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

Since y is an iterable I really do not see why the calls to append:

a = np.array(list(y))

will do and it's much faster:

import timeit

print timeit.timeit('list(s)', 's=set(x for x in xrange(1000))')
# 23.952975494633154

print timeit.timeit("""li=[]
for x in s: li.append(x)""", 's=set(x for x in xrange(1000))')
# 189.3826994248866