What does the function np.isreal does in a dataframe?

Question

Can anyone explain the below code?

pima_df[~pima_df.applymap(np.isreal).all(1)]

pima_df is a dataframe.

Answer 1

You are extracting rows in which atleast one complex number occurs.

e.g : pima_df =

    a   b
0   1   2
1   2   4+3j
2   3   5

result would be :

    a   b
1   2   (4+3j)

in short :

applymap - apply function on each and every element of dataframe.

np.isreal - returns true for real otherwise false

all - returns true if each element along an axis is true otherwise false.

~ - negates the boolean index

Answer 2

Please look at the doc or help(np.isreal).

Returns a bool array, where True if input element is real.

If element has complex type with zero complex part, the return value
for that element is True.

To be precise Numpy Provides a set of methods for comparing and performing operations on arrays elementwise:

np.isreal : Determines whether each element of array is real.
np.all :    Determines whether all array element of a specific array evaluate to True.
tilde(~) :  used for Boolean indexing which means not.
applymap:   applymap works element-wise on a DataFrame.
all()   :   used to find rows where all the values are True.

The ~ is the operator equivalent of the invert dunder which has been overridden explicitly for the purpose performing vectorized logical inversions on pd.DataFrame/pd.Series objects.

Example of boolean index (~):

>>> df 
    a  b  c  d
0   a  a  2  6
1   a  a  4  7
2   b  a  1  6
3   b  a  2  1
4   c  b  3  6
5   c  b  0  2
6   d  b  3  3
7   d  b  2  1
8   e  c  4  3
9   e  c  2  0
10  f  c  0  6
11  f  c  1  2

>>> df.query('a in b')
   a  b  c  d
0  a  a  2  6
1  a  a  4  7
2  b  a  1  6
3  b  a  2  1
4  c  b  3  6
5  c  b  0  2

OR

>>> df[~df.a.isin(df.b)]    # same as above
    a  b  c  d
6   d  b  3  3
7   d  b  2  1
8   e  c  4  3
9   e  c  2  0
10  f  c  0  6
11  f  c  1  2

hope this will help.