Adding and removing columns in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you add a new column to a data frame in R?

You can add a new column by assigning a vector to a new column name using the $ operator or bracket notation. For example, df$new_column <- c(1, 2, 3) adds a column named new_column.

beginner

What is the syntax to remove a column from a data frame in R?

You can remove a column by setting it to NULL: df$column_to_remove <- NULL. This deletes the column from the data frame.

intermediate

What happens if you assign a vector of different length than the number of rows when adding a column?

R will recycle the vector to match the number of rows, repeating the values to fill the column. A warning is given if its length does not divide evenly into the number of rows.

intermediate

How can you add multiple columns at once to a data frame?

You can add multiple columns by assigning a data frame or list to new columns. For example, df[c('col1', 'col2')] <- list(c(1,2), c(3,4)) adds two columns.

intermediate

Can you remove multiple columns at once? How?

Yes, by setting multiple columns to NULL using their names in brackets: df[c('col1', 'col2')] <- NULL removes both columns.

Which of the following adds a new column named 'age' with values 25, 30, 35 to data frame df?

A df$age = 25

B df['age'] = 25:35

C df$age <- c('25', '30', '35')

D df$age <- c(25, 30, 35)

Option D correctly assigns a numeric vector of length 3 to the new column 'age'.

How do you remove the column 'salary' from data frame df?

A df$salary <- NULL

B df$salary <- NA

C df <- df[-salary]

D remove(df$salary)

Setting df$salary to NULL deletes the column from the data frame.

What happens if you add a column with a vector shorter than the number of rows?

A The column is filled with NA

B R throws an error always

C R recycles the vector values to fill the column

D The data frame shrinks to match vector length

R recycles shorter vectors to fill the column, issuing a warning if the length does not divide evenly.

Which syntax adds two columns 'x' and 'y' at once to df?

A df[c('x', 'y')] <- list(c(1,2), c(3,4))

B df$x, df$y <- c(1,2), c(3,4)

C df[c('x', 'y')] <- c(1,2,3,4)

D df$xy <- c(1,2,3,4)

Assigning a list of vectors to multiple columns using bracket notation adds them correctly.

How to remove columns 'a' and 'b' from df in one step?

A df$a <- NULL; df$b <- NULL

B df[c('a', 'b')] <- NULL

C df <- df[-c('a', 'b')]

D remove(df$a, df$b)

Setting multiple columns to NULL at once removes them from the data frame.

Explain how to add a new column to a data frame in R and what happens if the vector length differs from the number of rows.

Think about how R handles vectors when adding columns.

Describe the methods to remove one or multiple columns from a data frame in R.

Consider how NULL affects data frame columns.