Interview Query

Previous NaN Values

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Given a dataframe with three columns:

  • client_id
  • ranking
  • value

Write a function to fill the NaN values in the value column with the previous non-NaN value from the same client_id ranked in ascending order.

If there doesn’t exist a previous client_id then return the previous value.

Input:

print(clients_df)
client_id ranking value
1001 1 1000
1001 2 NaN
1001 3 1200
1002 1 1500
1002 2 1250
1002 3 NaN
1003 1 1100
1003 2 NaN

Output:

client_id ranking value
1001 1 1000
1002 1 1500
1003 1 1100
1001 2 1000
1002 2 1250
1003 2 1100
1001 3 1200
1002 3 1250
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Python 3.9.6
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