Sevyns - 8 months ago 86

Python Question

I'm looking for a method that behaves similarly to coalesce in T-SQL. I have 2 columns (column A and B) that are sparsely populated in a pandas dataframe. I'd like to create a new column using the following rules:

- If the value in column A
, use that value for the new column C*is not null* - If the value in column A
, use the value in column B for the new column C*is null*

Like I mentioned, this can be accomplished in MS SQL Server via the coalesce function. I haven't found a good pythonic method for this; does one exist?

Answer

use combine_first():

```
In [16]: df = pd.DataFrame(np.random.randint(0, 10, size=(10, 2)), columns=list('ab'))
In [17]: df.ix[::2, 'a'] = np.nan
In [18]: df
Out[18]:
a b
0 NaN 0
1 5.0 5
2 NaN 8
3 2.0 8
4 NaN 3
5 9.0 4
6 NaN 7
7 2.0 0
8 NaN 6
9 2.0 5
In [19]: df['c'] = df.a.combine_first(df.b)
In [20]: df
Out[20]:
a b c
0 NaN 0 0.0
1 5.0 5 5.0
2 NaN 8 8.0
3 2.0 8 2.0
4 NaN 3 3.0
5 9.0 4 9.0
6 NaN 7 7.0
7 2.0 0 2.0
8 NaN 6 6.0
9 2.0 5 2.0
```