What does aggregation do to a data frame?
It combines rows into groups based on the values in one or more columns and then calculates summary statistics for each group.
Flip the card by pressing the q key.
What two dplyr functions are used to aggregate data?
group_by() to make the groups, followed by summarize() to calculate the summary statistics
df is short for Data Frame and represents the data table in the remaining cards
How do you calculate the mean of col_name_2 for each value of col_name_1?
df |>
group_by(col_name_1) |>
summarize(mean = mean(col_name_2))What does n() do inside summarize()?
Counts the number of rows in each group.
How do you group by more than one column?
Pass multiple column names to group_by()
For example,
df |>
group_by(col_name_1, col_name_2) |>
summarize(mean = mean(col_name_3))This produces one row for each unique combination of the two columns.
What do we use to combine two or more tables?
Joins
How do you combine df and df2 using their shared join_column?
inner_join(df, df2, join_by(join_column))Which rows does inner_join() keep?
Only rows that have matching values in both tables.
How do you join three tables together?
Join two tables and then join the result with the third table.
df |>
inner_join(df2, join_by(col_name_1)) |>
inner_join(df3, join_by(col_name_2))What are three ways to extract a single column (col_name) from df as a vector?
df$col_name
df[["col_name"]]
pull(df, col_name)You have the vectors vector_1, vector_2, and vector_3.
How do you combine them into a data frame?
df <- data.frame(
col_name_1 = vector_1,
col_name_2 = vector_2,
col_name_3 = vector_3
)or to make a tibble:
df <- tibble(
col_name_1 = vector_1,
col_name_2 = vector_2,
col_name_3 = vector_3
)How do you add a column that has the same value, 2026, in every row?
Use the single value in place of a vector.
df <- data.frame(
col_name_1 = vector_1,
col_name_2 = vector_2,
year = 2026
)