Initialise numpy array of unknown length

Question

I want to be able to \'build\' a numpy array on the fly, I do not know the size of this array in advance.

For example I want to do something like this:

Answer 1

Build a Python list and convert that to a Numpy array. That takes amortized O(1) time per append + O(n) for the conversion to array, for a total of O(n).

    a = []
    for x in y:
        a.append(x)
    a = np.array(a)