county_idx is just indexing into alpha. The result will be (1000, ). For example try:
import numpy as np
alpha = np.array([1.5, 2.5])
ix = [0,0,1,1,0,0,1,0,1]
alpha[ix]
>> array([1.5, 1.5, 2.5, 2.5, 1.5, 1.5, 2.5, 1.5, 2.5])
county_idx is just indexing into alpha. The result will be (1000, ). For example try:
import numpy as np
alpha = np.array([1.5, 2.5])
ix = [0,0,1,1,0,0,1,0,1]
alpha[ix]
>> array([1.5, 1.5, 2.5, 2.5, 1.5, 1.5, 2.5, 1.5, 2.5])