Append Numpy arrays

Question

I have the following problem. I want to extend a numpy array in a loop, so that each array is seperated from the next like a=[[1,2,3,4,5],[1,2,3,4,5]],b=[[1,2,3,4,5],[1,2,3,4,5]]-->[[[1,2,3,4,5],[1,2,3,4,5]],[[1,2,3,4,5],[1,2,3,4,5]]]

My approach so far:

count=0
for i in range(int(max(allCoo[:,4]))+1):

        mask1 = allCoo[:,4] == count 
        if count>0:
            trackList=np.vstack((trackList,np.array((allCoo[mask1]))))
        else:
            trackList=np.array((allCoo[mask1]))

        count+=1

But this just give me things like: [[1,2,3,4,5],[1,2,3,4,5],[1,2,3,4,5],[1,2,3,4,5]]

Best regards

Collect the arrays in a list, or list of lists, and do the stack/concatenate once at the end. — hpaulj
– hpaulj, Commented Apr 21, 2017 at 10:10

Daniel F · Accepted Answer · 2017-04-21 08:39:25Z

2

You want to use dstack instead of vstack if you want a new dimension

p.dstack([a,b]).swapaxes(1,2)

array([[[1, 2, 3, 4, 5],
        [1, 2, 3, 4, 5]],

       [[1, 2, 3, 4, 5],
        [1, 2, 3, 4, 5]]])

np.vstack([a,b])

array([[1, 2, 3, 4, 5],
       [1, 2, 3, 4, 5],
       [1, 2, 3, 4, 5],
       [1, 2, 3, 4, 5]])

answered Apr 21, 2017 at 8:39

Daniel F

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3 Comments

Varlor Over a year ago

Nice thank you. But what is for arrays with different array dumensions like np.dstack(([[[1,2,3,4,5],[1,2,3,4,5],[1,2,3,4,5]],[[1,2,3,4,5],[1,2,3,4,5]]])).swapaxes(1,2). Here i get the error: ValueError: all the input array dimensions except for the concatenation axis must match exactly

Daniel F Over a year ago

That output would be a ragged array (since the second dimension would not be fixed) and thus wouldn't be a valid numpy array. You'd be better off making a list of arrays then.

hpaulj Over a year ago

np.stack is a newer function with more flexibility in adding that new dimension.

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