How can I reorder a list

Question

If I have a list  a b c d e  how can I reorder the items in an arbitrary manner like  d c a b e    Edit  I don t want to shuffle them  I want to re-order them in a predefined manner   for example  I know that the 3rd element in the old list should become the first element in the new list

User · Answer

gt  gt  gt  import random  gt  gt  gt  x    1 2 3 4 5   gt  gt  gt  random shuffle x   gt  gt  gt  x  5  2  4  3  1

User · Answer

You can do it like this  mylist     a    b    c    d    e   myorder    3  2  0  1  4  mylist    mylist i  for i in myorder  print mylist            prints    d    c    a    b    e

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One more thing which can be considered is the other interpretation as pointed out by darkless  Code in Python 2 7  Mainly    Reorder by value - Already solved by AJ above Reorder by index  mylist     a    b    c    d    e   myorder    3  2  0  1  4   mylist   sorted zip mylist  myorder   key lambda x  x 1   print  item 0  for item in mylist     This will print   c    d    b    a    e

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gt  gt  gt  a   a   b   c   d   e    gt  gt  gt  a 0  a 3    a 3  a 0   gt  gt  gt  a   d    b    c    a    e

User · Answer

gt  gt  gt  a    1  2  3   gt  gt  gt  a 0   a 2    a 2   a 0   gt  gt  gt  a  3  2  1

User · Answer

Is the final order defined by a list of indices     gt  gt  gt  items    1  None   chicken   int   gt  gt  gt  order    3  0  1  2    gt  gt  gt  ordered list    items i  for i in order   gt  gt  gt  ordered list   lt type  int  gt   1  None   chicken     edit  meh  AJ was faster    How can I reorder a list in python

User · Answer

If you use numpy there s a neat way to do it   items   np array   a   b   c   d    indices   np arange items shape 0   np random shuffle indices  print indices  print items indices     This code returns    1 3 2 0    b   d   c   a

User · Answer

From what I understand of your question  it appears that you want to apply a permutation that you specify on a list  This is done by specifying another list  lets call it p  that holds the indices of the elements of the original list that should appear in the permuted list  You then use p to make a new list by simply substituting the element at each position by that whose index is in that position in p   def apply permutation lst  p       return  lst x  for x in p   arr list  abcde   new order  3 2 0 1 4   print apply permutation arr new order    This prints   d    c    a    b    e     This actually creates a new list  but it can be trivially modified to permute the original  in place

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This is what I used when I stumbled upon this problem    def order list item  i     reorder at index i     order at   list item index i      ordered list   list item order at     list item  order at      return ordered list   EX  for the the lowercase letters  order string ascii lowercase   h     gt  gt  gt   hijklmnopqrstuvwxyzabcdefg    It simply just shifts the list to a specified index

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newList    oldList 3   newList extend oldList  3   newList extend oldList 4

User · Answer

You can provide your own sort function to list sort     The sort   method takes optional arguments for controlling the comparisons   cmp specifies a custom comparison function of two arguments  list items  which should return a negative  zero or positive number depending on whether the first argument is considered smaller than  equal to  or larger than the second argument  cmp lambda x y  cmp x lower    y lower     The default value is None   key specifies a function of one argument that is used to extract a comparison key from each list element  key str lower  The default value is None   reverse is a boolean value  If set to True  then the list elements are sorted as if each comparison were reversed    In general  the key and reverse conversion processes are much faster than specifying an equivalent cmp function  This is because cmp is called multiple times for each list element while key and reverse touch each element only once

User · Answer

If you do not care so much about efficiency  you could rely on numpy s array indexing to make it elegant   a     123    abc   456  order    2  0  1  a2   list  np array a  dtype object  order

[python] How can I reorder a list?

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