CODEWITHSUNDEEP
pythonpandasnumpydataframe
hdatas
hdatas

Reputation: 1082

Re-occurrence count

I have a dataframe like the follow:

      Col1     
0      C        
1      A         
3      D         
4      A         
5      A

I would like to count the step/index that a certain value will re-occur so I would get the following: 

      Col1      Col2  
0      C        NaN
1      A         2
3      D        NaN
4      A         1
5      A        NaN

Any ideas on how to do it ? Thanks for help !

Upvotes: 1

Views: 43

Answers (1)

jezrael
jezrael

Reputation: 862511

Use GroupBy.cumcount and then replace 0 to NaNs:

df['Col2'] = df.groupby('Col1').cumcount(ascending=False).replace(0,np.nan)
print (df)
  Col1  Col2
0    C   NaN
1    A   2.0
3    D   NaN
4    A   1.0
5    A   NaN

Alternative solution with mask:

df['Col2'] = df.groupby('Col1').cumcount(ascending=False).mask(lambda x: x == 0)

Upvotes: 4

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