Counting duplicates in Pandas - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the duplicated() function in pandas do?

It returns a Boolean Series indicating whether each row is a duplicate of a previous row. <br>True means the row is a duplicate, False means it is unique so far.

beginner

How can you count the total number of duplicate rows in a DataFrame?

Use df.duplicated().sum(). <br>This counts how many rows are duplicates.

intermediate

What does the keep parameter in duplicated() control?

It controls which duplicates to mark as True: <br>- 'first' marks duplicates except the first occurrence <br>- 'last' marks duplicates except the last occurrence <br>- False marks all duplicates as True

intermediate

How do you count duplicates based on specific columns only?

Pass the column names to the subset parameter in duplicated(). <br>Example: df.duplicated(subset=['col1', 'col2']).sum()

beginner

What is the difference between duplicated() and drop_duplicates()?

duplicated() marks duplicate rows with True/False. <br>drop_duplicates() removes duplicate rows from the DataFrame.

Which pandas function returns a Boolean Series marking duplicate rows?

A duplicated()

B drop_duplicates()

C count_duplicates()

D is_duplicate()

duplicated() returns True for duplicate rows, False otherwise.

How do you count the number of duplicate rows in a DataFrame df?

A df.count_duplicates()

B df.drop_duplicates().count()

C df.duplicated().sum()

D df.duplicated().count()

df.duplicated() returns a Boolean Series; summing it counts duplicates.

What does df.duplicated(keep=False) do?

A Marks all duplicates as True, including first occurrences

B Marks only the first duplicate as True

C Marks only the last duplicate as True

D Removes duplicates from df

keep=False marks all duplicates as True, not keeping any unique.

To check duplicates based on columns 'A' and 'B' only, which is correct?

A df.duplicated(['A', 'B'])

B df.duplicated(columns=['A', 'B'])

C df.duplicated(cols=['A', 'B'])

D df.duplicated(subset=['A', 'B'])

The subset parameter specifies columns to check for duplicates.

Which function removes duplicate rows from a DataFrame?

A duplicated()

B drop_duplicates()

C remove_duplicates()

D delete_duplicates()

drop_duplicates() returns a DataFrame without duplicate rows.

Explain how to find and count duplicate rows in a pandas DataFrame.

Think about marking duplicates first, then counting them.

Describe the difference between duplicated() and drop_duplicates() in pandas.

One marks duplicates, the other removes them.