gistfile1.py

import numpy as np

def random_mask(a, n_choose):
    ret = a.copy()
    indices = np.random.choice(np.nonzero(a)[0], n_choose, replace=False)
    ret[:] = 0
    ret[indices] = a[indices]
    return ret

def random_flip(a, n_flip):
    ret = a.copy()
    indices = np.random.choice(np.nonzero(a)[0], n_flip, replace=False)
    ret[indices] = -a[indices]
    return ret

def magic(a, n_choose, n_flip):
    return random_flip(random_mask(a, n_choose), n_flip)

#n = 100
#a = np.random.randint(-1, 2, n)
#magic(a, 20, 5)