In Python, How can one subtract two non-unique, unordered lists? Say we have a = [0,1,2,1,0]
and b = [0, 1, 1]
I'd like to do something like c = a - b
and have c
be [2, 0]
or [0, 2]
order doesn't matter to me. This should throw an exception if a does not contain all elements in b.
Note this is different from sets! I'm not interested in finding the difference of the sets of elements in a and b, I'm interested in the difference between the actual collections of elements in a and b.
I can do this with a for loop, looking up the first element of b in a and then removing the element from b and from a, etc. But this doesn't appeal to me, it would be very inefficient (order of O(n^2)
time) while it should be no problem to do this in O(n log n)
time.
This question is related to
python
list
collections
c = [i for i in b if i not in a]
I attempted to find a more elegant solution, but the best I could do was basically the same thing that Dyno Fu said:
from copy import copy
def subtract_lists(a, b):
"""
>>> a = [0, 1, 2, 1, 0]
>>> b = [0, 1, 1]
>>> subtract_lists(a, b)
[2, 0]
>>> import random
>>> size = 10000
>>> a = [random.randrange(100) for _ in range(size)]
>>> b = [random.randrange(100) for _ in range(size)]
>>> c = subtract_lists(a, b)
>>> assert all((x in a) for x in c)
"""
a = copy(a)
for x in b:
if x in a:
a.remove(x)
return a
I would do it in an easier way:
a_b = [e for e in a if not e in b ]
..as wich wrote, this is wrong - it works only if the items are unique in the lists. And if they are, it's better to use
a_b = list(set(a) - set(b))
To prove jkp's point that 'anything on one line will probably be helishly complex to understand', I created a one-liner. Please do not mod me down because I understand this is not a solution that you should actually use. It is just for demonstrational purposes.
The idea is to add the values in a one by one, as long as the total times you have added that value does is smaller than the total number of times this value is in a minus the number of times it is in b:
[ value for counter,value in enumerate(a) if a.count(value) >= b.count(value) + a[counter:].count(value) ]
The horror! But perhaps someone can improve on it? Is it even bug free?
Edit: Seeing Devin Jeanpierre comment about using a dictionary datastructure, I came up with this oneliner:
sum([ [value]*count for value,count in {value:a.count(value)-b.count(value) for value in set(a)}.items() ], [])
Better, but still unreadable.
Python 2.7+ and 3.0 have collections.Counter (a.k.a. multiset). The documentation links to Recipe 576611: Counter class for Python 2.5:
from operator import itemgetter
from heapq import nlargest
from itertools import repeat, ifilter
class Counter(dict):
'''Dict subclass for counting hashable objects. Sometimes called a bag
or multiset. Elements are stored as dictionary keys and their counts
are stored as dictionary values.
>>> Counter('zyzygy')
Counter({'y': 3, 'z': 2, 'g': 1})
'''
def __init__(self, iterable=None, **kwds):
'''Create a new, empty Counter object. And if given, count elements
from an input iterable. Or, initialize the count from another mapping
of elements to their counts.
>>> c = Counter() # a new, empty counter
>>> c = Counter('gallahad') # a new counter from an iterable
>>> c = Counter({'a': 4, 'b': 2}) # a new counter from a mapping
>>> c = Counter(a=4, b=2) # a new counter from keyword args
'''
self.update(iterable, **kwds)
def __missing__(self, key):
return 0
def most_common(self, n=None):
'''List the n most common elements and their counts from the most
common to the least. If n is None, then list all element counts.
>>> Counter('abracadabra').most_common(3)
[('a', 5), ('r', 2), ('b', 2)]
'''
if n is None:
return sorted(self.iteritems(), key=itemgetter(1), reverse=True)
return nlargest(n, self.iteritems(), key=itemgetter(1))
def elements(self):
'''Iterator over elements repeating each as many times as its count.
>>> c = Counter('ABCABC')
>>> sorted(c.elements())
['A', 'A', 'B', 'B', 'C', 'C']
If an element's count has been set to zero or is a negative number,
elements() will ignore it.
'''
for elem, count in self.iteritems():
for _ in repeat(None, count):
yield elem
# Override dict methods where the meaning changes for Counter objects.
@classmethod
def fromkeys(cls, iterable, v=None):
raise NotImplementedError(
'Counter.fromkeys() is undefined. Use Counter(iterable) instead.')
def update(self, iterable=None, **kwds):
'''Like dict.update() but add counts instead of replacing them.
Source can be an iterable, a dictionary, or another Counter instance.
>>> c = Counter('which')
>>> c.update('witch') # add elements from another iterable
>>> d = Counter('watch')
>>> c.update(d) # add elements from another counter
>>> c['h'] # four 'h' in which, witch, and watch
4
'''
if iterable is not None:
if hasattr(iterable, 'iteritems'):
if self:
self_get = self.get
for elem, count in iterable.iteritems():
self[elem] = self_get(elem, 0) + count
else:
dict.update(self, iterable) # fast path when counter is empty
else:
self_get = self.get
for elem in iterable:
self[elem] = self_get(elem, 0) + 1
if kwds:
self.update(kwds)
def copy(self):
'Like dict.copy() but returns a Counter instance instead of a dict.'
return Counter(self)
def __delitem__(self, elem):
'Like dict.__delitem__() but does not raise KeyError for missing values.'
if elem in self:
dict.__delitem__(self, elem)
def __repr__(self):
if not self:
return '%s()' % self.__class__.__name__
items = ', '.join(map('%r: %r'.__mod__, self.most_common()))
return '%s({%s})' % (self.__class__.__name__, items)
# Multiset-style mathematical operations discussed in:
# Knuth TAOCP Volume II section 4.6.3 exercise 19
# and at http://en.wikipedia.org/wiki/Multiset
#
# Outputs guaranteed to only include positive counts.
#
# To strip negative and zero counts, add-in an empty counter:
# c += Counter()
def __add__(self, other):
'''Add counts from two counters.
>>> Counter('abbb') + Counter('bcc')
Counter({'b': 4, 'c': 2, 'a': 1})
'''
if not isinstance(other, Counter):
return NotImplemented
result = Counter()
for elem in set(self) | set(other):
newcount = self[elem] + other[elem]
if newcount > 0:
result[elem] = newcount
return result
def __sub__(self, other):
''' Subtract count, but keep only results with positive counts.
>>> Counter('abbbc') - Counter('bccd')
Counter({'b': 2, 'a': 1})
'''
if not isinstance(other, Counter):
return NotImplemented
result = Counter()
for elem in set(self) | set(other):
newcount = self[elem] - other[elem]
if newcount > 0:
result[elem] = newcount
return result
def __or__(self, other):
'''Union is the maximum of value in either of the input counters.
>>> Counter('abbb') | Counter('bcc')
Counter({'b': 3, 'c': 2, 'a': 1})
'''
if not isinstance(other, Counter):
return NotImplemented
_max = max
result = Counter()
for elem in set(self) | set(other):
newcount = _max(self[elem], other[elem])
if newcount > 0:
result[elem] = newcount
return result
def __and__(self, other):
''' Intersection is the minimum of corresponding counts.
>>> Counter('abbb') & Counter('bcc')
Counter({'b': 1})
'''
if not isinstance(other, Counter):
return NotImplemented
_min = min
result = Counter()
if len(self) < len(other):
self, other = other, self
for elem in ifilter(self.__contains__, other):
newcount = _min(self[elem], other[elem])
if newcount > 0:
result[elem] = newcount
return result
if __name__ == '__main__':
import doctest
print doctest.testmod()
Then you can write
a = Counter([0,1,2,1,0])
b = Counter([0, 1, 1])
c = a - b
print list(c.elements()) # [0, 2]
list(set([x for x in a if x not in b]))
a
and b
untouched.Here's a relatively long but efficient and readable solution. It's O(n).
def list_diff(list1, list2):
counts = {}
for x in list1:
try:
counts[x] += 1
except:
counts[x] = 1
for x in list2:
try:
counts[x] -= 1
if counts[x] < 0:
raise ValueError('All elements of list2 not in list2')
except:
raise ValueError('All elements of list2 not in list1')
result = []
for k, v in counts.iteritems():
result += v*[k]
return result
a = [0, 1, 1, 2, 0]
b = [0, 1, 1]
%timeit list_diff(a, b)
%timeit list_diff(1000*a, 1000*b)
%timeit list_diff(1000000*a, 1000000*b)
100000 loops, best of 3: 4.8 µs per loop
1000 loops, best of 3: 1.18 ms per loop
1 loops, best of 3: 1.21 s per loop
I'm not sure what the objection to a for loop is: there is no multiset in Python so you can't use a builtin container to help you out.
Seems to me anything on one line (if possible) will probably be helishly complex to understand. Go for readability and KISS. Python is not C :)
to use list comprehension:
[i for i in a if not i in b or b.remove(i)]
would do the trick. It would change b in the process though. But I agree with jkp and Dyno Fu that using a for loop would be better.
Perhaps someone can create a better example that uses list comprehension but still is KISS?
I know "for" is not what you want, but it's simple and clear:
for x in b:
a.remove(x)
Or if members of b
might not be in a
then use:
for x in b:
if x in a:
a.remove(x)
You can use the map
construct to do this. It looks quite ok, but beware that the map
line itself will return a list of None
s.
a = [1, 2, 3]
b = [2, 3]
map(lambda x:a.remove(x), b)
a
You can try something like this:
class mylist(list):
def __sub__(self, b):
result = self[:]
b = b[:]
while b:
try:
result.remove(b.pop())
except ValueError:
raise Exception("Not all elements found during subtraction")
return result
a = mylist([0, 1, 2, 1, 0] )
b = mylist([0, 1, 1])
>>> a - b
[2, 0]
You have to define what [1, 2, 3] - [5, 6] should output though, I guess you want [1, 2, 3] thats why I ignore the ValueError.
Edit:
Now I see you wanted an exception if a
does not contain all elements, added it instead of passing the ValueError.
Source: Stackoverflow.com