Shiny App reactivity issue and scoping issue - r

Question

I am new to shiny and I am trying to build an app but I have been stuck on this one issue for a while. The purpose of the app is so users can upload their data, select their independent and dependent variables, select their number of trees...etc and eventually have that run through a random forest script and display the outputs.
However, right now I am stuck on setting up the drop-down input where users can select their variables (headers from the data they uploaded). It needs to be reactive so they first upload their data and then the app automatically knows what to put in the drop-down menu because it would be NULL otherwise. Here are copies of my ui.R and server.R files. If you know what may be wrong, your help would greatly be appreciated. Also, thank you to the people who helped me last week. I did not upload the actual R code (just images) so it was extra challenging for them.
ui.R
library(shiny)
shinyUI(fluidPage(
headerPanel(title = "Upload File"),
sidebarLayout(
sidebarPanel(
fileInput("file","Upload the file"),
h5("Max file size is 5 MB"),
tags$hr(),
radioButtons("sep","Seperator", choices = c(Comma = ",", Period = ".", Tilde = "~",minus = "-")),
tags$hr(),
checkboxInput("header","Header", TRUE),
tags$hr(),
uiOutput("vx"),
tags$hr(),
uiOutput("vy"),
tags$hr(),
numericInput("MTRY", "Set the MTRY", 0, min = 0, max = 500, step = 1,
width = 100),
helpText("The MTRY should default to 0"),
numericInput("numtree", "Number of Trees", 500, min = 30, max = 10000, step = 100,
width = 100)
),
mainPanel(
tableOutput("input_file")
)
)
)
)
Server.R
library(shiny)
shinyServer(function(input, output) {
output$input_file <- renderTable({
file_to_read = input$file
if(is.null(file_to_read)){
return()
}
dat1 <- read.table(file_to_read$datapath, sep = input$sep, header = input$header)
return(dat1)
})
reactive1 = reactive({
if(is.null(dat1))
return()
D <- colnames(dat1)
reactive1[[1]] = D
reactive1[[2]] = D[1]
reactive1
})
output$vx <- renderUI({
selectInput("cols", "Select Dependent Variable",
choices = colnames(reactive1()[[1]]), selected = reactive1()[[2]][1])
})
output$vy <- renderUI({
selectInput("cols", "Select Independent Variables",
choices = colnames(reactive1()[[1]]), selected = reactive1()[[2]][1], multiple = T)
})
})
Here is what the app looks like after uploading a csv:
App

Answer 1

The key is to make the input data reactive and updateSelectInput based off of the reactive data frame. See below:
ui <- fluidPage(
titlePanel("Uploading Files"),
sidebarLayout(
sidebarPanel(
fileInput("file1", "Choose CSV File",
multiple = FALSE,
accept = c("text/csv",
"text/comma-separated-values,text/plain",
".csv")),
tags$hr(),
checkboxInput("header", "Header", TRUE),
radioButtons("sep", "Separator",
choices = c(Comma = ",",
Semicolon = ";",
Tab = "\t"),
selected = ","),
tags$hr(),
selectInput("vars", "Label",choices = c("NULL"))
),
mainPanel(
tableOutput("contents")
)
)
)
server <- function(input, output, session) {
df1 <- reactive({
req(input$file1)
read.csv(input$file1$datapath,
header = input$header,
sep = input$sep,
quote = input$quote)
})
observeEvent(df1(), {
updateSelectInput(session, "vars", choices = names(df1()))
})
}
shinyApp(ui, server)
Please comment if this answers your question.