Pandas: Group by the first column and get second column as lists in rows

Last update on April 03 2025 13:11:24 (UTC/GMT +8 hours)

55. Group by First Column to Get Lists

Write a Pandas program to group by the first column and get second column as lists in rows.

Sample Solution :

Python Code :

import pandas as pd
df = pd.DataFrame( {'col1':['C1','C1','C2','C2','C2','C3','C2'], 'col2':[1,2,3,3,4,6,5]})
print("Original DataFrame")
print(df)
df = df.groupby('col1')['col2'].apply(list)
print("\nGroup on the col1:")
print(df)

Sample Output:

Original DataFrame
  col1  col2
0   C1     1
1   C1     2
2   C2     3
3   C2     3
4   C2     4
5   C3     6
6   C2     5

Group on the col1:
col1
C1          [1, 2]
C2    [3, 3, 4, 5]
C3             [6]
Name: col2, dtype: object       

For more Practice: Solve these Related Problems:

• Write a Pandas program to group a DataFrame by the first column and aggregate the second column into lists, then sort the lists.
• Write a Pandas program to group by a categorical column and concatenate the values of another column into comma‐separated strings.
• Write a Pandas program to group a DataFrame by one column and apply a custom function to list unique values from another column.
• Write a Pandas program to group by the first column and then count the frequency of each unique list item from the second column.

Python-Pandas Code Editor:

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