filter() for row selection in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of filter() with multiple conditions

What is the output of the following R code using filter() from dplyr?

R Programming

library(dplyr)
df <- tibble(x = 1:5, y = c(10, 20, 30, 40, 50))
result <- filter(df, x > 2, y < 50)
print(result)

A

A tibble: 2 × 2
      x     y
  <int> <dbl>
1     3    30
2     4    40

B

A tibble: 3 × 2
      x     y
  <int> <dbl>
1     3    30
2     4    40
3     5    50

C

A tibble: 1 × 2
      x     y
  <int> <dbl>
1     4    40

D

A tibble: 4 × 2
      x     y
  <int> <dbl>
1     2    20
2     3    30
3     4    40
4     5    50

💡 Hint

Remember that filter() keeps rows where all conditions are TRUE.

The filter keeps rows where x > 2 and y < 50. Rows with x=3,y=30 and x=4,y=40 satisfy both conditions.

❓ Predict Output

intermediate

filter() with logical OR condition

What will be the output of this R code using filter()?

R Programming

library(dplyr)
df <- tibble(a = c('red', 'blue', 'green', 'red'), b = 1:4)
result <- filter(df, a == 'red' | b == 3)
print(result)

A

A tibble: 2 × 2
  a         b
  <chr> <int>
1 red       1
2 red       4

B

A tibble: 4 × 2
  a         b
  <chr> <int>
1 red       1
2 blue      2
3 green     3
4 red       4

C

A tibble: 3 × 2
  a         b
  <chr> <int>
1 red       1
2 blue      2
3 green     3

D

A tibble: 3 × 2
  a         b
  <chr> <int>
1 red       1
2 green     3
3 red       4

💡 Hint

The | operator means OR, so rows matching either condition are kept.

Rows where a is 'red' or b is 3 are kept. That includes rows 1 (red,1), 3 (green,3), and 4 (red,4).

🔧 Debug

advanced

Identify the error in filter() usage

What error will this R code produce when run?

R Programming

library(dplyr)
df <- tibble(x = 1:3, y = c(5, 6, 7))
filter(df, x > 1 & y < 7, z == 2)

A Error: unexpected symbol in 'filter(df, x > 1 & y < 7, z == 2)'

B No error, returns rows where x > 1, y < 7, and z == 2

C Error: object 'z' not found

D Warning: z is not a column in df, but filter runs anyway

💡 Hint

Check if all variables used in filter exist in the data frame.

The variable 'z' does not exist in df, so filter throws an error about 'z' not found.

🧠 Conceptual

advanced

Understanding filter() with NA values

Given this data frame, what will filter(df, x > 2) return?

R Programming

library(dplyr)
df <- tibble(x = c(1, 2, NA, 4))
result <- filter(df, x > 2)
print(result)

A A tibble with rows where x is 4 only

B A tibble with rows where x is 4 and NA

C An error because NA cannot be compared

D A tibble with rows where x is 1, 2, and 4

💡 Hint

Remember how comparisons with NA behave in R.

Comparisons with NA return NA, which filter treats as FALSE, so only rows with x > 2 and not NA are kept.

❓ Predict Output

expert

Complex filter() with grouped data

What is the output of this R code using filter() on grouped data?

R Programming

library(dplyr)
df <- tibble(group = c('A', 'A', 'B', 'B'), value = c(10, 20, 30, 40))
result <- df %>% group_by(group) %>% filter(value == max(value))
print(result)

A Error: max() not found in filter

B

A tibble: 2 × 2
Groups:   group [2]
  group value
  <chr> <dbl>
1 A        20
2 B        40

C

A tibble: 1 × 2
Groups:   group [1]
  group value
  <chr> <dbl>
1 B        40

D

A tibble: 4 × 2
Groups:   group [2]
  group value
  <chr> <dbl>
1 A        10
2 A        20
3 B        30
4 B        40

💡 Hint

filter() inside group_by keeps rows where the condition is TRUE per group.

For each group, filter keeps rows where value equals the maximum value in that group.