HW 3: dplyr Review (Koans 4–6)

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Multiple Choice (15)

1) What does filter() do?

a) Keeps rows that match a condition b) Keeps columns that match a condition c) Sorts rows d) Creates new columns

2) Which operator means “not equal to” in R?

a) = b) != c) == d) <>

3) What does %in% help you do inside filter()?

a) Sort values b) Check membership in a list/vector of values c) Rename columns d) Compute a summary statistic

4) Which line keeps only the columns country, year, and lifeExp?

gapminder %>%
  ___(country, year, lifeExp)

a) filter b) select c) summarize d) arrange

5) What does mutate() do?

a) Adds or transforms columns b) Reduces the tibble to one row c) Picks rows by position only d) Renames row numbers

6) What does this code do?

gapminder %>%
  arrange(desc(pop))

a) Sorts population from smallest to largest b) Sorts population from largest to smallest c) Keeps only the largest population row d) Filters to rows where pop is missing

7) What does slice(2:4) do?

a) Keeps columns 2 through 4 b) Keeps rows 2 through 4 c) Keeps the 2nd and 4th columns only d) Keeps rows where the value is between 2 and 4

8) Without group_by(), summarize() usually returns

a) One row b) One row per original row c) One row per column d) It always errors

9) What does group_by(continent) mainly change?

a) The actual values in the tibble b) How later verbs like summarize() operate c) The column names d) The data types of columns

10) Which pipeline correctly computes the median life expectancy for each year?

a) gapminder %>% summarize(median_lifeExp = median(lifeExp)) b) gapminder %>% arrange(year) %>% summarize(median_lifeExp = median(lifeExp)) c) gapminder %>% group_by(year) %>% summarize(median_lifeExp = median(lifeExp)) d) gapminder %>% select(year) %>% summarize(median_lifeExp = median(year))

11) What does slice_head(n = 5) do?

a) Returns the first 5 rows b) Returns the last 5 rows c) Returns 5 random rows d) Returns the 5 largest values of a column

12) What does count(country) return?

a) The number of columns named country b) A tibble with one row per country and a count of rows c) The number of unique years per country d) A sorted list of countries only

13) In filter(), separating conditions with a comma means

a) OR logic (either can be true) b) AND logic (both must be true) c) It creates a new column d) It sorts the data

14) What does filter(year >= 2000) keep?

a) Only year 2000 b) Years before 2000 c) Years 2000 and later d) The first 2000 rows

15) Which verb primarily reduces rows?

a) mutate() b) select() c) filter() d) rename()

Short Answer (5)

SA1) In one sentence, explain what filter() does.

SA2) What is the difference between select() and filter()?

SA3) Why is group_by() usually paired with summarize()?

SA4) What does mutate() do that summarize() does not?

SA5) Explain the difference between arrange() and slice().

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