
> Oliver
> -- 
> http://mail.python.org/mailman/listinfo/python-list
>