What is the preferred way to preallocate NumPy arrays?

Question

I am new to NumPy/SciPy. From the documentation, it seems more efficient to preallocate a single array rather than call append/insert/concatenate.

For example, to add a column of 1's to an array, i think that this:

ar0 = np.linspace(10, 20, 16).reshape(4, 4) ar0[:,-1] = np.ones_like(ar0[:,0]) 

is preferred to this:

ar0 = np.linspace(10, 20, 12).reshape(4, 3) ar0 = np.insert(ar0, ar0.shape[1], np.ones_like(ar0[:,0]), axis=1) 

my first question is whether this is correct (that the first is better), and my second question is, at the moment, I am just preallocating my arrays like this (which I noticed in several of the Cookbook examples on the SciPy Site):

np.zeros((8,5)) 

what is the 'NumPy-preferred' way to do this?

Answer 1

Preallocation mallocs all the memory you need in one call, while resizing the array (through calls to append,insert,concatenate or resize) may require copying the array to a larger block of memory. So you are correct, preallocation is preferred over (and should be faster than) resizing.

There are a number of "preferred" ways to preallocate numpy arrays depending on what you want to create. There is np.zeros, np.ones, np.empty, np.zeros_like, np.ones_like, and np.empty_like, and many others that create useful arrays such as np.linspace, and np.arange.

So

ar0 = np.linspace(10, 20, 16).reshape(4, 4) 

is just fine if this comes closest to the ar0 you desire.

However, to make the last column all 1's, I think the preferred way would be to just say

ar0[:,-1]=1 

Since the shape of ar0[:,-1] is (4,), the 1 is broadcasted to match this shape.