How to efficiently subset a dataframe in R Shiny? - r

Question

In the below example code I reactively subset the mtcars dataframe inside the renderPlot() function. However, in my larger App with many render functions in the server section I am having to repeat the same rv$x[1:input$samples], etc., over and over in many places. How would I apply this subsetting instead "at the top", into the rv <- reactiveValues(...) function itself or equivalent "master function"? I tried subsetting inside the reactiveValues() and got the message "Warning: Error in : Can't access reactive value 'samples' outside of reactive consumer. Do you need to wrap inside reactive() or observer()?" I assumed incorrectly that the reactiveValues() function is a "reactive consumer".
If someone can answer this basic understanding question, please explain the logic for correctly subsetting "at the top" because I am getting very embarrassed by my repeated questions about Shiny reactivity.
library(shiny)
ui <- fluidPage(
sliderInput('samples','Nbr of samples:',min=2,max=32,value=16),
plotOutput("p")
)
server <- function(input, output, session) {
rv <- reactiveValues(
x = mtcars$mpg,
y = mtcars$wt
)
output$p <- renderPlot({plot(rv$x[1:input$samples],rv$y[1:input$samples])})
}
shinyApp(ui, server)

Answer 1

There are multiple ways you can handle this.
Here is one way to create new subset reactive values inside observe.
library(shiny)
ui <- fluidPage(
sliderInput('samples','Nbr of samples:',min=2,max=32,value=16),
plotOutput("p")
)
server <- function(input, output, session) {
rv <- reactiveValues(
x = mtcars$mpg,
y = mtcars$wt
)
observe({
rv$x_sub <- rv$x[1:input$samples]
rv$y_sub <- rv$y[1:input$samples]
})
output$p <- renderPlot({plot(rv$x_sub,rv$y_sub)})
}
shinyApp(ui, server)

Answer 2

I'd use reactiveValues only if you need them to be modified in different places.
reactive is shiny's basic solution for this:
library(shiny)
library(datasets)
ui <- fluidPage(
sliderInput(
'samples',
'Nbr of samples:',
min = 2,
max = 32,
value = 16
),
plotOutput("p")
)
server <- function(input, output, session) {
reactive_mtcars <- reactive({mtcars[1:input$samples,]})
output$p <- renderPlot({
plot(reactive_mtcars()$mpg, reactive_mtcars()$wt)
})
}
shinyApp(ui, server)