r shinyapp: Change dataframe using numericInput

Question

I have the following shiny app, which consists of a numeric input and as outputs two ggplot-graphics.

library(shiny)

n <- 100
dat <- data.frame(var1 = round(rnorm(n, 50, 10),0),
                  var2 = sample(c("A", "B"), n, replace = TRUE))

# USER INTERFACE
ui <- fluidPage(

    titlePanel("My Sample App"),

    sidebarLayout(
        sidebarPanel(
            numericInput("n", "Number of cases", value=100)
        ),


        mainPanel(
           plotOutput("boxplot"),
           plotOutput("distribution")
        )
    )
)

# SERVER
server <- function(input, output) {

    output$boxplot <- renderPlot({
        ggplot(data = dat, aes(x = var2, y = var1)) + geom_boxplot() + ggtitle("Boxplot")
    })
    output$distribution <- renderPlot({
        ggplot(data = dat, aes(var1)) + geom_histogram() + ggtitle("Histogram")
    })
}

# Run the application 
shinyApp(ui = ui, server = server)

I've been trying to replace n = 10 with n = input$n. However it didn't work and I am unsure, where exactly I have to define the data.frame (inside the server function?). Can someone help please?

starja · Accepted Answer · 2020-08-15 07:20:43Z

input$n is a reactive variable that can only be used in a reactive context. You can only define a reactive context in the server function, e.g. using reactive. Have a look here for an explanation.

library(shiny)
library(ggplot2)


# USER INTERFACE
ui <- fluidPage(
  
  titlePanel("My Sample App"),
  
  sidebarLayout(
    sidebarPanel(
      numericInput("n", "Number of cases", value=100)
    ),
    
    
    mainPanel(
      plotOutput("boxplot"),
      plotOutput("distribution")
    )
  )
)

# SERVER
server <- function(input, output) {
  
  dat <- reactive({
    data.frame(var1 = round(rnorm(input$n, 50, 10),0),
               var2 = sample(c("A", "B"), input$n, replace = TRUE))
  })
  
  output$boxplot <- renderPlot({
    ggplot(data = dat(), aes(x = var2, y = var1)) + geom_boxplot() + ggtitle("Boxplot")
  })
  output$distribution <- renderPlot({
    ggplot(data = dat(), aes(var1)) + geom_histogram() + ggtitle("Histogram")
  })
}

# Run the application 
shinyApp(ui = ui, server = server)

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