Chapter 4 Basic Reactivity


Draw the reactive graph for the following server functions:

server1 <- function(input, output, session) {
  c <- reactive(input$a + input$b)
  e <- reactive(c() + input$d)
  output$f <- renderText(e())

server2 <- function(input, output, session) {
  x <- reactive(input$x1 + input$x2 + input$x3)
  y <- reactive(input$y1 + input$y2)
  output$z <- renderText(x() / y())

server3 <- function(input, output, session) {
  d <- reactive(c() ^ input$d)
  a <- reactive(input$a * 10)
  c <- reactive(b() / input$c)
  b <- reactive(a() + input$b)


To create the reactive graph we need to consider the inputs, reactive expressions, and outputs of the app.

For server1 we have the following objects:

  • inputs: input$a, input$b, and input$d
  • reactives: c() and e()
  • outputs: output$f

Inputs input$a and input$b are used to create c(), which is combined with input$d to create e(). The output depends only on e().

reactive graph - server 1

For server2 we have the following objects:

  • inputs: input$y1, input$y2, input$x1, input$x2, input$x3
  • reactives: y() and x()
  • outputs: output$z

Inputs input$y1 and input$y2 are needed to create the reactive y(). In addition, inputs input$x1, input$x2, and input$x3 are required to create the reactive x(). The output depends on both x() and y().

reactive graph - server 2

For server3 we have the following objects:

  • inputs: input$a, input$b, input$c, input$d
  • reactives: a(), b(), c(), d()

As we can see below, a() relies on input$a, b() relies on both a() and input$b, and c() relies on both b() and input$c. The final output depends on both c() and input$d.

reactive graph - server 3


Can the reactive graph contain a cycle? Why/why not?


No! This will create circular references and a recursion loop!


Use reactive expressions to reduce the duplicated code in the following simple apps.


Unclear about the apps mentioned in the exercise.