[FIXED] pandas: multiple conditions while indexing data frame - unexpected behavior

Issue

I am filtering rows in a dataframe by values in two columns.

For some reason the OR operator behaves like I would expect AND operator to behave and vice versa.

My test code:

df = pd.DataFrame({'a': range(5), 'b': range(5) })

# let's insert some -1 values
df['a'][1] = -1
df['b'][1] = -1
df['a'][3] = -1
df['b'][4] = -1

df1 = df[(df.a != -1) & (df.b != -1)]
df2 = df[(df.a != -1) | (df.b != -1)]

print(pd.concat([df, df1, df2], axis=1,
                keys = [ 'original df', 'using AND (&)', 'using OR (|)',]))

And the result:

      original df      using AND (&)      using OR (|)    
             a  b              a   b             a   b
0            0  0              0   0             0   0
1           -1 -1            NaN NaN           NaN NaN
2            2  2              2   2             2   2
3           -1  3            NaN NaN            -1   3
4            4 -1            NaN NaN             4  -1

[5 rows x 6 columns]

As you can see, the AND operator drops every row in which at least one value equals -1. On the other hand, the OR operator requires both values to be equal to -1 to drop them. I would expect exactly the opposite result. Could anyone explain this behavior?

I am using pandas 0.13.1.

Solution

As you can see, the AND operator drops every row in which at least one value equals -1. On the other hand, the OR operator requires both values to be equal to -1 to drop them.

That's right. Remember that you're writing the condition in terms of what you want to keep, not in terms of what you want to drop. For df1:

df1 = df[(df.a != -1) & (df.b != -1)]

You're saying "keep the rows in which df.a isn't -1 and df.b isn't -1", which is the same as dropping every row in which at least one value is -1.

For df2:

df2 = df[(df.a != -1) | (df.b != -1)]

You're saying "keep the rows in which either df.a or df.b is not -1", which is the same as dropping rows where both values are -1.

PS: chained access like df['a'][1] = -1 can get you into trouble. It's better to get into the habit of using .loc and .iloc.

Answered By - DSM