Numpy 数组相等 boolean

Question

The following two approaches should yield an identical result, but it seems they do not:

Approach 1 :

#Generate an array with 12 annual fractions corresponding to each month
ann_frac = np.arange(1,13,1).reshape([12,1])
ann_frac = ann_frac[:,0]/12

Output:

array([0.08333333, 0.16666667, 0.25      , 0.33333333, 0.41666667,
       0.5       , 0.58333333, 0.66666667, 0.75      , 0.83333333,
       0.91666667, 1.        ])

Approach 2 :

i = np.arange(1,13,1)
ann_frac2 = (i/12).reshape([12,1])

Output:

array([[0.08333333],
       [0.16666667],
       [0.25      ],
       [0.33333333],
       [0.41666667],
       [0.5       ],
       [0.58333333],
       [0.66666667],
       [0.75      ],
       [0.83333333],
       [0.91666667],
       [1.        ]])

Comparing approaches :

ann_frac2==ann_frac

Output:

array([[False],
       [False],
       [False],
       [False],
       [False],
       [False],
       [False],
       [False],
       [False],
       [False],
       [False],
       [False]])

A bit strange it seems. Any explanation? Obviously, if one just wants to compare numbers in two different arrays for equlity, the example above demonstrates that even though the numbers can be identical, the way the numbers were created and stored can produce an unexpected behaviour.

Answer 1

I think you are comparing arrays with different dimensionality. What I guess you want is

ann_frac = np.arange(1,13,1).reshape([12,1])
ann_frac1 = ann_frac[:,0]/12
i = np.arange(1,13,1)
ann_frac2 = (i/12).reshape([12,1])
ann_frac3 = (i/12).reshape(12)

and the correct comparison should be

ann_frac3==ann_frac1

with output

array([ True,  True,  True,  True,  True,  True,  True,  True,  True,
    True,  True,  True])

Even better would be

(ann_frac3==ann_frac1).all()

with output

True