Python numpy nan在带有字符串的数组中与true相比较

Question

I am trying to compare two numpy arrays which contain numbers, string and nans. I want to know how many items in the array are equal.

When comparing these two arrays:

c =np.array([1,np.nan]);
d =np.array([2,np.nan]);
print (c==d)
[False False]

Which is the expected behaviour.

However, when comparing:

a =np.array([1,'x', np.nan]);
b =np.array([1,'x', np.nan]);
print (a==b)
[ True  True  True]

That makes no sense to me, how can adding a string to the array change the way nans are compared? Any ideas?

Thanks!

Answer 1

If you examine the arrays, you'll see that np.nan has been converted to string ( 'n' ):

In [48]: a = np.array([1, 'x', np.nan])

In [49]: a
Out[49]: 
array(['1', 'x', 'n'], 
      dtype='|S1')

And 'n' == 'n' is True .

What I don't understand is why changing the array's dtype to object doesn't change the result of the comparison:

In [72]: a = np.array([1, 'x', np.nan], dtype=object)

In [73]: b = np.array([1, 'x', np.nan], dtype=object)

In [74]: a == b
Out[74]: array([ True,  True,  True], dtype=bool)

In [75]: a[2] == b[2]
Out[75]: False

In [76]: type(a[2])
Out[76]: float

In [77]: type(b[2])
Out[77]: float

It's almost as if the two NaN objects are compared by reference rather than by value:

In [79]: id(a[2])
Out[79]: 26438340

In [80]: id(b[2])
Out[80]: 26438340