Converting a list to a set changes element order

Question

Recently I noticed that when I am converting a list to set the order of elements is changed and is sorted by character   Consider this example   x  1 2 20 6 210  print x     1  2  20  6  210    the order is same as initial order  set x    set  1  2  20  210  6     in the set x  output order is sorted   My questions are -   Why is this happening  How can I do set operations  especially Set Difference  without losing the initial order

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Here s an easy way to do it   x  1 2 20 6 210  print sorted set x

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It s interesting that people always use  real world problem  to make joke on the definition in theoretical science    If set has order  you first need to figure out the following problems  If your list has duplicate elements  what should the order be when you turn it into a set  What is the order if we union two sets  What is the order if we intersect two sets with different order on the same elements   Plus  set is much faster in searching for a particular key which is very good in sets operation  and that s why you need a set  but not list    If you really care about the index  just keep it as a list  If you still want to do set operation on the elements in many lists  the simplest way is creating a dictionary for each list with the same keys in the set along with a value of list containing all the index of the key in the original list   def indx dic l       dic          for i in range len l            if l i  in dic              dic get l i   append i          else              dic l i      i      return dic   a    1 2 3 4 5 1 3 2  set a    set a  dic a   indx dic a   print dic a     1   0  5   2   1  7   3   2  6   4   3   5   4   print set a     1  2  3  4  5

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An implementation of the highest score concept above that brings it back to a list   def SetOfListInOrder incominglist       from collections import OrderedDict     outtemp   OrderedDict       for item in incominglist          outtemp item    None     return list outtemp     Tested  briefly  on Python 3 6 and Python 2 7

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A set is an unordered data structure  so it does not preserve the insertion order  This depends on your requirements  If you have an normal list  and want to remove some set of elements while preserving the order of the list  you can do this with a list comprehension    gt  gt  gt  a    1  2  20  6  210   gt  gt  gt  b   set  6  20  1    gt  gt  gt   x for x in a if x not in b   2  210    If you need a data structure that supports both fast membership tests and preservation of insertion order  you can use the keys of a Python dictionary  which starting from Python 3 7 is guaranteed to preserve the insertion order    gt  gt  gt  a   dict fromkeys  1  2  20  6  210    gt  gt  gt  b   dict fromkeys  6  20  1    gt  gt  gt  dict fromkeys x for x in a if x not in b   2  None  210  None    b doesn t really need to be ordered here     you could use a set as well  Note that a keys   - b keys   returns the set difference as a set  so it won t preserve the insertion order   In older versions of Python  you can use collections OrderedDict instead    gt  gt  gt  a   collections OrderedDict fromkeys  1  2  20  6  210    gt  gt  gt  b   collections OrderedDict fromkeys  6  20  1    gt  gt  gt  collections OrderedDict fromkeys x for x in a if x not in b  OrderedDict   2  None    210  None

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In Python 3 6  set   now should keep the order  but there is another solution for Python 2 and 3    gt  gt  gt  x    1  2  20  6  210   gt  gt  gt  sorted set x   key x index   1  2  20  6  210

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As denoted in other answers  sets are data structures   and mathematical concepts  that do not preserve the element order -   However  by using a combination of sets and dictionaries  it is possible that you can achieve wathever you want - try using these snippets     save the element order in a dict  x dict   dict x y for y  x in enumerate my list    x set   set my list   perform desired set operations      retrieve ordered list from the set  new list    None    len new set  for element in new set     new list x dict element     element

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In case you have a small number of elements in your two initial lists on which you want to do set difference operation  instead of using collections OrderedDict which complicates the implementation and makes it less readable  you can use     initial lists on which you want to do set difference  gt  gt  gt  nums    1 2 2 3 3 4 4 5   gt  gt  gt  evens    2 4 4 6   gt  gt  gt  evens set   set evens   gt  gt  gt  result       gt  gt  gt  for n in nums        if not n in evens set and not n in result          result append n        gt  gt  gt  result  1  3  5     Its time complexity is not that good but it is neat and easy to read

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Building on Sven s answer  I found using collections OrderedDict like so helped me accomplish what you want plus allow me to add more items to the dict   import collections  x  1 2 20 6 210  z collections OrderedDict fromkeys x  z OrderedDict   1  None    2  None    20  None    6  None    210  None      If you want to add items but still treat it like a set you can just do   z  nextitem   None   And you can perform an operation like z keys   on the dict and get the set   z keys    1  2  20  6  210

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Remove duplicates and preserve order by below function def unique sequence       seen   set       return  x for x in sequence if not  x in seen or seen add x     How to remove duplicates from a list while preserving order in Python

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In mathematics  there are sets and ordered sets  osets     set  an unordered container of unique elements  Implemented  oset  an ordered container of unique elements  NotImplemented    In Python  only sets are directly implemented  We can emulate osets  with regular dict keys  3 7     Given  a    1  2  20  6  210  2  1  b    2  6    Code  oset   dict fromkeys a  keys     dict keys  1  2  20  6  210     Demo  Replicates are removed  insertion-order is preserved   list oset     1  2  20  6  210    Set-like operations on dict keys   oset - b    1  20  210   oset   b    1  2  5  6  20  210   oset  amp  b    2  6   oset   b    1  5  20  210      Details  Note  an unordered structure does not preclude ordered elements   Rather  maintained order is not guaranteed   Example    assert  1  2  3      2  3  1                       sets  order is ignored      assert  1  2  3      2  3  1                       lists  order is guaranteed    One may be pleased to discover that a list and multiset  mset  are two more fascinating  mathematical data structures    list  an ordered container of elements that permits replicates  Implemented  mset  an unordered container of elements that permits replicates  NotImplemented     Summary  Container   Ordered   Unique   Implemented ---------- --------- -------- ------------ set            n         y         y oset           y         y         n list           y         n         y mset           n         n         n       A multiset can be indirectly emulated with collections Counter    a dict-like mapping of multiplicities  counts

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Answering your first question  a set is a data structure optimized for set operations  Like a mathematical set  it does not enforce or maintain any particular order of the elements  The abstract concept of a set does not enforce order  so the implementation is not required to  When you create a set from a list  Python has the liberty to change the order of the elements for the needs of the internal implementation it uses for a set  which is able to perform set operations efficiently

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You can remove the duplicated values and keep the list order of insertion with one line of code  Python 3 8 2 mylist     b    b    a    d    d    c     results   list  value    for value in mylist    print results         b    a    d    c    results   list dict fromkeys mylist    print results         b    a    d    c

[python] Converting a list to a set changes element order

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