python - Count occurrences of items in Series in each row of a DataFrame

Question

I have a pandas.DataFrame that looks like this.

COL1    COL2    COL3
C1      None    None
C1      C2      None
C1      C1      None
C1      C2      C3

For each row in this dataframe I would like to count the occurrences of each of C1, C2, C3 and append this information as columns to this dataframe. For instance, the first row has 1 C1, 0 C2 and 0 C3. The final data frame should look like this

COL1    COL2    COL3    C1  C2  C3
C1      None    None    1   0   0
C1      C2      None    1   1   0
C1      C1      None    2   0   0
C1      C2      C3      1   1   1

So, I have created a Series with C1, C2 and C3 as the values - one way top count this is to loop over the rows and columns of the DataFrame and then over this Series and increment the counter if it matches. But is there an apply approach that can achieve this in a compact fashion?

Answer 1

You could apply value_counts:

In [11]: df.apply(pd.Series.value_counts, axis=1)
Out[11]: 
   C1  C2  C3  None
0   1 NaN NaN     2
1   1   1 NaN     1
2   2 NaN NaN     1
3   1   1   1   NaN

So you can fill the NaN and applend just the base values you want:

In [12]: df.apply(pd.Series.value_counts, axis=1)[['C1', 'C2', 'C3']].fillna(0)
Out[12]: 
   C1  C2  C3
0   1   0   0
1   1   1   0
2   2   0   0
3   1   1   1

Note: there's an open issue to have a value_counts method directly for a DataFrame (which I think should be introduced by pandas 0.15).