duplicated() for finding duplicates in Pandas - Step-by-Step Execution

Concept Flow - duplicated() for finding duplicates

Start with DataFrame

↓

Call duplicated()

↓

Check each row if duplicate

↓

Mark True for duplicates, False for first occurrences

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Return Boolean Series

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Use result to filter or analyze duplicates

The duplicated() method checks each row in a DataFrame and marks True if it is a duplicate of a previous row, otherwise False.

Execution Sample

Pandas

import pandas as pd

df = pd.DataFrame({
    'A': [1, 2, 2, 3, 1],
    'B': ['x', 'y', 'y', 'z', 'x']
})

result = df.duplicated()

This code creates a DataFrame and uses duplicated() to find which rows are duplicates of earlier rows.

Execution Table

Step	Row Index	Row Data	Is Duplicate?	Reason
1	0	{'A': 1, 'B': 'x'}	False	First occurrence of this row
2	1	{'A': 2, 'B': 'y'}	False	First occurrence of this row
3	2	{'A': 2, 'B': 'y'}	True	Duplicate of row 1
4	3	{'A': 3, 'B': 'z'}	False	First occurrence of this row
5	4	{'A': 1, 'B': 'x'}	True	Duplicate of row 0
6	End			All rows checked

💡 All rows processed, duplicated() returns Boolean Series marking duplicates

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	Final
result	empty	[False]	[False, False]	[False, False, True]	[False, False, True, False]	[False, False, True, False, True]	[False, False, True, False, True]

Key Moments - 2 Insights

Why is the first occurrence of a row marked False, not True?

duplicated() marks only repeated rows as True. The first time a row appears, it is not a duplicate, so it is False (see execution_table rows 1 and 2).

How does duplicated() decide which rows are duplicates?

It compares each row to all previous rows. If it matches any earlier row exactly, it marks True (see execution_table row 3 and 5).

Visual Quiz - 3 Questions

Look at the execution_table, what is the 'Is Duplicate?' value for row index 3?

A True

B False

C None

D Error

💡 Hint

Check execution_table row 4 for row index 3's duplicate status.

At which step does duplicated() first mark a row as True?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look at execution_table rows 2 and 3 to see when True first appears.

If the DataFrame had no repeated rows, what would the final 'result' variable look like?

A [True, True, True, True, True]

B [False, True, False, True, False]

C [False, False, False, False, False]

D [True, False, True, False, True]

💡 Hint

duplicated() marks only duplicates as True, first occurrences are False (see variable_tracker final).

Concept Snapshot

duplicated() checks each row in a DataFrame
Returns a Boolean Series: True for duplicates, False for first occurrences
By default, compares all columns
Use to filter or analyze repeated rows
Example: df.duplicated()

Full Transcript

The duplicated() method in pandas helps find duplicate rows in a DataFrame. It looks at each row and checks if it has appeared before. If yes, it marks that row as True, meaning it is a duplicate. If not, it marks it as False, meaning it is the first time this row appears. This method returns a Boolean Series that matches the DataFrame's rows. You can use this result to filter out duplicates or analyze them. For example, in the sample DataFrame, rows 2 and 4 are duplicates of earlier rows, so duplicated() marks them True. The first occurrences are always False. This helps you quickly spot repeated data.