action/submit button for multiple numeric inputs in shiny - r

Question

In the shiny application multiple numeric input widgets are generated dynamically each having an initial value inside which are row-column numbers. The sum of those values are also displayed in main panel.
As the user changes the numeric input values the sum updates accordingly. Currently it is instantly. I want to delay the process of inputs in main panel for all numeric inputs by adding action/submit button till I change more than one numeric Inputs.
But I am getting the following error if action/submit buttons are used inside the render functions.
Warning: Error in <<-: number of items to replace is not a multiple of replacement length.
If not inside render functions where else to place as these widgets are generated inside renderUI only.
library(shiny)
ui <- shinyUI(fluidPage(
titlePanel(title = "Use of action/submit button for multiple inputs"),
sidebarLayout(
sidebarPanel(numericInput("rows","Input No. of rows",value = 3,min=1),
br(),
numericInput("col","Input No. of cols",value = 1,min=1)),
mainPanel(textOutput("display"),
uiOutput("plo")
))))
Server.r
server <- function(input,output){
# creating input widgets dynamically
output$plo <- renderUI({
z <- input$col
lapply(seq(input$col), function(j){
column(width=3,
lapply(seq(input$rows),function(i){
numericInput(inputId = paste0("range",paste0(i,j)),label = j,value = paste0(i,j))
})
)
})
})
# capturing the value of input widgets in a matrix
cm <- reactive({
c <- input$col
r <- input$rows
changed_m <- matrix(nrow = r,ncol = c)
lapply(seq(input$col), function(j){
lapply(seq(input$rows),function(i){
changed_m[i,j] <<- input[[paste0("range",paste0(i,j))]]
})
})
changed_m
})
# display the sum
output$display <- renderText({
paste0("Sum of matrix: ",sum(cm()))
})
}

Answer 1

Here is a working example of a possible solution. You can store the string to display (or just the sum of course) in a reactiveVal, and update this only when the user clicks the button, or display an alternative text when one of the inputs has changed so the user knows the sum is no longer correct.
Hope this helps!
library(shiny)
ui <- shinyUI(fluidPage(
titlePanel(title = "Use of action/submit button for multiple inputs"),
sidebarLayout(
sidebarPanel(numericInput("rows","Input No. of rows",value = 3,min=1),
br(),
numericInput("col","Input No. of cols",value = 1,min=1),
actionButton('update' ,'update!')),
mainPanel(textOutput("display"),
uiOutput("plo")
))))
server <- function(input,output){
# creating input widgets dynamically
output$plo <- renderUI({
z <- input$col
lapply(seq(input$col), function(j){
column(width=3,
lapply(seq(input$rows),function(i){
numericInput(inputId = paste0("range",paste0(i,j)),label = j,value = paste0(i,j))
})
)
})
})
# capturing the value of input widgets in a matrix
cm <- reactive({
c <- input$col
r <- input$rows
changed_m <- matrix(nrow = r,ncol = c)
lapply(seq(input$col), function(j){
lapply(seq(input$rows),function(i){
x=input[[paste0("range",paste0(i,j))]]
changed_m[i,j] <<- ifelse(!is.null(x),x,0)
})
})
changed_m
})
# initialize our reactiveVal with an empty string
my_sum <- reactiveVal('')
# observer that listens to the button click, then updates the sum string.
observeEvent(input$update,{
my_sum(paste0("Sum of matrix: ",sum(cm())))
})
# observer that listens to changes in the input, then updates the sum string.
observeEvent(cm(),ignoreNULL = T,ignoreInit = T, {
isolate(my_sum('invalidated. Press button to update.'))
})
# display the sum string
output$display <- renderText({
my_sum()
})
}
shinyApp(ui,server)