Merging on index in Pandas - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does merging on index mean in pandas?

Merging on index means combining two data tables using their row labels (index) instead of columns. It matches rows based on their index values.

beginner

How do you merge two DataFrames on their indexes in pandas?

Use the pandas merge function with the parameters left_index=True and right_index=True to merge on the indexes of both DataFrames.

intermediate

What is the difference between merging on columns and merging on index?

Merging on columns uses matching values in specified columns to combine rows, while merging on index uses the row labels (index) to match rows.

intermediate

What happens if indexes do not match when merging on index?

Rows with indexes that do not match will be included or excluded depending on the type of merge (inner, outer, left, right). Non-matching rows may have missing values.

beginner

Write a simple pandas code snippet to merge two DataFrames df1 and df2 on their indexes.

merged = df1.merge(df2, left_index=True, right_index=True)

Which parameters do you set to True to merge two DataFrames on their indexes?

A left_index=True, right_index=True

B on='index'

C index=True

D left_on='index', right_on='index'

To merge on indexes, set left_index=True and right_index=True in pandas merge.

What type of merge includes all rows from both DataFrames, matching where possible?

A inner

B right

C left

D outer

An outer merge includes all rows from both DataFrames, filling missing matches with NaN.

If you want to merge on a column instead of index, which parameter do you use?

A on='column_name'

B right_index=True

C left_index=True

D index=True

Use on='column_name' to merge on a specific column.

What will happen if indexes do not match in an inner merge on index?

A All rows are kept

B Only rows with matching indexes are kept

C Rows with unmatched indexes get NaN values

D Merge will fail

Inner merge keeps only rows with matching indexes.

Which pandas function is used to merge DataFrames?

A concat()

B join()

C merge()

D append()

The merge() function is used to combine DataFrames based on columns or indexes.

Explain how to merge two pandas DataFrames on their indexes and what parameters are needed.

Think about how row labels are used instead of columns.

Describe the difference between merging on columns and merging on index in pandas.

Consider what pandas uses to align rows in each case.