dates = pd.date_range(start='20191101', end='20191124', freq='D')
a1 = pd.DataFrame(np.random.randn(24, 4), index=dates, columns=list('ABCD'))
a2 = pd.DataFrame(np.random.rand(24, 4))
(np.random.randn(24, 4)和(np.random.rand(24, 4)) 中的randn与rand有什么区别