idx_b = data[:,1] == 0 and data[:,4] == 1
用and会提示错误， 因为and 最终只给定一个bool值， 要么用.all() 或 .any()
所以这里用& 是正确， 会给定一个bool值的array, 它是elementwise的
https://stackoverflow.com/questions/10062954/valueerror-the-truth-value-of-an-array-with-more-than-one-element-is-ambiguous
