initialize a numpy array

Is there way to initialize a numpy array of a shape and add to it? I will explain what I need with a list example. If I want to create a list of objects generated in a loop, I can do:

a = []
for i in range(5):
    a.append(i)

I want to do something similar with a numpy array. I know about vstack, concatenate etc. However, it seems these require two numpy arrays as inputs. What I need is:

big_array # Initially empty. This is where I don't know what to specify
for i in range(5):
    array i of shape = (2,4) created.
    add to big_array

The big_array should have a shape (10,4). How to do this?

EDIT:

I want to add the following clarification. I am aware that I can define big_array = numpy.zeros((10,4)) and then fill it up. However, this requires specifying the size of big_array in advance. I know the size in this case, but what if I do not? When we use the .append function for extending the list in python, we don't need to know its final size in advance. I am wondering if something similar exists for creating a bigger array from smaller arrays, starting with an empty array.

Solution 1:

numpy.zeros

Return a new array of given shape and type, filled with zeros.

or

numpy.ones

Return a new array of given shape and type, filled with ones.

or

numpy.empty

Return a new array of given shape and type, without initializing entries.

However, the mentality in which we construct an array by appending elements to a list is not much used in numpy, because it's less efficient (numpy datatypes are much closer to the underlying C arrays). Instead, you should preallocate the array to the size that you need it to be, and then fill in the rows. You can use numpy.append if you must, though.

Solution 2:

The way I usually do that is by creating a regular list, then append my stuff into it, and finally transform the list to a numpy array as follows :

import numpy as np
big_array = [] #  empty regular list
for i in range(5):
    arr = i*np.ones((2,4)) # for instance
    big_array.append(arr)
big_np_array = np.array(big_array)  # transformed to a numpy array

of course your final object takes twice the space in the memory at the creation step, but appending on python list is very fast, and creation using np.array() also.

Solution 3:

Introduced in numpy 1.8:

numpy.full

Return a new array of given shape and type, filled with fill_value.

Examples:

>>> import numpy as np
>>> np.full((2, 2), np.inf)
array([[ inf,  inf],
       [ inf,  inf]])
>>> np.full((2, 2), 10)
array([[10, 10],
       [10, 10]])