pivot_longer (wide to long) in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this pivot_longer operation?

Given the data frame df below, what will be the output after applying the pivot_longer function as shown?

R Programming

library(tidyr)
df <- data.frame(
  id = 1:2,
  math = c(90, 80),
  science = c(85, 95)
)
result <- pivot_longer(df, cols = c(math, science), names_to = "subject", values_to = "score")
print(result)

A

  id  subject score
1  1     math    90
2  1  science    85
3  2     math    80
4  2  science    95

B

  id  subject score
1  1  science    85
2  1     math    90
3  2  science    95
4  2     math    80

C

  id  subject score
1  1     math    90
2  2     math    80
3  1  science    85
4  2  science    95

D

  id  subject score
1  1     math    90
2  1  science    85
3  2  science    95
4  2     math    80

💡 Hint

Remember that pivot_longer stacks the selected columns in order, keeping the original row order.

pivot_longer takes the columns math and science and stacks them under a new column 'subject'. The values go into 'score'. The order preserves the original row order and the order of columns selected.

🧠 Conceptual

intermediate

Which statement best describes the purpose of pivot_longer?

What does the pivot_longer function do in data manipulation?

A It converts data from a wide format to a long format by gathering multiple columns into key-value pairs.

B It converts data from a long format to a wide format by spreading key-value pairs into multiple columns.

C It filters rows based on a condition in one or more columns.

D It sorts the data frame by one or more columns.

💡 Hint

Think about how columns are transformed into rows.

pivot_longer gathers multiple columns into two columns: one for the column names (keys) and one for the values, changing wide data into long data.

🔧 Debug

advanced

What error does this pivot_longer code produce?

Consider the code below. What error will it raise when run?

R Programming

library(tidyr)
df <- data.frame(
  id = 1:2,
  math = c(90, 80),
  science = c(85, 95)
)
result <- pivot_longer(df, cols = math science, names_to = "subject", values_to = "score")

A Error: unused argument (names_to = "subject")

B Error: unexpected symbol in "pivot_longer(df, cols = math science"

C Error: object 'math science' not found

D No error, runs successfully

💡 Hint

Check the syntax for specifying multiple columns in the cols argument.

The cols argument requires columns to be separated by commas or use c(). Writing 'math science' without a comma or c() causes a syntax error.

❓ Predict Output

advanced

What is the output of pivot_longer with regex selection?

Given the data frame df below, what will be the output of the pivot_longer call that selects columns starting with 'score_'?

R Programming

library(tidyr)
df <- data.frame(
  id = 1:2,
  score_math = c(90, 80),
  score_science = c(85, 95),
  age = c(15, 16)
)
result <- pivot_longer(df, cols = starts_with("score_"), names_to = "subject", values_to = "score")
print(result)

A

  id       subject score
1  1   score_math    90
2  1 score_science    85
3  2 score_science    95
4  2   score_math    80

B

  id       subject score
1  1   score_math    90
2  2   score_math    80
3  1 score_science    85
4  2 score_science    95

C

  id       subject score
1  1   score_math    90
2  1 score_science    85
3  2   score_math    80
4  2 score_science    95

D

  id       subject score
1  1 score_science    85
2  1   score_math    90
3  2 score_science    95
4  2   score_math    80

💡 Hint

The order of rows follows the original row order and the order of selected columns.

pivot_longer stacks columns selected by starts_with("score_") in the order they appear, preserving row order.

🚀 Application

expert

How many rows are in the result after pivot_longer?

You have a data frame with 3 rows and 4 columns: one ID column and three measurement columns. After applying pivot_longer to gather the three measurement columns, how many rows will the resulting data frame have?

A 12

B 4

C 3

D 9

💡 Hint

Each original row will produce one row per gathered column.

pivot_longer stacks the selected columns into rows. With 3 original rows and 3 columns gathered, the result has 3 * 3 = 9 rows.