Add and Remove elements in Shiny - r

Question

The code worked only when I added variables and removed for the first time. After I removed that variable, it went back to the select "Add to the plot", then I couldn't add it back, I think when I updated updateSelectInput, there is something wrong. Plus selectRemove needed to remove when it NULL. How I can update by both ways?
library(shiny)
mtcars_1 <- mtcars[,c("mpg", "disp", "hp", "drat", "wt", "qsec")]
runApp(list(
ui=pageWithSidebar(headerPanel("Adding and Removing Variables"),
sidebarPanel(
selectInput(inputId = "selectAdd", label = "Add to the plot",
choices = c(names(mtcars_1)),
selected = names(mtcars_1)[1]),
actionButton(inputId = "add", label = "Add to the plot")
),
mainPanel(
textOutput("text"),hr(),
uiOutput("remove_list")
)
),
server=function(input, output, session) {
rv <- reactiveValues(add_v = c())
observeEvent(input$add,{
rv$add_v <- rbind(rv$add_v,input$selectAdd)
})
rv <- reactiveValues(rem_v = c())
observeEvent(input$remove,{
rv$rem_v <- rbind(rv$rem_v,input$selectRemove)
})
observe({
value_add <- c(names(mtcars_1)[!names(mtcars_1) %in% rv$add_v ],rv$rem_v)
value_rem <-c(rv$add_v[! rv$add_v %in% rv$rem_v])
updateSelectInput(session,"selectAdd",choices = value_add)
updateSelectInput(session,"selectRemove",choices = value_rem)
})
output$remove_list <- renderUI({
if(length(rv$add_v) > 0){
tagList(
selectInput(inputId = "selectRemove", label = "Remove to the plot",
choices = c(rv$add_v),
selected = rv$add_v[1]),
actionButton(inputId = "remove", label = "Remove to the plot")
)
}
})
output$text <- renderText({
c(rv$add_v[! rv$add_v %in% rv$rem_v])
})
}))

Answer 1

Welcome to stackoverflow!
You were almost there - however, your updating logic for the reactiveValues wasn't complete. For each button click you'll have to add an object to one value and remove it from the other. Please check the following:
library(shiny)
mtcars_1 <- mtcars[, c("mpg", "disp", "hp", "drat", "wt", "qsec")]
runApp(list(
ui = pageWithSidebar(
headerPanel("Adding and Removing Variables"),
sidebarPanel(
selectInput(
inputId = "selectAdd",
label = "Add to the plot",
choices = names(mtcars_1),
selected = names(mtcars_1)[1]
),
actionButton(inputId = "add", label = "Add to the plot")
),
mainPanel(textOutput("text"), hr(),
uiOutput("remove_list"))
),
server = function(input, output, session) {
rv <- reactiveValues(add_v = NULL, rem_v = names(mtcars_1))
observeEvent(input$add, {
rv$rem_v <- setdiff(rv$rem_v, input$selectAdd)
rv$add_v <- union(rv$add_v, input$selectAdd)
})
observeEvent(input$remove, {
rv$add_v <- setdiff(rv$add_v, input$selectRemove)
rv$rem_v <- union(rv$rem_v, input$selectRemove)
})
observe({
updateSelectInput(session, "selectAdd", choices = rv$rem_v)
updateSelectInput(session, "selectRemove", choices = rv$add_v)
})
output$remove_list <- renderUI({
if (length(rv$add_v) > 0) {
tagList(
selectInput(
inputId = "selectRemove",
label = "Remove to the plot",
choices = c(rv$add_v),
selected = rv$add_v[1]
),
actionButton(inputId = "remove", label = "Remove to the plot")
)
}
})
output$text <- renderText({
c(rv$add_v[!rv$add_v %in% rv$rem_v])
})
}
))