List of lists into numpy array

Question

How do I convert a simple list of lists into a numpy array  The rows are individual sublists and each row contains the elements in the sublist

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Again  after searching for the problem of converting nested lists with N levels into an N-dimensional array I found nothing  so here s my way around it   import numpy as np  new array np array    coord for coord in xk  for xk in xj  for xj in xi   ndmin 3   this case for N 3

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As this is the top search on Google for converting a list of lists into a Numpy array  I ll offer the following despite the question being 4 years old    gt  gt  gt  x     1  2    1  2  3    1    gt  gt  gt  y   numpy hstack x   gt  gt  gt  print y   1 2 1 2 3 1    When I first thought of doing it this way  I was quite pleased with myself because it s soooo simple   However  after timing it with a larger list of lists  it is actually faster to do this    gt  gt  gt  y   numpy concatenate  numpy array i  for i in x    gt  gt  gt  print y   1 2 1 2 3 1    Note that  Bastiaan s answer  1 doesn t make a single continuous list  hence I added the concatenate      Anyway   I prefer the hstack approach for it s elegant use of Numpy

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It s as simple as    gt  gt  gt  lists     1  2    3  4    gt  gt  gt  np array lists  array   1  2           3  4

User · Answer

If your list of lists contains lists with varying number of elements then the answer of Ignacio Vazquez-Abrams will not work  Instead there are at least 3 options   1  Make an array of arrays   x   1 2   1 2 3   1   y numpy array  numpy array xi  for xi in x   type y   gt  gt  gt  lt type  numpy ndarray  gt  type y 0    gt  gt  gt  lt type  numpy ndarray  gt    2  Make an array of lists   x   1 2   1 2 3   1   y numpy array x  type y   gt  gt  gt  lt type  numpy ndarray  gt  type y 0    gt  gt  gt  lt type  list  gt    3  First make the lists equal in length   x   1 2   1 2 3   1   length   max map len  x   y numpy array  xi  None   length-len xi   for xi in x   y  gt  gt  gt array   1  2  None    gt  gt  gt         1  2  3    gt  gt  gt         1  None  None    dtype object

User · Answer

Just use pandas  list pd DataFrame listofstuff  melt   values    this only works for a list of lists  if you have a list of list of lists you might want to try something along the lines of  lists pd DataFrame listofstuff  melt   apply pd Series  melt   values

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I had a list of lists of equal length  Even then Ignacio Vazquez-Abrams s answer didn t work out for me  I got a 1-D numpy array whose elements are lists  If you faced the same problem  you can use the below method  Use numpy vstack  import numpy as np  np array   np empty  0 4   dtype  float   for i in range 10       row data           get row data as list      np array   np vstack  np array  np array row data

User · Answer

gt  gt  gt  numpy array   1  2    3  4     array   1  2    3  4

[python] List of lists into numpy array

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