CODEWITHSUNDEEP
pythonnumpy
sachinruk
sachinruk

Reputation: 9869

ndarray concatenate error

I wish to concatenate the following arrays:

a=np.array([[1,2],[1],[2,3,4]])    
b=np.array([[20,2]])    
np.concatenate((a,b),axis=0)

but I get the following error:

ValueError                                Traceback (most recent call last)
<ipython-input-40-42253341965b> in <module>()
----> 1 np.concatenate((a,b),axis=0)

ValueError: all the input arrays must have same number of dimensions

I was expecting the answer to be [[1,2],[1],[2,3,4],[20,2]]. If b=np.array([20,2]) instead the concatenation works fine except I get the answer: [[1,2],[1],[2,3,4],20,2]

Upvotes: 1

Views: 74

Answers (1)

user707650
user707650

Reputation:

Check the dtype, ndim and shape of a: you'll find that those are numpy.object, 1 and (3,), respectively. This is because array a contains lists of different lengths, so each list is treated as an object, and a is a one dimensional array of objects. I don't know what you are aiming for, but if you want a to have an ndim of 2, you'll need to make sure all the lists have the same length.

Of course, b has an ndim of 2, since it contains only one (nested) lists, which will always result in a regular n-dimensional array.

The error message is then obvious: you're trying to concatenate two arrays with different dimensions: that won't work.

To get the answer you're seeking, [[1,2],[1],[2,3,4],[20,2]], you'll need to convert the inner list of b into an object as well: then you're concatenating two 1 dimensional arrays of objects.

Upvotes: 3

Related Questions