# python – Initialise numpy array of unknown length-ThrowExceptions

Exception or error:

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:

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

Which would result in a containing all the elements of x, obviously this is a trivial answer. I am just curious whether this is possible?

How to solve:

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)
``````

You can do this:

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

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
``````

For posterity, I think this is quicker:

``````a = np.array([np.array(list()) for _ in y])
``````

You might even be able to pass in a generator (i.e. [] -> ()), in which case the inner list is never fully stored in memory.

Responding to comment below:

``````>>> import numpy as np
>>> y = range(10)
>>> a = np.array([np.array(list) for _ in y])
>>> a
array([array(<type 'list'>, dtype=object),
array(<type 'list'>, dtype=object),
array(<type 'list'>, dtype=object),
array(<type 'list'>, dtype=object),
array(<type 'list'>, dtype=object),
array(<type 'list'>, dtype=object),
array(<type 'list'>, dtype=object),
array(<type 'list'>, dtype=object),
array(<type 'list'>, dtype=object),
array(<type 'list'>, dtype=object)], dtype=object)
``````

``````a = np.empty(0)