groupby and find the first non-zero value

I have the following df, the last column is the desired output. thanks!

group  date  value    desired_first_nonzero

  1    jan2019    0        2                      
  1    jan2019    2        2
  1    feb2019    3        2
  1    mar2019    4        2
  1    mar2019    5        2
  2    feb2019    0        4
  2    feb2019    0        4
  2    mar2019    0        4
  2    mar2019    4        4
  2    apr2019    5        4

I want to group by "group" and find the first non-zero value

>Solution :

You can use GroupBy.transform with a custom function to get the index of the first non-zero value with idxmax (that return the first True value here):

df['desired_first_nonzero'] = (df.groupby('group')['value']
                                 .transform(lambda s: s[s.ne(0).idxmax()])
                              )

alternatively, using an intermediate Series:

s = df.set_index('group')['value']
df['desired_first_nonzero'] = df['group'].map(s[s.ne(0)].groupby(level=0).first())

output:

   group     date  value  desired_first_nonzero
0      1  jan2019      0                      2
1      1  jan2019      2                      2
2      1  feb2019      3                      2
3      1  mar2019      4                      2
4      1  mar2019      5                      2
5      2  feb2019      0                      4
6      2  feb2019      0                      4
7      2  mar2019      0                      4
8      2  mar2019      4                      4
9      2  apr2019      5                      4