How to add a new row to an empty numpy array

Question

Using standard Python arrays  I can do the following   arr      arr append  1 2 3   arr append  4 5 6     arr is now   1 2 3   4 5 6     However  I cannot do the same thing in numpy  For example   arr   np array     arr   np append arr  np array  1 2 3    arr   np append arr  np array  4 5 6      arr is now  1 2 3 4 5 6    I also looked into vstack  but when I use vstack on an empty array  I get   ValueError  all the input array dimensions except for the concatenation axis must match exactly   So how do I do append a new row to an empty array in numpy

User · Answer

In this case you might want to use the functions np.hstack and np.vstack

arr = np.array([])
arr = np.hstack((arr, np.array([1,2,3])))
# arr is now [1,2,3]

arr = np.vstack((arr, np.array([4,5,6])))
# arr is now [[1,2,3],[4,5,6]]

You also can use the np.concatenate function.

Cheers

User · Answer

The way to  start  the array that you want is   arr   np empty  0 3   int    Which is an empty array but it has the proper dimensionality    gt  gt  gt  arr array     shape  0  3   dtype int64    Then be sure to append along axis 0   arr   np append arr  np array   1 2 3     axis 0  arr   np append arr  np array   4 5 6     axis 0    But   jonrsharpe is right   In fact  if you re going to be appending in a loop  it would be much faster to append to a list as in your first example  then convert to a numpy array at the end  since you re really not using numpy as intended during the loop   In  210     timeit           l                for i in xrange 1000                 l append  3 i 1 3 i 2 3 i 3             l   np asarray l             1000 loops  best of 3  1 18 ms per loop  In  211     timeit           a   np empty  0 3   int            for i in xrange 1000                 a   np append a  3 i np array   1 2 3     0             100 loops  best of 3  18 5 ms per loop  In  214   np allclose a  l  Out 214   True   The numpythonic way to do it depends on your application  but it would be more like   In  220   timeit n   np arange 1 3001  reshape 1000 3  100000 loops  best of 3  5 93   s per loop  In  221   np allclose a  n  Out 221   True

User · Answer

I want to do a for loop  yet with askewchan s method it does not work well  so I have modified it  x   np empty  0 3   y   np array  1 2 3   for i in         x   np vstack  x y

User · Answer

using an custom dtype definition  what worked for me was   import numpy    define custom dtype type1   numpy dtype    freq   numpy float64  1     amplitude   numpy float64  1      declare empty array  zero rows but one column arr   numpy empty  0 1  dtype type1    store row data  maybe inside a loop row   numpy array   0 0001  0 002    dtype type1    append row to the main array arr   numpy row stack  arr  row     print values stored in the row 0 print float arr 0   freq    print float arr 0   amplitude

User · Answer

Here is my solution   arr      arr append  1 2 3   arr append  4 5 6   np arr   np array arr

User · Answer

In case of adding new rows for array in loop  Assign the array directly for firsttime in loop instead of initialising an empty array   for i in range 0 len 0 100        SOMECALCULATEDARRAY               if i  0           finalArrayCollection   SOMECALCULATEDARRAY     else          finalArrayCollection   np vstack finalArrayCollection SOMECALCULATEDARRAY    This is mainly useful when the shape of the array is unknown

[python] How to add a new row to an empty numpy array

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