[Tutor] comparing lists of strings with each other

[Lloyd Kvam]

Good thing you are watching Rich.  I scanned right past the use
of filter.  The variable name "list" convinced me I was looking
at a list.  (Actually list is also a bad choice for variable name
because it hides the builtin list.)

Rich Krauter wrote:

> If you trace out each step of what filter is doing,
> you'll see why you are getting what you are getting.
> 
> By using filter, you are passing each element of the
> first list into your comparison function. Your
> comparison  function compares the passed-in item with
> each item in the second list, and returns the item (a
> true value) when the two are not equal:
> 
> 'a' in list1 != 'c' in list2, so 'a' is returned
> 'b' in list1 != 'a' in list2, so 'b' is returned
> 'c' in list1 != 'a' in list2, so 'c' is returned
> 
> Filter is documented here:
> http://www.python.org/doc/current/lib/built-in-funcs.html#l2h-26
> 
> You can do this without filter, using list membership
> tests:
> 
>>>>newlist = []
>>>>for x in list1:
> 
> ... 	if x not in list2:
> ... 		newlist.append(x)
> 
> or
> 
> 
>>>>f = lambda i: i not in list2
>>>>filter(f,list1)
>>>>['b']
> 
> 
> One way you can do what you want in python 2.3 is with
> sets:  
> 
>>>>import sets
>>>>a1 = ['a','b','c']
>>>>a2 = ['a','c']
>>>>s1 = sets.Set(a1)
>>>>s2 = sets.Set(a2)
> 
> 
> #find all elements of s1 not in s2 
> 
>>>>list(s1-s2)
> 
> ['b']
> 
> Rich
> 
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