shiny/_includes/tutorial/shiny-text.md

The Shiny Text application demonstrates printing R objects directly as well as displaying data-frames using HTML tables. To run the example type:

> library(shiny)
> runExample("02_text")

The first example had a single numeric input specified using a slider and a single plot output. This example has a bit more going on: two inputs and two types of textual output.

If you try changing the number of observations to another value you'll see a demonstration of one of the most important attributes of Shiny applications -- inputs and outputs are connected together "live" and changes are propagated immediately (like a spreadsheet). In this case rather than the entire page being reloaded just the table view is updated when the number of observations change.

Here is the user interface definition for the application. Notice in particular that the mainPanel function now takes two arguments (corresponding to the two types of output displayed):

ui.R

library(shiny)

# Define UI for dataset viewer application
shinyUI(pageWithSidebar(

  # Application title
  headerPanel("Shiny Text"),

  # Sidebar with controls to select a dataset and specify the number
  # of observations to view
  sidebarPanel(
    selectInput("dataset", "Choose a dataset:", 
                choices = c("rock", "pressure", "cars")),

    numericInput("obs", "Number of observations to view:", 10)
  ),

  # Show a summary of the dataset and an HTML table with the requested
  # number of observations
  mainPanel(
    verbatimTextOutput("summary"),

    tableOutput("view")
  )
))

The server side of the application has also gotten a bit more complicated. Now rather than just computing a single output we see the definition of a reactive function to return the dataset corresponding to the user choice and two other reactive functions (reactivePrint and reactiveTable) that return the output$summary and output$view values.

These reactive functions work similarly to the reactivePrint function used in the first example: by declaring a reactive function you tell Shiny that it should only be executed when it's dependencies change. In this case that's either one of the user input values.

server.R

library(shiny)
library(datasets)

# Define server logic required to summarize and view the selected dataset
shinyServer(function(input, output) {

  # Return the requested dataset
  datasetInput <- reactive(function() {
    switch(input$dataset,
           "rock" = rock,
           "pressure" = pressure,
           "cars" = cars)
  })

  # Generate a summary of the dataset
  output$summary <- reactivePrint(function() {
    dataset <- datasetInput()
    summary(dataset)
  })

  # Show the first "n" observations
  output$view <- reactiveTable(function() {
    head(datasetInput(), n = input$obs)
  })
})

The next example will start with this one as a baseline and expand significantly on how reactive functions work in Shiny.