How to fill nans with multiple if-else conditions? - pandas

Question

I have a dataset:
value score
0 0.0 8
1 0.0 7
2 NaN 4
3 1.0 11
4 2.0 22
5 NaN 12
6 0.0 4
7 NaN 15
8 0.0 5
9 2.0 24
10 1.0 12
11 1.0 15
12 0.0 5
13 2.0 26
14 NaN 28
There are some NaNs in it. I want to fill those NaNs with these conditions:
If 'score' is less than 10, then fill nan with 0.0
If 'score' is between 10 and 20, then fill nan with 1.0
If 'score' is greater than 20, then fill nan with 2.0
How do I do this in pandas?
Here is an example dataframe:
value = [0,0,np.nan,1,2,np.nan,0,np.nan,0,2,1,1,0,2,np.nan]
score = [8,7,4,11,22,12,4,15,5,24,12,15,5,26,28]
pd.DataFrame({'value': value, 'score':score})

Answer 1

Do with cut then fillna
df.value.fillna(pd.cut(df.score,[-np.Inf,10,20,np.Inf],labels = [0,1,2]).astype(int),inplace=True)
df
Out[6]:
value score
0 0.0 8
1 0.0 7
2 0.0 4
3 1.0 11
4 2.0 22
5 1.0 12
6 0.0 4
7 1.0 15
8 0.0 5
9 2.0 24
10 1.0 12
11 1.0 15
12 0.0 5
13 2.0 26
14 2.0 28

Answer 2

You could use numpy.select with conditions on <10, 10≤score<20, etc. but a more efficient version could be to use a floor division to have values below 10 become 0, below 20 -> 1, etc.
df['value'] = df['value'].fillna(df['score'].floordiv(10))
with numpy.select:
df['value'] = df['value'].fillna(np.select([df['score'].lt(10),
df['score'].between(10, 20),
df['score'].ge(20)],
[0, 1, 2])
)
output:
value score
0 0.0 8
1 0.0 7
2 0.0 4
3 1.0 11
4 2.0 22
5 1.0 12
6 0.0 4
7 1.0 15
8 0.0 5
9 2.0 24
10 1.0 12
11 1.0 15
12 0.0 5
13 2.0 26
14 2.0 28

Answer 3

use np.select or pd.cut to map the intervals to values, then fillna:
mapping = np.select((df['score'] < 10, df['score'] > 20),
(0, 2), 1)
df['value'] = df['value'].fillna(mapping)