mutate() for new columns in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of mutate() adding a new column

What is the output of this R code using dplyr::mutate() to add a new column?

R Programming

library(dplyr)
df <- tibble(x = 1:3, y = 4:6)
df_new <- df %>% mutate(z = x + y)
df_new

A Error: object 'z' not found

B A tibble with columns x, y, z where z is the product of x and y: rows (1,4,4), (2,5,10), (3,6,18)

C A tibble with columns x, y, z where z is the sum of x and y: rows (1,4,5), (2,5,7), (3,6,9)

D A tibble with columns x, y only, no z column added

💡 Hint

mutate() adds new columns by evaluating expressions using existing columns.

mutate() creates a new column 'z' by adding columns 'x' and 'y' element-wise.

❓ Predict Output

intermediate

Using mutate() with conditional new column

What is the output of this R code that adds a new column with a condition inside mutate()?

R Programming

library(dplyr)
df <- tibble(score = c(45, 75, 85))
df_new <- df %>% mutate(pass = ifelse(score >= 50, "yes", "no"))
df_new

A A tibble with columns score and pass: (45, "yes"), (75, "no"), (85, "no")

B A tibble with columns score and pass: (45, "no"), (75, "yes"), (85, "yes")

C A tibble with only the score column, no pass column added

D Error: unexpected symbol in ifelse statement

💡 Hint

ifelse(condition, true_value, false_value) returns values based on condition for each row.

The pass column is "yes" if score >= 50, otherwise "no".

🔧 Debug

advanced

Identify the error in mutate() code

What error does this R code produce when trying to add a new column with mutate()?

R Programming

library(dplyr)
df <- tibble(a = 1:3)
df %>% mutate(b = a + c)

A No error, adds column b with values 1, 2, 3

B Error: unexpected symbol in mutate()

C Error: column 'a' not found

D Error: object 'c' not found

💡 Hint

Check if all variables used inside mutate() exist in the data frame or environment.

The variable 'c' is not defined anywhere, so mutate() cannot compute 'b = a + c'.

❓ Predict Output

advanced

Result of mutate() with multiple new columns

What is the output of this R code that adds two new columns using mutate()?

R Programming

library(dplyr)
df <- tibble(x = 2:4)
df_new <- df %>% mutate(y = x^2, z = y + 1)
df_new

A A tibble with columns x, y, z: rows (2,4,5), (3,9,10), (4,16,17)

B A tibble with columns x, y, z: rows (2,4,4), (3,9,9), (4,16,16)

C Error: object 'y' not found inside mutate()

D A tibble with columns x and y only, no z column

💡 Hint

mutate() evaluates new columns in order, so 'z' can use 'y' defined earlier.

Column y is x squared, then z is y plus 1, so z values are 5, 10, 17.

🧠 Conceptual

expert

Understanding mutate() column evaluation order

Which statement correctly describes how mutate() evaluates new columns when multiple are added?

A New columns are evaluated sequentially from left to right, so later columns can use earlier ones.

B New columns are evaluated in reverse order, so earlier columns can use later ones.

C mutate() only allows adding one new column at a time; multiple columns cause an error.

D All new columns are evaluated simultaneously, so none can use others defined in the same mutate().

💡 Hint

Think about how you can use a new column just created in the same mutate() call.

mutate() evaluates new columns in order, so each can use columns created before it.