How do you extract a column from a multi-dimensional array

Question

Does anybody know how to extract a column from a multi-dimensional array in Python

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gt  gt  gt  x   arange 20  reshape 4 5   gt  gt  gt  x array    0   1   2   3   4             5   6   7   8   9            10  11  12  13  14            15  16  17  18  19      if you want the second column you can use   gt  gt  gt  x    1  array   1   6  11  16

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let s say we have n X m matrix n rows and m columns  say 5 rows and 4 columns  matrix     1 2 3 4   5 6 7 8   9 10 11 12   13 14 15 16   17 18 19 20     To extract the columns in python  we can use list comprehension like this     row i  for row in matrix  for in range 4      You can replace 4 by whatever number of columns your matrix has  The result is     1 5 9 13 17   2 10 14 18   3 7 11 15 19   4 8 12 16 20

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If you want to grab more than just one column just use slice       a   np array   1  2  3   4  5  6   7  8  9        print a     1  2      2 3   5 6   8 9

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The itemgetter operator can help too  if you like map-reduce style python  rather than list comprehensions  for a little variety     tested in 2 4 from operator import itemgetter def column matrix i       f   itemgetter i      return map f matrix   M    range x x 5  for x in range 10   assert column M 1     range 1 11

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If you have a two-dimensional array in Python  not numpy   you can extract all the columns like so    data       a   1  2      b   3  4      c   5  6     columns   list zip  data    print  column 0        format columns 0    print  column 1        format columns 1    print  column 2        format columns 2       Executing this code will yield     gt  gt  gt  print  column 0        format columns 0    column 0      a    b    c     gt  gt  gt  print  column 1        format columns 1    column 1     1  3  5    gt  gt  gt  print  column 2        format columns 2    column 2     2  4  6    Of course  you can extract a single column by index  e g  columns 0

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I think you want to extract a column from an array such as an array below  import numpy as np A   np array   1 2 3 4   5 6 7 8   9 10 11 12      Now if you want to get the third column in the format  D array  3    7    11     Then you need to first make the array a matrix  B np asmatrix A  C B   2  D asarray C    And now you can do element wise calculations much like you would do in excel

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You can use this as well   values   np array   1 2 3   4 5 6    values     0    first column   1 4    Note  This is not working for built-in array and not aligned  e g  np array   1 2 3   4 5 6 7

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I prefer the next hint  having the matrix named matrix a and use column number  for example   import numpy as np matrix a   np array   1 2 3 4   5 6 7 8   9 10 11 12    column number 2    you can get the row from transposed matrix - it will be a column  col matrix a transpose   column number

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check it out   a     1  2    2  3    3  4   a2   zip  a  a2 0    it is the same thing as above except somehow it is neater the zip does the work but requires single arrays as arguments  the  a syntax unpacks the multidimensional array into single array arguments

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Well a  bit  late      In case performance matters and your data is shaped rectangular  you might also store it in one dimension and access the columns by regular slicing e g       A     1 2 3 4   5 6 7 8         lt  assume this 4x2-matrix B   reduce  operator add  A     lt  get it one-dimensional  def column1d  matrix  dimX  colIdx      return matrix colIdx  dimX   def row1d  matrix  dimX  rowIdx      return matrix rowIdx rowIdx dimX     gt  gt  gt  column1d  B  4  1    2  6   gt  gt  gt  row1d  B  4  1    2  3  4  5    The neat thing is this is really fast  However  negative indexes don t work here  So you can t access the last column or row by index -1   If you need negative indexing you can tune the accessor-functions a bit  e g   def column1d  matrix  dimX  colIdx      return matrix colIdx   dimX  dimX   def row1d  matrix  dimX  dimY  rowIdx      rowIdx    rowIdx   dimY    dimX   return matrix rowIdx rowIdx dimX

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One more way using matrices   gt  gt  gt  from numpy import matrix  gt  gt  gt  a      1 2 3   4 5 6   7 8 9     gt  gt  gt  matrix a  transpose   1  getA   0  array  2  5  8    gt  gt  gt  matrix a  transpose   0  getA   0  array  1  4  7

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All columns from a matrix into a new list   N   len matrix   column list      matrix row  column  for row in range N   for column in range N

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def get col arr  col       return map lambda x   x col   arr   a     1 2 3 4    5 6 7 8    9 10 11 12   13 14 15 16    print get col a  3    map function in Python is another way to go

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matrix i  column  for i in range len matrix

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Just use transpose    then you can get the colummns as easy as you get rows  matrix np array originalMatrix  transpose   print matrix NumberOfColum

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Despite using zip  iterable  to transpose a nested list  you can also use the following if the nested lists vary in length   map None     1 2 3     4 5     6       results in     1  4  6    2  5  None    3  None  None     The first column is thus   map None     1 2 3     4 5     6     0    gt  1  4  6

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gt  gt  gt  import numpy as np  gt  gt  gt  A   np array   1 2 3 4   5 6 7 8      gt  gt  gt  A array   1  2  3  4        5  6  7  8      gt  gt  gt  A   2    returns the third columm array  3  7     See also   numpy arange  and  reshape  to allocate memory  Example   Allocating a array with shaping of matrix  3x4    nrows   3 ncols   4 my array   numpy arange nrows ncols  dtype  double   my array   my array reshape nrows  ncols

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If you have an array like   a     1  2    2  3    3  4     Then you extract the first column like that    row 0  for row in a    So the result looks like this    1  2  3

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array     1 2 3 4   5 6 7 8   9 10 11 12   13 14 15 16    col1    val 1  for val in array  col2    val 2  for val in array  col3    val 3  for val in array  col4    val 4  for val in array  print col1  print col2  print col3  print col4   Output   1  5  9  13   2  6  10  14   3  7  11  15   4  8  12  16

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Could it be that you re using a NumPy array  Python has the array module  but that does not support multi-dimensional arrays  Normal Python lists are single-dimensional too   However  if you have a simple two-dimensional list like this   A     1 2 3 4         5 6 7 8     then you can extract a column like this   def column matrix  i       return  row i  for row in matrix    Extracting the second column  index 1     gt  gt  gt  column A  1   2  6    Or alternatively  simply    gt  gt  gt   row 1  for row in A   2  6

[python] How do you extract a column from a multi-dimensional array?

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