Sketch.num_na() is broken for float

[guyinbar]

According to documentation, Sketch.num_na() should return the exact number of missing values (https://apple.github.io/turicreate/docs/api/generated/turicreate.Sketch.html#turicreate-sketch), however, when dtype is float, it always returns 0.

sf = tc.SFrame({'A':[1.0, 2.0] + [float('nan')]*100})
print sf['A'].summary().num_na() 
print len(sf['A']) - len(sf['A'].dropna()) 

sf = tc.util.generate_random_sframe(10000, 'R', random_seed=11)
print sf['X1-R'].dtype
print sf['X1-R'].summary().num_na() 
print len(sf) - len(sf.dropna())
print sum(sf['X1-R'].apply(lambda x : math.isnan(x)))

[hoytak]

The name of this should be changed. A float('nan') is not a missing value; that would be a python None. dropna() and num_na() are not consistent in that they handle NAN differently.

[hoytak]

Let's change num_na to be num_missing.