Comparing Lists - Dev Shed

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July 27th, 2004, 06:10 PM

bquad20

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Comparing Lists

I have a question I need some help with about lists.
I have 2 lists (one being generated from a file and the other one from a database) that I need to compare against each other.

Can some one help me:

- compare the lists against each other
- take the differences between the lists and append that information to a 3rd list so I can print it out

Code:

# generated from reading a file
list 1 ['testName', 'test@email.com', '123 street', '/usr/local/bin/myScript.sh', 'None']

# generated from a database table
list 2 ['testName', 'test@email.com', '456 street', '/usr/local/bin/orTest9d.sh', 'None']

# shows the differences between lists -- HELP!
list 3 []
print "differences:", list 3

July 27th, 2004, 08:57 PM

Strike

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I suggest you use Python 2.3's sets and then just take the difference of each of the two sets (lists can be coerced into sets simply by calling Set() constructor on them)

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July 27th, 2004, 11:24 PM

caroundw5h

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why don't you check out the zip() function in your python document. the

PHP Code:

not

operator and the basic lists methods, like append. then write a function that iterates through the contents of the lists and appends to the datalist you want it to.

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July 28th, 2004, 02:45 AM

DevCoach

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If the order of the elements is not important then Strike is right, sets are what you want.

Code:

>>> import sets
>>> list1 = ['testName', 'test@email.com', '123 street', '/usr/local/bin/myScript.sh', 'None']
>>> list2 = ['testName', 'test@email.com', '456 street', '/usr/local/bin/orTest9d.sh', 'None']
>>> 
>>> print sets.Set(list1) ^ sets.Set(list2)
Set(['/usr/local/bin/myScript.sh', '456 street', '/usr/local/bin/orTest9d.sh', '123 street'])
>>> print sets.Set(list1) - sets.Set(list2)
Set(['/usr/local/bin/myScript.sh', '123 street'])
>>> print sets.Set(list1) | sets.Set(list2)
Set(['456 street', '/usr/local/bin/orTest9d.sh', 'None', '/usr/local/bin/myScript.sh', 'testName', '123 street', 'test@email.com'])
>>> print sets.Set(list1) & sets.Set(list2)
Set(['None', 'test@email.com', 'testName'])
>>> 
>>> #convert a set back into a list:
>>> set = sets.Set(list1) ^ sets.Set(list2)
>>> list(set)
['/usr/local/bin/myScript.sh', '456 street', '/usr/local/bin/orTest9d.sh', '123 street']
>>>

Dave - The Developers' Coach

Last edited by DevCoach : July 28th, 2004 at 02:47 AM.

July 28th, 2004, 02:52 AM

DevCoach

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If the order is important, i.e. you only want to compare list1[0] with list2[0], then you can use a zip and compare the entries, as caroundw5h said.

e.g. This will create a list of tuples of the non-matching entries:

Code:

>>> print [ (x, y) for (x, y) in zip(list1, list2) if x != y]
[('123 street', '456 street'), ('/usr/local/bin/myScript.sh', '/usr/local/bin/orTest9d.sh')]
>>>

Dave - The Developers' Coach

July 28th, 2004, 06:46 AM

netytan

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Yet another way to do this, originally written for working with multiple iterators. But still, itll work prefectly in here too.

Code:

#!/usr/bin/env python

def group(*iterators):
    while True:
        yield [iterator.next() for iterator in iterators]
        
if __name__ == '__main__':

    list1 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
    list2 = [1, 2, 3, 4, 4, 6, 7, 8, 9, 11]
    list3 = []

    for item1, item2 in group(iter(list1), iter(list2)):
        if item1 != item2:
            list3.append((item1, item2))
    print list3

Pretty easy to use..

Quote:

>>> def group(*iterators):
... while True:
... yield [iterator.next() for iterator in iterators]
...
>>> l1 = [1, 2, 3,4 , 5, 6, 7, 8, 9]
>>> l2 = [2, 3, 4, 5, 6, 7, 9, 8, 9]
>>> [(x, y) for x, y in group(iter(l1), iter(l2)) if x != y]
[(1, 2), (2, 3), (3, 4), (4, 5), (5, 6), (6, 7), (7, 9)]
>>>

Still, i think Dave has the best sugestion, if order maters to you, use the zip() function!

Mark.

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July 28th, 2004, 12:55 PM

caroundw5h

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Quote:

Originally Posted by DevCoach

Code:

>>> print [ (x, y) for (x, y) in zip(list1, list2) if x != y]
[('123 street', '456 street'), ('/usr/local/bin/myScript.sh', '/usr/local/bin/orTest9d.sh')]
>>>

Dave - The Developers' Coach

Nice list comprehension. I didn't even think of that. Very pythonic.

July 31st, 2004, 05:15 AM

DevCoach

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Quote:

Originally Posted by netytan

Code:

#!/usr/bin/env python

def group(*iterators):
    while True:
        yield [iterator.next() for iterator in iterators]

This functionality is already built into the standard library, as itertools.izip. From the docs:

Quote:

izip( *iterables)

Make an iterator that aggregates elements from each of the iterables. Like zip() except that it returns an iterator instead of a list. Used for lock-step iteration over several iterables at a time. Equivalent to:

Code:

     def izip(*iterables):
         iterables = map(iter, iterables)
         while iterables:
             result = [i.next() for i in iterables]
             yield tuple(result)

Changed in version 2.3.1: When no iterables are specified, returns a zero length iterator instead of raising a TypeError exception.

Of course there are imap and ifilter functions as well. The itertool functions are all written in C, so should be much faster than the Python equivalents.

If you do any functional programming with lists, generators etc, then it is well worth while getting to know the itertools module, since there are a lot of interesting goodies in there.

Dave - The Developers' Coach

July 31st, 2004, 06:24 AM

netytan

Hello World :)

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Thanks Dave, i never once thought about the itertools module when i wrote it

. And then once i had it i didnt bother looking for an alternative

.

Mark.

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