How can I compare two lists in python and return matches

Question

I want to take two lists and find the values that appear in both   a    1  2  3  4  5  b    9  8  7  6  5   returnMatches a  b    would return  5   for instance

User · Answer

another a bit more functional way to check list equality for list 1  lst1  and list 2  lst2  where objects have depth one and which keeps the order is   all i    j for i  j in zip lst1  lst2

User · Answer

I just used the following and it worked for me   group1    1  2  3  4  5  group2    9  8  7  6  5   for k in group1      for v in group2          if k    v              print k    this would then print 5 in your case  Probably not great performance wise though

User · Answer

I prefer the set based answers  but here s one that works anyway   x for x in a if x in b

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You can use   a    1  3  4  5  9  6  7  8  b    1  7  0  9  same values   set a   amp  set b  print same values   Output   set  1  7  9

User · Answer

You can use   def returnMatches a b          return list set a   amp  set b

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The following solution works for any order of list items and also supports both lists to be different length   import numpy as np def getMatches a  b       matches          unique a   np unique a      unique b   np unique b      for a in unique a          for b in unique b              if a    b                  matches append a      return matches print getMatches  1  2  3  4  5    9  8  7  6  5  9      displays  5  print getMatches  1  2  3    3  4  5  1      displays  1  3

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a    1  2  3  4  5  b    9  8  7  6  5   lista  set a  listb  set b     print listb intersection lista     returnMatches   set   5     output   print     join str return  for return in returnMatches     remove the set        5         final output

User · Answer

Do you want duplicates  If not maybe you should use sets instead   gt  gt  gt  set  1  2  3  4  5   intersection set  9  8  7  6  5    set  5

User · Answer

Not the most efficient one  but by far the most obvious way to do it is    gt  gt  gt  a    1  2  3  4  5   gt  gt  gt  b    9  8  7  6  5   gt  gt  gt  set a   amp  set b   5    if order is significant you can do it with list comprehensions like this    gt  gt  gt   i for i  j in zip a  b  if i    j   5     only works for equal-sized lists  which order-significance implies

User · Answer

Use set intersection    it s fast and readable    gt  gt  gt  set a  intersection b  set  5

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gt  gt  gt  s     a   b   c       gt  gt  gt  f     a   b   d   c      gt  gt  gt  ss  set s     gt  gt  gt  fs  set f     gt  gt  gt  print ss intersection fs          set   a    c    b         gt  gt  gt  print ss union fs               set   a    c    b    d         gt  gt  gt  print ss union fs   - ss intersection fs          set   d

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If you want a boolean value    gt  gt  gt  a    1  2  3  4  5   gt  gt  gt  b    9  8  7  6  5   gt  gt  gt  set b     set a    amp  set b  and set a     set a   amp  set b  False  gt  gt  gt  a    3 1 2   gt  gt  gt  b    1 2 3   gt  gt  gt  set b     set a    amp  set b  and set a     set a   amp  set b  True

User · Answer

Quick way   list set a  intersection set b

User · Answer

Also you can try this by keeping common elements in a new list    new list      for element in a      if element in b          new list append element

User · Answer

A quick performance test showing Lutz s solution is the best   import time  def speed test func       def wrapper  args    kwargs           t1   time time           for x in xrange 5000               results   func  args    kwargs          t2   time time           print   s took  0 3f ms     func func name   t2-t1  1000 0          return results     return wrapper   speed test def compare bitwise x  y       set x   frozenset x      set y   frozenset y      return set x  amp  set y   speed test def compare listcomp x  y       return  i for i  j in zip x  y  if i    j    speed test def compare intersect x  y       return frozenset x  intersection y     Comparing short lists a    1  2  3  4  5  b    9  8  7  6  5  compare bitwise a  b  compare listcomp a  b  compare intersect a  b     Comparing longer lists import random a   random sample xrange 100000   10000  b   random sample xrange 100000   10000  compare bitwise a  b  compare listcomp a  b  compare intersect a  b    These are the results on my machine     Short list  compare bitwise took 10 145 ms compare listcomp took 11 157 ms compare intersect took 7 461 ms    Long list  compare bitwise took 11203 709 ms compare listcomp took 17361 736 ms compare intersect took 6833 768 ms   Obviously  any artificial performance test should be taken with a grain of salt  but since the set   intersection   answer is at least as fast as the other solutions  and also the most readable  it should be the standard solution for this common problem

User · Answer

Using   and   attribute method also works    gt  gt  gt  a    1  2  3  4  5   gt  gt  gt  b    9  8  7  6  5   gt  gt  gt  set a    and   set b   set  5     or simply   gt  gt  gt  set  1  2  3  4  5     and   set  9  8  7  6  5    set  5    gt  gt  gt

User · Answer

you can   for set union and  amp  for set intersection  for example       set1  1 2 3      set2  3 4 5      print set1 amp set2      output 3      set1  1 2 3      set2  3 4 5      print set1 set2      output 1 2 3 4 5  curly braces in the answer

User · Answer

The easiest way to do that is to use sets    gt  gt  gt  a    1  2  3  4  5   gt  gt  gt  b    9  8  7  6  5   gt  gt  gt  set a   amp  set b  set  5

User · Answer

Can use itertools product too    gt  gt  gt  common elements     gt  gt  gt  for i in list itertools product a b            if i 0     i 1               common elements append i 0

[python] How can I compare two lists in python and return matches

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