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)
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)