r一个循环中闪亮的renderUI

Question

I'd like to output several tables as a one uiOutput. If I put them together in a list using a loop then all outputs are equal to the last one. Example:

library(shiny)

ui <- fluidPage(
  mainPanel(
    uiOutput("tables")
  )
)

server <- function(input, output) {
  output$tables <- renderUI({
    data=array(rnorm(150),c(10,5,3))

    tfc = function(m){
#      x = m[1,1]
      renderTable({m})
    }

    result=list()
    for(i in 1:3)
      result[[i]] = tfc(data[,,i])

    return(result)
  })
}

shinyApp(ui = ui, server = server)

If I remove the commented line (x = m[1,1]) I get the desired result.

I can live with this workaround but is there a reason why shiny behaves like that or is there a different way to do it?

Answer 1

I usually use lapply for such usecases. This way, you don't run into issues with lazy evaluation.

library(shiny)

ui <- fluidPage(
  mainPanel(
    uiOutput("tables")
  )
)

server <- function(input, output) {
  output$tables <- renderUI({
    data=array(rnorm(150),c(10,5,3))

    tfc = function(m){renderTable({m})}

    lapply(1:3, function(i){tfc(data[,,i])})
  })
}

shinyApp(ui = ui, server = server)

If you want to use a reacive table, you can use something like

tfc = function(m, output, id){
  output[[id]] <- renderTable({m()})
  tableOutput(id)
}

instead.

Answer 2

To get around this, you can force evaluation of function arguments:

tfc = function(m) {
  force(m)
  renderTable(m)
}

or

create a local scope for each loop iteration:

for (i in 1:3) {
  local({
    i <- i
    result[[i]] <<- tfc(data[,,i])
  })
}

lapply works as well, but only for R versions 3.2 and above: https://cran.r-project.org/bin/windows/base/old/3.2.0/NEWS.R-3.2.0.html