Balanced Random Assignment in Python

Question

It has been a very long time since I've used Python.

What I'm looking for: I would like to create a 6-by-6 random matrix where each component is either 1 or 2, so that there are 18 ones and 18 twos.

Attempt:

import numpy
import random
array = numpy.zeros((6, 6))
while (array == 1).sum() != 18 or (array == 2).sum() != 18: 
 for i in range(0, 6):
 for j in range(0, 6):
 array[i][j] = random.randint(1, 2) 
print array 

Sure, this works. But this seems to me to be extremely inefficient, as the code is literally having to insert new matrix values until the number of ones/twos is 18. How can I make this more efficient?

Answer 1

Just create a list of 18 ones and 18 twos, shuffle it, then reshape to 6x6:

from random import shuffle
from numpy import reshape
nums = [1]*18 + [2]*18
shuffle(nums)
arr = reshape(nums, (6, 6))

Produces (for example):

array([[1, 2, 2, 1, 2, 1],
 [2, 2, 2, 2, 2, 1],
 [2, 2, 2, 1, 1, 2],
 [1, 1, 1, 1, 2, 2],
 [2, 1, 1, 2, 1, 1],
 [1, 2, 1, 1, 1, 2]])