Impute Missing Values with Group Median in Pandas

Question

Given an employee DataFrame with `department` and `salary`, impute missing salaries using the median salary of each employee's specific department.

Accepted Answer

Optimal Solution:
import pandas as pd

# Group-wise median imputation with transform
df['salary'] = df['salary'].fillna(
    df.groupby('department')['salary'].transform('median')
)

Technical Explanation:
`groupby('department')['salary'].transform('median')` computes group medians and expands the result back to the original index shape, allowing direct fillna.

name	department	salary
Aarav	IT	90000
Bhavna	IT	NULL
Chirag	HR	60000
Divya	HR	NULL

name	department	salary
Aarav	IT	90000
Bhavna	IT	90000
Chirag	HR	60000
Divya	HR	60000