How to convert list of numpy arrays into single numpy array

Question

Suppose I have    LIST     array  1  2  3  4  5    array  1  2  3  4  5   1 2 3 4 5      inner lists are numpy arrays   I try to convert   array   1  2  3  4  5           1  2  3  4  5           1  2  3  4  5     I am solving it by iteration on vstack right now but it is really slow for especially large LIST  What do you suggest for the best efficient way

User · Answer

Starting in NumPy version 1 10  we have the method stack  It can stack arrays of any dimension  all equal      List of arrays  L    np random randn 5 4 2 5 1 2  for i in range 10      Stack them using axis 0  M   np stack L  M shape       10 5 4 2 5 1 2  np all M    L       True  M   np stack L  axis 1  M shape       5 10 4 2 5 1 2  np all M    L       False  Don t Panic     This are all true     np all M   0       L 0        True all np all M   i       L i   for i in range 10        True   Enjoy

User · Answer

I checked some of the methods for speed performance and find that there is no difference  The only difference is that using some methods you must carefully check dimension   Timing    ------------ ---------------- -------------------                 shape  10000      shape  1 10000      ------------ ---------------- -------------------    np concat       0 18280            0 17960         ------------ ---------------- -------------------     np stack       0 21501            0 16465         ------------ ---------------- -------------------    np vstack       0 21501            0 17181         ------------ ---------------- -------------------     np array       0 21656            0 16833         ------------ ---------------- -------------------    As you can see I tried 2 experiments - using np random rand 10000  and np random rand 1  10000  And if we use 2d arrays than np stack and np array create additional dimension - result shape is  1 10000 10000  and  10000 1 10000  so they need additional actions to avoid this   Code   from time import perf counter from tqdm import tqdm notebook import numpy as np l      for i in tqdm notebook range 10000        new np   np random rand 10000      l append new np     start   perf counter   stack   np stack l  axis 0   print f np stack   perf counter   - start  5f     start   perf counter   vstack   np vstack l  print f np vstack   perf counter   - start  5f     start   perf counter   wrap   np array l  print f np array   perf counter   - start  5f     start   perf counter   l    el reshape 1 -1  for el in l  conc   np concatenate l  axis 0   print f np concatenate   perf counter   - start  5f

User · Answer

In general you can concatenate a whole sequence of arrays along any axis   numpy concatenate  LIST  axis 0     but you do have to worry about the shape and dimensionality of each array in the list  for a 2-dimensional 3x5 output  you need to ensure that they are all 2-dimensional n-by-5 arrays already     If you want to concatenate 1-dimensional arrays as the rows of a 2-dimensional output  you need to expand their dimensionality   As Jorge s answer points out  there is also the function stack  introduced in numpy 1 10   numpy stack  LIST  axis 0     This takes the complementary approach  it creates a new view of each input array and adds an extra dimension  in this case  on the left  so each n-element 1D array becomes a 1-by-n 2D array  before concatenating  It will only work if all the input arrays have the same shape   even along the axis of concatenation   vstack  or equivalently row stack  is often an easier-to-use solution because it will take a sequence of 1- and or 2-dimensional arrays and expand the dimensionality automatically where necessary and only where necessary  before concatenating the whole list together  Where a new dimension is required  it is added on the left  Again  you can concatenate a whole list at once without needing to iterate   numpy vstack  LIST     This flexible behavior is also exhibited by the syntactic shortcut numpy r   array1        arrayN    note the square brackets    This is good for concatenating a few explicitly-named arrays but is no good for your situation because this syntax will not accept a sequence of arrays  like your LIST   There is also an analogous function column stack and shortcut c        for horizontal  column-wise  stacking  as well as an almost-analogous function hstack   although for some reason the latter is less flexible  it is stricter about input arrays  dimensionality  and tries to concatenate 1-D arrays end-to-end instead of treating them as columns    Finally  in the specific case of vertical stacking of 1-D arrays  the following also works   numpy array  LIST        because arrays can be constructed out of a sequence of other arrays  adding a new dimension to the beginning

[python] How to convert list of numpy arrays into single numpy array?

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