Merging data frames in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of merging data frames in R?

Merging data frames combines two or more data frames based on common columns or row names, allowing you to integrate related data into one table.

beginner

What function in R is commonly used to merge two data frames?

The merge() function is used to combine two data frames by matching rows based on one or more common columns.

beginner

What does the argument by specify in the merge() function?

The by argument specifies the column name(s) used to match rows between the two data frames during the merge.

intermediate

Explain the difference between all=TRUE and all=FALSE in merge().

all=TRUE performs a full outer join, keeping all rows from both data frames. all=FALSE (default) performs an inner join, keeping only rows with matching keys in both data frames.

intermediate

How can you merge data frames by different column names in R?

Use by.x and by.y arguments in merge() to specify the column names to match in the first and second data frames respectively.

Which function is used to merge two data frames in R?

A bind()

B combine()

C merge()

D join()

The merge() function is the standard way to merge two data frames in R.

What does all=TRUE do in the merge() function?

A Keeps all rows from both data frames

B Drops all rows

C Keeps only matching rows

D Merges only the first data frame

all=TRUE performs a full outer join, keeping all rows from both data frames.

How do you specify different column names to merge on in two data frames?

A keys

B by

C columns

D by.x and by.y

Use by.x and by.y to specify different column names in each data frame.

If you want to keep only rows that appear in both data frames, which argument should you use?

A all=TRUE

B all=FALSE

C all.x=TRUE

D all.y=TRUE

all=FALSE (default) keeps only rows with matching keys in both data frames (inner join).

What happens if you do not specify the by argument in merge()?

A Merges on all common column names automatically

B Merge fails with error

C Merges on row names only

D Merges randomly

If by is not specified, merge() uses all common column names to merge.

Describe how to merge two data frames in R when the key columns have different names.

Think about how to tell R which columns to match in each data frame.

Explain the difference between inner join and full outer join when merging data frames in R.

Consider which rows appear in the result for each join type.