Understanding the set function

Question

In python  set   is an unordered collection with no duplicate elements  However  I am not able to understand how it generates the output    For example  consider the following     gt  gt  gt  x    1  1  2  2  2  2  2  3  3   gt  gt  gt  set x  set  1  2  3     gt  gt  gt  y    1  1  6  6  6  6  6  8  8   gt  gt  gt  set y  set  8  1  6     gt  gt  gt  z    1  1  6  6  6  6  6  7  7   gt  gt  gt  set z  set  1  6  7     Shouldn t the output of set y  be  set  1  6  8    I tried the above two in Python 2 6

User · Accepted Answer

Sets are unordered  as you say  Even though one way to implement sets is using a tree  they can also be implemented using a hash table  meaning getting the keys in sorted order may not be that trivial     If you d like to sort them  you can simply perform   sorted set y     which will produce a sorted list containing the set s elements   Not a set  Again  sets are unordered    Otherwise  the only thing guaranteed by set is that it makes the elements unique  nothing will be there more than once     Hope this helps

User · Answer

As an unordered collection type  set  8  1  6   is equivalent to set  1  6  8     While it might be nicer to display the set contents in sorted order  that would make the repr   call more expensive   Internally  the set type is implemented using a hash table  a hash function is used to separate items into a number of buckets to reduce the number of equality operations needed to check if an item is part of the set   To produce the repr   output it just outputs the items from each bucket in turn  which is unlikely to be the sorted order

User · Answer

Python s sets  and dictionaries  will iterate and print out in some order  but exactly what that order will be is arbitrary  and not guaranteed to remain the same after additions and removals   Here s an example of a set changing order after a lot of values are added and then removed    gt  gt  gt  s   set  1 6 8    gt  gt  gt  print s   8  1  6   gt  gt  gt  s update range 10 100000    gt  gt  gt  for v in range 10  100000       s remove v   gt  gt  gt  print s   1  6  8    This is implementation dependent though  and so you should not rely upon it

User · Answer

As  Volatility and yourself pointed out  sets are unordered  If you need the elements to be in order  just call sorted on the set    gt  gt  gt  y    1  1  6  6  6  6  6  8  8   gt  gt  gt  sorted set y    1  6  8

User · Answer

After reading the other answers  I still had trouble understanding why the set comes out un-ordered   Mentioned this to my partner and he came up with this metaphor  take marbles  You put them in a tube a tad wider than marble width   you have a list  A set  however  is a bag  Even though you feed the marbles one-by-one into the bag  when you pour them from a bag back into the tube  they will not be in the same order  because they got all mixed up in a bag

[python] Understanding the set() function

Examples related to python

Examples related to set