Chapter 8 Project Solutions

The Data

Run the following R command:

load("/com/threshold_states.rda")

Take a look at the data:

View(threshold_states)

Each row is a state in the United States, on a particular day. Only states that have experienced 500 or more cases of COVID-19 by April 4, 2020, the most recent day covered by this data. The variables are:

• date: the date
• state: the name of the state
• fips: a standard geographic code for the state
• cases: the number of confirmed cases of COVID-19 in the state, up to the given date
• deaths: number of deaths in the state due to COVID-19, up to the given date
• day_number: number of days since the 500-case mark was reached in the state

The data frame threshold_states comes from public data provided by the New York Times. For more information see:

https://github.com/nytimes/covid-19-data

Problem 1

Write a function called state_cases_compare() that makes line plots of the number of confirmed cases against the number of days since reaching the threshold of 500 or more confirmed cases, for any set of states that the user specifies. The function should take one parameter called states, a character vector of the names of the states that are to figure in the graph. If one of the requested states has not yet experienced 500 cases, then the function should return a message to that effect, and not produce a graph. Examples of use should be as follows:

state_cases_compare(
  state = c("New Jersey", "Louisiana", "Washington")
)

Here is an example where one of the states has not experienced 500 cases by April 4, 2020:

state_cases_compare(
  state = c("Alaska", "Arkansas", "West Virginia")
)

## Alaska has not yet had 500 cases.
## West Virginia has not yet had 500 cases.
## Try other states!

state_cases_compare <- function(states) {
  
  ## first take care of validation:
  graphable_states <- unique(threshold_states$state)
  good_to_go <- TRUE
  for (state in states) {
    if (!(state %in% graphable_states)) {
      cat(state, " has not yet had 500 cases.\n", sep = "")
      good_to_go <- FALSE
    }
  }
  if (!good_to_go) {
    return(cat("Try other states!"))
  }
  
  ## if we made it this far, our input is OK,
  
  ## now get the all rows for the states to graph:
  df <- subset(threshold_states, state %in% states)
  
  ## now make the graph:
  ggplot(df, aes(x = day_number, y = cases)) +
    geom_line(aes(color = state)) +
    labs(
      x = "days since reaching 500 cases",
      y = "number of conformed cases"
    )
}

Problem 2

threshold_states$mortality_rate <- 
  with(
    threshold_states,
    deaths / cases * 100
  )
df <- threshold_states[, c("date", "state", "cases", "mortality_rate")]

df2 <- subset(
  df,
  date == "2020-04-04",
  select = c(state, cases, mortality_rate) 
)
df3 <- df2[order(df2$mortality_rate, decreasing = TRUE), ]