r shiny renderUI in a loop - r

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