Group list by values

Question

Let s say I have a list like this  mylist      quot A quot  0     quot B quot  1     quot C quot  0     quot D quot  2     quot E quot  2    How can I most elegantly group this to get this list output in Python     quot A quot    quot C quot      quot B quot      quot D quot    quot E quot     So the values are grouped by the secound value but the order is preserved

User · Answer

Howard s answer is concise and elegant  but it s also O n 2  in the worst case  For large lists with large numbers of grouping key values  you ll want to sort the list first and then use itertools groupby     gt  gt  gt  from itertools import groupby  gt  gt  gt  from operator import itemgetter  gt  gt  gt  seq      A  0     B  1     C  0     D  2     E  2    gt  gt  gt  seq sort key   itemgetter 1    gt  gt  gt  groups   groupby seq  itemgetter 1    gt  gt  gt    item 0  for item in data  for  key  data  in groups     A    C      B      D    E      Edit   I changed this after seeing eyequem s answer   itemgetter 1  is nicer than lambda x  x 1

User · Answer

values   set map lambda x x 1   mylist   newlist     y 0  for y in mylist if y 1   x  for x in values

User · Answer

gt  gt  gt  import collections  gt  gt  gt  D1   collections defaultdict list   gt  gt  gt  for element in L1          D1 element 1   append element 0         gt  gt  gt  L2   D1 values    gt  gt  gt  print L2    A    C      B      D    E     gt  gt  gt

User · Answer

len   max key for  item  key  in list  newlist       for i in range len 1   for item key in list    newlist key  append item    You can do it in a single list comprehension  perhaps more elegant but O n  2      item for  item key  in list if key  i  for i in range max key for  item key  in list  1

User · Answer

from operator import itemgetter from itertools import groupby  lki      A  0     B  1     C  0     D  2     E  2   lki sort key itemgetter 1    glo     x for x y in g         for k g in  groupby lki key itemgetter 1     print glo      EDIT  Another solution that needs no import   is more readable  keeps the orders  and is 22   shorter than the preceding one   oldlist      A  0     B  1     C  0     D  2     E  2    newlist  dicpos         for val k in oldlist      if k in dicpos          newlist dicpos k   extend val      else          newlist append  val           dicpos k    len dicpos   print newlist

User · Answer

I don t know about elegant  but it s certainly doable   oldlist      A  0     B  1     C  0     D  2     E  2     change into  list      A    C      B      D    E     order    dic dict   for value key in oldlist    try      dic key  append value    except KeyError      order append key      dic key   value  newlist map dic get  order   print newlist   This preserves the order of the first occurence of each key  as well as the order of items for each key  It requires the key to be hashable  but does not otherwise assign meaning to it

User · Answer

gt  gt  gt  xs      A  0     B  1     C  0     D  2     E  2    gt  gt  gt  xs sort key lambda x  x 1    gt  gt  gt  reduce lambda l  x   l append  x   if l -1  0  1     x 1  else l -1  append x   or l  xs 1      xs 0     if xs else        A   0     C   0       B   1       D   2     E   2      Basically  if the list is sorted  it is possible to reduce by looking at the last group constructed by the previous steps - you can tell if you need to start a new group  or modify an existing group  The     or l bit is a trick that enables us to use lambda in Python   append returns None  It is always better to return something more useful than None  but  alas  such is Python

[python] Group list by values

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