访问Numpy数组中四个元素的最快方法？

Question

Suppose I have a Numpy array, such as

rand = np.random.randn(6, 6)

I need the central four values in the array, since it has axes of even length. If it had been odd, such as 5 by 5, then there would only be one central value. What is the simplest/fastest/easiest way of retrieving these four entries? I can obtain them very crudely with indices, but I'm looking for a faster way than calling a bunch of functions and performing a bunch of calculations.

For example, consider the following:

array([[ 0.25659355, -0.75456113,  0.39467396,  0.50805361],
       [-0.77218172,  1.00016061, -0.70389486,  1.67632146],
       [-0.41106158, -0.63757421,  1.70390504, -0.79073362],
       [-0.2016959 ,  0.55316318, -1.55280823,  0.45740193]])

I want the following:

array([[1.00016061, -0.70389486],
       [-0.63757421,  1.70390504]])

But not just for a 4 by 4 array - if it is even by even, I want the central four elements, as above.

Answer 1

Is something like this too complicated?

def get_middle(arr):
    n = arr.shape[0] / 2.0
    n_int = int(n)
    if n % 2 == 1:
        return arr[[n_int], [n_int]]
    else:
        return arr[n_int:n_int + 2, n_int:n_int + 2]

Answer 2

You can do this with a single slicing operation:

rand = np.random.randn(n,n)
# assuming n is even
center = rand[n/2-1:n/2+1, n/2-1:n/2+1]

I'm abusing order of operations by leaving out the parens, just to make it a little less messy.

Answer 3

Given array a:

 import numpy as np a = np.array([[ 0.25659355, -0.75456113, 0.39467396, 0.50805361], [-0.77218172, 1.00016061, -0.70389486, 1.67632146], [-0.41106158, -0.63757421, 1.70390504, -0.79073362], [-0.2016959 , 0.55316318, -1.55280823, 0.45740193]])

The easiest way to get the central 4 values is:

 ax, ay = a.shape a[int(ax/2)-1:int(ax/2)+1, int(ay/2)-1:int(ay/2)+1]

This works if you have even numbers for the dimensions of the array. In case of odd numbers, there won't be a central 4 values.

Answer 4

Could you just use indexing? Like:

A = np.array([[ 0.25659355, -0.75456113,  0.39467396,  0.50805361],
[-0.77218172,  1.00016061, -0.70389486,  1.67632146],
[-0.41106158, -0.63757421,  1.70390504, -0.79073362],
[-0.2016959 ,  0.55316318, -1.55280823,  0.45740193]])
])

A[1:3,1:3]

Or if matrix A had odd dimensions, say 5x5 then:

A[2,2]