Nesting and unnesting in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested list unnesting with dplyr

What is the output of the following R code using dplyr and tidyr to unnest a nested list column?

R Programming

library(dplyr)
library(tidyr)
df <- tibble(id = 1:2, data = list(tibble(x = 1:2), tibble(x = 3:4)))
unnest(df, data)

A A tibble with 2 rows: id 1 and 2, x column with list of vectors

B A tibble with 2 rows: id 1 and 2, data column still nested

C Error: object 'unnest' not found

D A tibble with 4 rows: id repeated twice for each nested tibble, x values 1,2,3,4

💡 Hint

Think about how unnest() expands nested tibbles into rows.

The unnest() function expands the nested data column, which contains tibbles, into multiple rows. Each row in the nested tibble becomes a separate row in the output, repeating the id accordingly.

❓ Predict Output

intermediate

Result of nesting and unnesting with multiple columns

Given this R code, what is the output after nesting and then unnesting?

R Programming

library(dplyr)
library(tidyr)
df <- tibble(group = c('A', 'A', 'B'), value = 1:3)
nested <- df %>% group_by(group) %>% nest()
unnest(nested, data)

A Dataframe with 3 rows, group column repeated, value column expanded

B Dataframe with 2 rows, group column and nested data column

C Original dataframe with columns group and value, 3 rows

D Error: object 'nest' not found

💡 Hint

Nesting groups rows, unnesting expands them back.

After nesting, the data is grouped by group with nested tibbles. Unnesting expands these nested tibbles back into rows, repeating the group values accordingly.

🔧 Debug

advanced

Identify the error in unnesting a list-column

What error does this R code produce and why?

R Programming

library(tidyr)
df <- tibble(id = 1:2, vals = list(1:3, 4:5))
unnest(df, vals, keep_empty = TRUE)

A Error: object 'unnest' not found

B No error, returns 5 rows with id repeated accordingly

C Error: unused argument (keep_empty = TRUE)

D Error: vals must be a data frame or list of data frames

💡 Hint

Check the arguments supported by the current unnest() function version.

The unnest() function in tidyr does not have a keep_empty argument, so passing it causes an unused argument error.

📝 Syntax

advanced

Correct syntax to unnest a nested list-column

Which option correctly unnests the nested list-column data in this tibble?

R Programming

library(tidyr)
df <- tibble(id = 1:2, data = list(tibble(a = 1:2), tibble(a = 3:4)))

A unnest(df, data = data)

B unnest(df, cols = data)

C unnest(df, c(data))

D unnest(df, data)

💡 Hint

unnest() uses tidyselect syntax (like select()) to specify columns.

The unnest() function uses tidyselect syntax so unnest(df, cols = data) correctly specifies the column to unnest. Other options use incorrect or deprecated syntax.

🚀 Application

expert

How many rows after nesting and unnesting complex data?

Consider this R code that nests and unnests a dataframe with multiple grouping variables and nested tibbles. How many rows does the final dataframe have?

R Programming

library(dplyr)
library(tidyr)
df <- tibble(
  group1 = c('X', 'X', 'Y', 'Y'),
  group2 = c('A', 'B', 'A', 'B'),
  val = 1:4
)
nested <- df %>% group_by(group1) %>% nest()
unnested <- nested %>% unnest(data)

A 4 rows, same as original dataframe

B 2 rows, one per group1 value

C 8 rows, double the original rows

D Error due to ambiguous grouping

💡 Hint

Nesting groups by group1 nests all rows per group1, unnesting expands them back.

Nesting by group1 creates two nested tibbles, each containing rows for that group. Unnesting expands these back, resulting in the original 4 rows.