R for Data Science 2nd Edition Exercises (Wickham, Mine Çetinkaya-Rundel and Grolemund, 2023)

Workflow Code Style

Run the code in your script for the answers! I'm just exploring as I go.

Workflow Code Style Exercises

Packages to load

library(nycflights13)
library(tidyverse)

1. Restyle the following pipelines using what you have learned in this section:

#1
flights|>filter(dest=="IAH")|>group_by(year,month,day)|>summarize(n=n(),
delay=mean(arr_delay,na.rm=TRUE))|>filter(n>10)

#2
flights|>filter(carrier=="UA",dest%in%c("IAH","HOU"),sched_dep_time>
0900,sched_arr_time<2000)|>group_by(flight)|>summarize(delay=mean(
arr_delay,na.rm=TRUE),cancelled=sum(is.na(arr_delay)),n=n())|>filter(n>10)

• Restyling:

  • Insert a space before and after each pipe operator (|>).

  • Insert a linebreak after each pipe operator (|>).

  • Insert a space before and after all operators in the code ==, =, >, <, ,.

#1
flights |>
  filter(dest == "IAH") |>
  group_by(year, month, day) |>
  summarize(
    n = n(),
    delay = mean(arr_delay, na.rm = TRUE)
  ) |>
  filter(n > 10)

#2
flights |>
  filter(
    carrier == "UA", 
    dest %in% c("IAH", "HOU"), 
    sched_dep_time > 0900, 
    sched_arr_time < 2000
  ) |>
  group_by(flight) |>
  summarize(
    delay = mean(arr_delay, na.rm = TRUE), 
    cancelled = sum(is.na(arr_delay)), n = n()
  ) |>
  filter(n > 10)

Data Tidying

Data Tidying Exercises

Packages to load

library(tidyverse)

Clarifications:

• variables = columns.

• observations = rows.

• values = cells.

tidyr provides two functions for pivoting data: pivot_longer() and pivot_wider().

• pivot_longer() makes datasets longer by increasing the number of rows and decreasing the number of columns.

  • Commonly needed to tidy wide datasets as they often optimise for ease of data entry or ease of comparison rather than ease of analysis.

• pivot_wider()makes a dataset wider by increasing the number of columns and decreasing the number of rows.

  • Relatively rare to need pivot_wider() to make tidy data, but it’s often useful for creating summary tables for presentation, or data in a format needed by other tools.

1. For each of the sample tables, describe what each observation and each column represents.

table1

table1

table2

table2

table3

table3

• In each of table1, table2, and table3, each observation represents a country.
• In table1, country = country name, year = year of data collection, cases = number of people with the disease in that year, and population = number of people in each country in that year.
• In table2, country and year are the same as in table1, type = type of number, and count = number of observations (either cases or population depending on type).
• Finally, in table3, country and year are again the same as in table1, and rate = rate of disease (cases divided by population).

2. Sketch out the process you’d use to calculate the rate for table2 and table3. You will need to perform four operations:

   1. Extract the number of TB cases per country per year.

   2. Extract the matching population per country per year.

   3. Divide cases by population, and multiply by 10000.

   4. Store back in the appropriate place.

For table2, we need to reshape the data to have a column for cases and a column for population and then divide the two to calculate the rate.

table2 |>
  pivot_wider(
    names_from = type,
    values_from = count
  ) |> 
  mutate(rate = cases / population * 10000)

For table3, we need to separate cases and population into their own columns and then divide them.

table3 |>
  separate_wider_delim(
    cols = rate, 
    delim = "/", 
    names = c("cases", "population"),
  ) |>
  mutate(
    cases = as.numeric(cases),
    population = as.numeric(population),
    rate = cases / population * 10000
  )

Reference

Wickham, H., Mine Çetinkaya-Rundel and Grolemund, G. (2023) R for data science. 2nd ed. Sebastopol, CA: O’Reilly Media.