Combining two lists and removing duplicates without removing duplicates in original list

Question

I have two lists that i need to combine where the second list has any duplicates of the first list ignored     A bit hard to explain  so let me show an example of what the code looks like  and what i want as a result   first list    1  2  2  5   second list    2  5  7  9     The result of combining the two lists should result in this list  resulting list    1  2  2  5  7  9    You ll notice that the result has the first list  including its two  2  values  but the fact that second list also has an additional 2 and 5 value is not added to the first list   Normally for something like this i would use sets  but a set on first list would purge the duplicate values it already has  So i m simply wondering what the best fastest way to achieve this desired combination   Thanks

User · Answer

Based on the recipe        resulting list   list set   union first list  second list

User · Answer

resulting list   first list    i for i in second list if i not in first list

User · Answer

resulting list   list first list  resulting list extend x for x in second list if x not in resulting list

User · Answer

You can also combine RichieHindle s and Ned Batchelder s responses for an average-case O m n  algorithm that preserves order   first list    1  2  2  5  second list    2  5  7  9   fs   set first list  resulting list   first list    x for x in second list if x not in fs   assert resulting list     1  2  2  5  7  9     Note that x in s has a worst-case complexity of O m   so the worst-case complexity of this code is still O m n

User · Answer

You can bring this down to one single line of code if you use numpy   a    1 2 3 4 5 6 7  b    2 4 7 8 9 10 11 12   sorted np unique a b     gt  gt  gt   1 2 3 4 5 6 7 8 9 10 11 12

User · Answer

first list    1  2  2  5      second list    2  5  7  9       newList        for i in first list          newList append i      for z in second list          if z not in newList              newList append z      newList sort       print newList    1  2  2  5  7  9

User · Answer

You can use sets   first list    1  2  2  5  second list    2  5  7  9   resultList  list set first list    set second list    print resultList    Results in   resultList    1 2 5 7 9

User · Answer

Simplest to me is   first list    1  2  2  5  second list    2  5  7  9   merged list   list set first list second list   print merged list    prints  1  2  5  7  9

User · Answer

You need to append to the first list those elements of the second list that aren t in the first - sets are the easiest way of determining which elements they are  like this   first list    1  2  2  5  second list    2  5  7  9   in first   set first list  in second   set second list   in second but not in first   in second - in first  result   first list   list in second but not in first  print result     Prints  1  2  2  5  9  7    Or if you prefer one-liners 8-   print first list   list set second list  - set first list

User · Answer

first list    1  2  2  5  second list    2  5  7  9   print  set  first list   second list

User · Answer

This might help     def union a b       for e in b          if e not in a              a append e    The union function merges the second list into first  with out duplicating an element of a  if it s already in a  Similar to set union operator  This function does not change b  If a  1 2 3  b  2 3 4   After union a b  makes a  1 2 3 4  and b  2 3 4

[python] Combining two lists and removing duplicates, without removing duplicates in original list

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