如何找到列表的交点？

如果顺序不重要，你不需要担心重复，那么你可以使用set intersection:

>>> a = [1,2,3,4,5]
>>> b = [1,3,5,6]
>>> list(set(a) & set(b))
[1, 3, 5]

如果布尔与是指同时出现在两个列表中的项，例如交集，那么你应该看看Python的set和frozenset类型。

从较大的集合中创建一个集合:

_auxset = set(a)

然后,

c = [x for x in b if x in _auxset]

将做你想做的(保留b的顺序，而不是a的顺序——不一定能保留这两个)，并执行快。(在列表推导中使用if x in a作为条件也可以工作，并避免需要构建_auxset，但不幸的是，对于相当长的列表，它会慢得多)。

如果你想对结果进行排序，而不是保持列表的顺序，一个更整洁的方法可能是:

c = sorted(set(a).intersection(b))

如果将两个列表中较大的一个转换为一个集合，则可以使用intersection()获取该集合与任何可迭代对象的交集:

a = [1,2,3,4,5]
b = [1,3,5,6]
set(a).intersection(b)

a = [1,2,3,4,5]
b = [1,3,5,6]
c = list(set(a).intersection(set(b)))

应该像做梦一样工作。并且，如果可以的话，使用集合而不是列表来避免所有这些类型更改!

对我来说，使用列表推导式是一个非常明显的方法。不确定性能如何，但至少能保持列表。

[x for x in a if x in b]

或者，如果x值在B"中，则A中的所有x值。

下面是一些Python 2 / Python 3代码，用于为基于列表和基于集的方法生成时间信息，以查找两个列表的交集。

纯粹的列表理解算法是O(n²)，因为列表上的in是一个线性搜索。基于集合的算法是O(n)，因为集合搜索是O(1)，集合创建是O(n)(将集合转换为列表也是O(n))。因此，对于足够大的n，基于集合的算法更快，但对于较小的n，创建集合的开销使它们比纯列表比较算法慢。

#!/usr/bin/env python


''' Time list- vs set-based list intersection
See http://stackoverflow.com/q/3697432/4014959
Written by PM 2Ring 2015.10.16
'''


from __future__ import print_function, division
from timeit import Timer


setup = 'from __main__ import a, b'
cmd_lista = '[u for u in a if u in b]'
cmd_listb = '[u for u in b if u in a]'


cmd_lcsa = 'sa=set(a);[u for u in b if u in sa]'


cmd_seta = 'list(set(a).intersection(b))'
cmd_setb = 'list(set(b).intersection(a))'


reps = 3
loops = 50000


def do_timing(heading, cmd, setup):
t = Timer(cmd, setup)
r = t.repeat(reps, loops)
r.sort()
print(heading, r)
return r[0]


m = 10
nums = list(range(6 * m))


for n in range(1, m + 1):
a = nums[:6*n:2]
b = nums[:6*n:3]
print('\nn =', n, len(a), len(b))
#print('\nn = %d\n%s %d\n%s %d' % (n, a, len(a), b, len(b)))
la = do_timing('lista', cmd_lista, setup)
lb = do_timing('listb', cmd_listb, setup)
lc = do_timing('lcsa ', cmd_lcsa, setup)
sa = do_timing('seta ', cmd_seta, setup)
sb = do_timing('setb ', cmd_setb, setup)
print(la/sa, lb/sa, lc/sa, la/sb, lb/sb, lc/sb)

输出

n = 1 3 2
lista [0.082171916961669922, 0.082588911056518555, 0.0898590087890625]
listb [0.069530963897705078, 0.070394992828369141, 0.075379848480224609]
lcsa  [0.11858987808227539, 0.1188349723815918, 0.12825107574462891]
seta  [0.26900982856750488, 0.26902294158935547, 0.27298116683959961]
setb  [0.27218389511108398, 0.27459001541137695, 0.34307217597961426]
0.305460649521 0.258469975867 0.440838458259 0.301898526833 0.255455833892 0.435697630214


n = 2 6 4
lista [0.15915989875793457, 0.16000485420227051, 0.16551494598388672]
listb [0.13000702857971191, 0.13060092926025391, 0.13543915748596191]
lcsa  [0.18650484085083008, 0.18742108345031738, 0.19513416290283203]
seta  [0.33592700958251953, 0.34001994132995605, 0.34146714210510254]
setb  [0.29436492919921875, 0.2953648567199707, 0.30039691925048828]
0.473793098554 0.387009751735 0.555194537893 0.540689066428 0.441652573672 0.633583767462


n = 3 9 6
lista [0.27657914161682129, 0.28098297119140625, 0.28311991691589355]
listb [0.21585917472839355, 0.21679902076721191, 0.22272896766662598]
lcsa  [0.22559309005737305, 0.2271728515625, 0.2323150634765625]
seta  [0.36382699012756348, 0.36453008651733398, 0.36750602722167969]
setb  [0.34979605674743652, 0.35533690452575684, 0.36164689064025879]
0.760194128313 0.59330170819 0.62005595016 0.790686848184 0.61710008036 0.644927481902


n = 4 12 8
lista [0.39616990089416504, 0.39746403694152832, 0.41129183769226074]
listb [0.33485794067382812, 0.33914685249328613, 0.37850618362426758]
lcsa  [0.27405810356140137, 0.2745978832244873, 0.28249192237854004]
seta  [0.39211201667785645, 0.39234519004821777, 0.39317893981933594]
setb  [0.36988520622253418, 0.37011313438415527, 0.37571001052856445]
1.01034878821 0.85398540833 0.698928091731 1.07106176249 0.905302334456 0.740927452493


n = 5 15 10
lista [0.56792402267456055, 0.57422614097595215, 0.57740211486816406]
listb [0.47309303283691406, 0.47619009017944336, 0.47628307342529297]
lcsa  [0.32805585861206055, 0.32813096046447754, 0.3349759578704834]
seta  [0.40036201477050781, 0.40322518348693848, 0.40548801422119141]
setb  [0.39103078842163086, 0.39722800254821777, 0.43811702728271484]
1.41852623806 1.18166313332 0.819398061028 1.45237674242 1.20986133789 0.838951479847


n = 6 18 12
lista [0.77897095680236816, 0.78187918663024902, 0.78467702865600586]
listb [0.629547119140625, 0.63210701942443848, 0.63321495056152344]
lcsa  [0.36563992500305176, 0.36638498306274414, 0.38175487518310547]
seta  [0.46695613861083984, 0.46992206573486328, 0.47583580017089844]
setb  [0.47616910934448242, 0.47661614418029785, 0.4850609302520752]
1.66818870637 1.34819326075 0.783028414812 1.63591241329 1.32210827369 0.767878297495


n = 7 21 14
lista [0.9703209400177002, 0.9734041690826416, 1.0182771682739258]
listb [0.82394003868103027, 0.82625699043273926, 0.82796716690063477]
lcsa  [0.40975093841552734, 0.41210508346557617, 0.42286920547485352]
seta  [0.5086359977722168, 0.50968098640441895, 0.51014018058776855]
setb  [0.48688101768493652, 0.4879908561706543, 0.49204087257385254]
1.90769222837 1.61990115188 0.805587768483 1.99293236904 1.69228211566 0.841583309951


n = 8 24 16
lista [1.204819917678833, 1.2206029891967773, 1.258256196975708]
listb [1.014998197555542, 1.0206191539764404, 1.0343101024627686]
lcsa  [0.50966787338256836, 0.51018595695495605, 0.51319599151611328]
seta  [0.50310111045837402, 0.50556015968322754, 0.51335406303405762]
setb  [0.51472997665405273, 0.51948785781860352, 0.52113485336303711]
2.39478683834 2.01748351664 1.01305257092 2.34068341135 1.97190418975 0.990165516871


n = 9 27 18
lista [1.511646032333374, 1.5133969783782959, 1.5639569759368896]
listb [1.2461750507354736, 1.254518985748291, 1.2613379955291748]
lcsa  [0.5565330982208252, 0.56119203567504883, 0.56451296806335449]
seta  [0.5966339111328125, 0.60275578498840332, 0.64791703224182129]
setb  [0.54694414138793945, 0.5508568286895752, 0.55375313758850098]
2.53362406013 2.08867620074 0.932788243907 2.76380331728 2.27843203069 1.01753187594


n = 10 30 20
lista [1.7777848243713379, 2.1453688144683838, 2.4085969924926758]
listb [1.5070111751556396, 1.5202279090881348, 1.5779800415039062]
lcsa  [0.5954139232635498, 0.59703707695007324, 0.60746097564697266]
seta  [0.61563014984130859, 0.62125110626220703, 0.62354087829589844]
setb  [0.56723213195800781, 0.57257509231567383, 0.57460403442382812]
2.88774814689 2.44791645689 0.967161734066 3.13413984189 2.6567803378 1.04968299523

使用2GHz的单核机器和2GB的RAM在Debian风格的Linux上运行Python 2.6.6 (Firefox在后台运行)生成。

这些数字只是一个粗略的指南，因为各种算法的实际速度受到两个源列表中元素的比例的不同影响。

函数式方法可以使用filter和lambda操作符实现。

list1 = [1,2,3,4,5,6]


list2 = [2,4,6,9,10]


>>> list(filter(lambda x:x in list1, list2))


[2, 4, 6]

编辑:它过滤掉了同时存在于list1和list中的x，集差异也可以使用:

>>> list(filter(lambda x:x not in list1, list2))
[9,10]

Edit2: python3 filter返回一个过滤器对象，用list封装它返回输出列表。

这是一个示例，当您需要在结果中的每个元素出现的次数应该与它在两个数组中显示的次数相同。

def intersection(nums1, nums2):
#example:
#nums1 = [1,2,2,1]
#nums2 = [2,2]
#output = [2,2]
#find first 2 and remove from target, continue iterating


target, iterate = [nums1, nums2] if len(nums2) >= len(nums1) else [nums2, nums1] #iterate will look into target


if len(target) == 0:
return []


i = 0
store = []
while i < len(iterate):


element = iterate[i]


if element in target:
store.append(element)
target.remove(element)


i += 1




return store

这可能是晚了，但我只是认为我应该分享的情况下，你需要手动做(显示工作-哈哈)或当你需要所有元素出现尽可能多的次数或当你也需要它是唯一的。

请注意，还为它编写了测试。

from nose.tools import assert_equal


'''
Given two lists, print out the list of overlapping elements
'''


def overlap(l_a, l_b):
'''
compare the two lists l_a and l_b and return the overlapping
elements (intersecting) between the two
'''


#edge case is when they are the same lists
if l_a == l_b:
return [] #no overlapping elements


output = []


if len(l_a) == len(l_b):
for i in range(l_a): #same length so either one applies
if l_a[i] in l_b:
output.append(l_a[i])


#found all by now
#return output #if repetition does not matter
return list(set(output))


else:
#find the smallest and largest lists and go with that
sm = l_a if len(l_a)  len(l_b) else l_b


for i in range(len(sm)):
if sm[i] in lg:
output.append(sm[i])


#return output #if repetition does not matter
return list(set(output))


## Test the Above Implementation


a = [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
b = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]
exp = [1, 2, 3, 5, 8, 13]


c = [4, 4, 5, 6]
d = [5, 7, 4, 8 ,6 ] #assuming it is not ordered
exp2 = [4, 5, 6]


class TestOverlap(object):


def test(self, sol):
t = sol(a, b)
assert_equal(t, exp)
print('Comparing the two lists produces')
print(t)


t = sol(c, d)
assert_equal(t, exp2)
print('Comparing the two lists produces')
print(t)


print('All Tests Passed!!')


t = TestOverlap()
t.test(overlap)

你也可以使用计数器!它不会保留顺序，但会考虑副本:

>>> from collections import Counter
>>> a = [1,2,3,4,5]
>>> b = [1,3,5,6]
>>> d1, d2 = Counter(a), Counter(b)
>>> c = [n for n in d1.keys() & d2.keys() for _ in range(min(d1[n], d2[n]))]
>>> print(c)
[1,3,5]

这样可以得到两个列表的交集，也可以得到公共重复项。

>>> from collections import Counter
>>> a = Counter([1,2,3,4,5])
>>> b = Counter([1,3,5,6])
>>> a &= b
>>> list(a.elements())
[1, 3, 5]

这里的大多数解决方案都不考虑列表中元素的顺序，而是将列表视为集合。另一方面，如果希望找到两个列表中包含的一个最长子序列，则可以尝试以下代码。

def intersect(a, b):
if a == [] or b == []:
return []
inter_1 = intersect(a[1:], b)
if a[0] in b:
idx = b.index(a[0])
inter_2 = [a[0]] + intersect(a[1:], b[idx+1:])
if len(inter_1) <= len(inter_2):
return inter_2
return inter_1

对于a=[1,2,3]和b=[3,1,4,2]，返回[1,2]而不是[1,2,3]。注意，这样的子序列不是唯一的，因为[1]、[2]、[3]都是a=[1,2,3]和b=[3,2,1]的解。

在这种情况下，你有一个列表的列表map很方便:

>>> lists = [[1, 2, 3], [2, 3, 4], [2, 3, 5]]
>>> set(lists.pop()).intersection(*map(set, lists))
{2, 3}

适用于类似的迭代对象:

>>> lists = ['ash', 'nazg']
>>> set(lists.pop()).intersection(*map(set, lists))
{'a'}

如果列表为空，pop将会爆炸，所以你可能想要在函数中进行包装:

def intersect_lists(lists):
try:
return set(lists.pop()).intersection(*map(set, lists))
except IndexError: # pop from empty list
return set()

当我们使用tuple时，我们想要交叉

a=([1,2,3,4,5,20], [8,3,9,5,1,4,20])


for i in range(len(a)):


b=set(a[i-1]).intersection(a[i])
print(b)
{1, 3, 4, 5, 20}