From R for Data Science
Exercises 4.2.5
1-Compare dep_time, sched_dep_time, and dep_delay. How would you expect those three numbers to be related?
dep_delay is the difference between dep_time and sched_dep_time. dep_time - sched_dep_time
2-Brainstorm as many ways as possible to select dep_time, dep_delay, arr_time, and arr_delay from flights.
> flights |>
+ select(dep_time, dep_delay, arr_time, arr_delay)
> flights |>
+ select(dep_time, dep_delay, arr_time, arr_delay)
3-What happens if you specify the name of the same variable multiple times in a select() call?
It only is shown one time
4-What does the any_of() function do? Why might it be helpful in conjunction with this vector?
variables <- c("year", "month", "day", "dep_delay", "arr_delay")
any_of() selects any of the columns listed. It’s helpful with the variables vector because variables might be used multiple times, and would be useful instead of writing them out.
From the documentation: any_of() doesn’t check for missing variables. It is especially useful with negative selections, when you would like to make sure a variable is removed.
My example
> flights |>
+ select(any_of(variables))
# A tibble: 336,776 × 5
year month day dep_delay arr_delay
<int> <int> <int> <dbl> <dbl>
1 2013 1 1 2 11
2 2013 1 1 4 20
3 2013 1 1 2 33
4 2013 1 1 -1 -18
5 2013 1 1 -6 -25
6 2013 1 1 -4 12
7 2013 1 1 -5 19
8 2013 1 1 -3 -14
9 2013 1 1 -3 -8
10 2013 1 1 -2 8
# ℹ 336,766 more rows
# ℹ Use `print(n = ...)` to see more rows
5-Does the result of running the following code surprise you? How do the select helpers deal with upper and lower case by default? How can you change that default?
flights |> select(contains("TIME"))
The results are all columns that contain the word “time” in them.
The default is to ignore case. You can change that with changing ignore.case=FALSE
contains(match, ignore.case = FALSE, vars = NULL)
6-Rename air_time to air_time_min to indicate units of measurement and move it to the beginning of the data frame.
> flights |>
+ rename(air_time_min=air_time) %>%
+ relocate(air_time_min)
# A tibble: 336,776 × 19
air_time_min year month day dep_time sched_dep_time
<dbl> <int> <int> <int> <int> <int>
1 227 2013 1 1 517 515
2 227 2013 1 1 533 529
3 160 2013 1 1 542 540
4 183 2013 1 1 544 545
5 116 2013 1 1 554 600
6 150 2013 1 1 554 558
7 158 2013 1 1 555 600
8 53 2013 1 1 557 600
9 140 2013 1 1 557 600
10 138 2013 1 1 558 600
# ℹ 336,766 more rows
# ℹ 13 more variables: dep_delay <dbl>, arr_time <int>,
# sched_arr_time <int>, arr_delay <dbl>, carrier <chr>,
# flight <int>, tailnum <chr>, origin <chr>, dest <chr>,
# distance <dbl>, hour <dbl>, minute <dbl>, time_hour <dttm>
# ℹ Use `print(n = ...)` to see more rows
7-Why doesn’t the following work, and what does the error mean?
flights |>
select(tailnum) |>
arrange(arr_delay)
#> Error in `arrange()`:
#> ℹ In argument: `..1 = arr_delay`.
#> Caused by error:
#> ! object 'arr_delay' not found
It doesn’t work because select(tailnum) changes flights to only include the tailnum column, and arrange is trying to sort a column that no longer exists in the flights dataframe.