library(shiny)
library(bslib)
library(datasets)

# Data pre-processing ----
# Tweak the "am" variable to have nicer factor labels -- since this
# doesn't rely on any user inputs, we can do this once at startup
# and then use the value throughout the lifetime of the app
mpgData <- mtcars
mpgData$am <- factor(mpgData$am, labels = c("Automatic", "Manual"))


# Define UI for miles per gallon app ----
ui <- page_sidebar(

  # App title ----
  title = "Miles Per Gallon",

  # Sidebar panel for inputs ----
  sidebar = sidebar(

    # Input: Selector for variable to plot against mpg ----
    selectInput(
      "variable",
      "Variable:",
      c(
        "Cylinders" = "cyl",
        "Transmission" = "am",
        "Gears" = "gear"
      )
    ),

    # Input: Checkbox for whether outliers should be included ----
    checkboxInput("outliers", "Show outliers", TRUE)
  ),

  # Output: Formatted text for caption ----
  h3(textOutput("caption")),

  # Output: Plot of the requested variable against mpg ----
  plotOutput("mpgPlot")
)

# Define server logic to plot various variables against mpg ----
server <- function(input, output) {

  # Compute the formula text ----
  # This is in a reactive expression since it is shared by the
  # output$caption and output$mpgPlot functions
  formulaText <- reactive({
    paste("mpg ~", input$variable)
  })

  # Return the formula text for printing as a caption ----
  output$caption <- renderText({
    formulaText()
  })

  # Generate a plot of the requested variable against mpg ----
  # and only exclude outliers if requested
  output$mpgPlot <- renderPlot({
    boxplot(
      as.formula(formulaText()),
      data = mpgData,
      outline = input$outliers,
      col = "#75AADB",
      pch = 19
    )
  })
}

# Create Shiny app ----
shinyApp(ui, server)