bli bli - 1 month ago 5
Python Question

How to make a numpy array from an array of arrays?

I'm experimenting in

ipython3
, where I created an array of arrays:

In [105]: counts_array
Out[105]:
array([array([ 17, 59, 320, ..., 1, 7, 0], dtype=uint32),
array([ 30, 71, 390, ..., 12, 20, 6], dtype=uint32),
array([ 7, 145, 214, ..., 4, 12, 0], dtype=uint32),
array([ 23, 346, 381, ..., 15, 19, 5], dtype=uint32),
array([ 51, 78, 270, ..., 3, 0, 2], dtype=uint32),
array([212, 149, 511, ..., 19, 31, 8], dtype=uint32)], dtype=object)

In [106]: counts_array.shape
Out[106]: (6,)

In [107]: counts_array[0].shape
Out[107]: (1590,)


I would like to obtain a plain
shape=(6, 1590), dtype=uint32
array from this monster I created.

How can I do that?

Answer

You can use np.vstack -

np.vstack(counts_array)

Another way with np.concatenate -

np.concatenate(counts_array).reshape(len(counts_array),-1)

Sample run -

In [23]: a
Out[23]: 
array([array([68, 92, 84, 35, 14, 71, 55, 40, 21, 41]),
       array([30, 90, 52, 64, 86, 68, 61, 85, 26, 98]),
       array([98, 64, 23, 49, 13, 17, 52, 96, 97, 19]),
       array([54, 26, 25, 22, 95, 77, 20, 73, 22, 80]),
       array([15, 84, 91, 54, 25, 21, 37, 19, 25, 25]),
       array([87, 17, 49, 74, 11, 34, 27, 23, 22, 83])], dtype=object)

In [24]: np.vstack(a)
Out[24]: 
array([[68, 92, 84, 35, 14, 71, 55, 40, 21, 41],
       [30, 90, 52, 64, 86, 68, 61, 85, 26, 98],
       [98, 64, 23, 49, 13, 17, 52, 96, 97, 19],
       [54, 26, 25, 22, 95, 77, 20, 73, 22, 80],
       [15, 84, 91, 54, 25, 21, 37, 19, 25, 25],
       [87, 17, 49, 74, 11, 34, 27, 23, 22, 83]])