Transpose list of lists

Question

Let s take   l     1  2  3    4  5  6    7  8  9     The result I m looking for is  r     1  4  7    2  5  8    3  6  9     and not  r     1  4  7    2  5  8    3  6  9     Much appreciated

User · Answer

One way to do it is with NumPy transpose. For a list, a:

>>> import numpy as np
>>> np.array(a).T.tolist()
[[1, 4, 7], [2, 5, 8], [3, 6, 9]]

Or another one without zip:

>>> map(list,map(None,*a))
[[1, 4, 7], [2, 5, 8], [3, 6, 9]]

User · Answer

Three options to choose from   1  Map with Zip  solution1   map list  zip  l     2  List Comprehension  solution2    list i  for i in zip  l     3  For Loop Appending  solution3      for i in zip  l       solution3 append  list i      And to view the results   print  solution1  print  solution2  print  solution3      1  4  7    2  5  8    3  6  9

User · Answer

Import functions from library     from numpy import size  array      Transpose a 2D list     def transpose list 2d list in mat           list out mat              array in mat   array list in mat          array out mat   array in mat T         nb lines   size array out mat  0          for i line out in range 0  nb lines               array out line   array out mat i line out              list out line   list array out line              list out mat append list out line          return list out mat

User · Answer

Here is a solution for transposing a list of lists that is not necessarily square   maxCol   len l 0   for row in l      rowLength   len row      if rowLength  gt  maxCol          maxCol   rowLength lTrans      for colIndex in range maxCol       lTrans append         for row in l          if colIndex  lt  len row               lTrans colIndex  append row colIndex

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matrix     1 2 3              1 2 3              1 2 3              1 2 3              1 2 3              1 2 3              1 2 3        rows   len matrix  cols   len matrix 0    transposed      while len transposed   lt  cols      transposed append         while len transposed -1    lt  rows          transposed -1  append 0   for i in range rows       for j in range cols           transposed j  i    matrix i  j   for i in transposed      print i

User · Answer

just for fun  valid rectangles and assuming that m 0  exists   gt  gt  gt  m     1 2 3   4 5 6   7 8 9    gt  gt  gt    row i  for row in m  for i in range len m 0       1  4  7    2  5  8    3  6  9

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How about map list  zip  l   -- gt    1  4  7    2  5  8    3  6  9    For python 3 x users can use list map list  zip  l      short circuits at shortest nested list if table is jagged list map list  itertools zip longest  l  fillvalue None      discards no data if jagged and fills short nested lists with None  Explanation  There are two things we need to know to understand what s going on   The signature of zip  zip  iterables  This means zip expects an arbitrary number of arguments each of which must be iterable  E g  zip  1  2    3  4    5  6    Unpacked argument lists  Given a sequence of arguments args  f  args  will call f such that each element in args is a separate positional argument of f  itertools zip longest does not discard any data if the number of elements of the nested lists are not the same  homogenous   and instead fills in the shorter nested lists then zips them up   Coming back to the input from the question l     1  2  3    4  5  6    7  8  9    zip  l  would be equivalent to zip  1  2  3    4  5  6    7  8  9    The rest is just making sure the result is a list of lists instead of a list of tuples

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Methods 1 and 2 work in Python 2 or 3  and they work on ragged  rectangular 2D lists  That means the inner lists do not need to have the same lengths as each other  ragged  or as the outer lists  rectangular   The other methods  well  it s complicated  the setup import itertools import six  list list     1 2 3    4 5 6  6 1  6 2  6 3    7 8 9     method 1     map    zip longest    gt  gt  gt  list map list  six moves zip longest  list list  fillvalue  -       1  4  7    2  5  8    3  6  9     -   6 1   -      -   6 2   -      -   6 3   -     six moves zip longest   becomes  itertools izip longest   in Python 2 itertools zip longest   in Python 3  The default fillvalue is None  Thanks to  jena s answer  where map   is changing the inner tuples to lists  Here it is turning iterators into lists  Thanks to  Oregano s and  badp s comments  In Python 3  pass the result through list   to get the same 2D list as method 2   method 2     list comprehension  zip longest    gt  gt  gt   list row  for row in six moves zip longest  list list  fillvalue  -      1  4  7    2  5  8    3  6  9     -   6 1   -      -   6 2   -      -   6 3   -     The  inspectorG4dget alternative   method 3     map   of map       broken in Python 3 6  gt  gt  gt  map list  map None   list list     1  4  7    2  5  8    3  6  9    None  6 1  None    None  6 2  None    None  6 3  None    This extraordinarily compact  SiggyF second alternative works with ragged 2D lists  unlike his first code which uses numpy to transpose and pass through ragged lists  But None has to be the fill value   No  the None passed to the inner map   is not the fill value  It means there is no function to process each column  The columns are just passed through to the outer map   which converts them from tuples to lists   Somewhere in Python 3  map   stopped putting up with all this abuse  the first parameter cannot be None  and ragged iterators are just truncated to the shortest  The other methods still work because this only applies to the inner map     method 4     map   of map   revisited  gt  gt  gt  list map list  map lambda  args  args   list list      1  4  7    2  5  8    3  6  9        Python 2 7   1  4  7    2  5  8    3  6  9    None  6 1  None    None  6 2  None    None  6 3  None      3 6   Alas the ragged rows do NOT become ragged columns in Python 3  they are just truncated  Boo hoo progress

User · Answer

One more way for square matrix  No numpy  nor itertools  use  effective  in-place elements exchange  def transpose m       for i in range 1  len m            for j in range i               m i  j   m j  i    m j  i   m i  j

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Equivalently to Jena s solution    gt  gt  gt  l   1 2 3   4 5 6   7 8 9    gt  gt  gt   list i  for i in zip  l         1  4  7    2  5  8    3  6  9

User · Answer

Maybe not the most elegant solution  but here s a solution using nested while loops   def transpose lst       newlist          i   0     while i  lt  len lst           j   0         colvec              while j  lt  len lst               colvec append lst j  i               j   j   1         newlist append colvec          i   i   1     return newlist

User · Answer

import numpy as np r   list map list  np transpose l

User · Answer

more itertools unzip   is easy to read  and it also works with generators   import more itertools l     1  2  3    4  5  6    7  8  9   r   more itertools unzip l    a tuple of generators  r   list map list  r          a list of lists   or equivalently  import more itertools l   more itertools chunked range 1 10   3  r   more itertools unzip l    a tuple of generators  r   list map list  r          a list of lists

[python] Transpose list of lists

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