Dynamically load a file and subset data from it (any column) in Shiny

Question

I'm trying to build an app that A) can load any given csv file B) from that loaded file, select 2 columns to then subset the data by selected attributes from these 2 columns. Obviously everything has to be done dynamically as the columns names and csv files would be different every time. I've had a go and am able to load the csv files, and select columns I want to display + those 2 that I wish to select upon but somehow the -breakdown- choices from these 2 columns do not seem to follow through the process and no list is visible when i hit the drop down input selector....

Any help would be much appreciated. Thank you very much

library(dplyr)
library(shinyWidgets)

fpath <- '/dbfs/May2022'

# Define UI
  ui <- fluidPage(theme = shinytheme("spacelab"),
    navbarPage(
      "MyQA Machine Learning Platform",
      tabPanel(
        "Select File",
  sidebarPanel(
    selectInput('selectfile','Select File',choice = list.files(fpath, pattern = ".csv")),
    mainPanel("Main Panel",dataTableOutput("ftxtout"),style = "font-size:50%") # mainPanel
  ), #sidebarPanel
              ), #tabPanel
      tabPanel("Subset Data",
               sidebarPanel(                
#                selectInput("columns", "Select Columns", choices = NULL,multiple = TRUE), # no choices before uploading 
#                ), #
                 dropdown(
    label = "Please Select Columns to Display", 
    icon = icon("sliders"),
    status = "primary",
    pickerInput(
      inputId = "columns",
#       label = "Select Columns",
      choices = NULL,
      multiple = TRUE
    )#pickerInput
  ), #dropdown
    selectInput("v_attribute1", "First Attribute to Filter Data", choices = NULL), 
    selectInput("v_attribute2", "Second Attribute to Filter Data", choices = NULL),
    selectInput("v_filter1", "First Filter", choices = NULL),
    selectInput("v_filter2", "Second Filter", choices = NULL),
                 
               ), #sidebarPanel
               
               mainPanel(tags$br(),tags$br(),
                            h4("Data Selection"),
                            dataTableOutput("txtout"),style = "font-size:70%"
                        ) # mainPanel
               
      ), # Navbar 1, tabPanel
      tabPanel("Create Label", "This panel is intentionally left blank")     
  
    ) # navbarPage
  ) # fluidPage

  
  # Define server function  
  server <- function(input, output, session) {     
        output$fileselected<-renderText({
      paste0('You have selected: ', input$selectfile)
    })
    
     info <- eventReactive(input$selectfile, {
#     inFile <- input$selectfile
    # Instead # if (is.null(inFile)) ... use "req"
    req(input$selectfile)
    fullpath <- file.path(fpath,input$selectfile)
    df <- read.csv(fullpath, header = TRUE,  sep = ",")
    vars <- names(df)
    # Update select input immediately after clicking on the action button. 
    updatePickerInput(session, "columns","Select Columns", choices = vars)
    updateSelectInput(session, "v_attribute1","First Attribute to Filter Data", choices = vars)
    updateSelectInput(session, "v_attribute2","Second Attribute to Filter Data", choices = vars) #everything seems to work until here...
    var1 = paste0(input$v_attribute1)
    choicesvar1=unique(df$var1)
    req(choicesvar1)
    updateSelectInput(session, "v_filter1","First Filter", choices = choicesvar1)
    var2 = paste0(input$v_attribute2)
    choicesvar2=unique(df$var2)
    req(choicesvar2)
    updateSelectInput(session, "v_filter2","Second Filter", choices = choicesvar2)
    df
  })   
    
    output$ftxtout <- renderDataTable({
      req(input$selectfile)
      fullpath <- file.path(fpath,input$selectfile)
      df <- read.csv(fullpath, header = TRUE,  sep = ",")
      head(df)
      }, options =list(pageLength = 5))
    
    output$txtout <- renderDataTable({
    f <- info()
    f <- subset(f, select = input$columns)
    f <- filter(f, input$v_attribute1 == input$v_filter1 & input$v_attribute1 == input$v_filter2)
    head(f)
  }, options =list(pageLength = 5)
                                            )
    
  } # server

  # Create Shiny object
  shinyApp(ui = ui, server = server)

Answer 1

As suggested by @Limey, you need to perform data wrangling separately. Try this

# Define UI
ui <- fluidPage(theme = shinytheme("spacelab"),
                navbarPage(
                  "MyQA Machine Learning Platform",
                  tabPanel(
                    "Select File",
                    sidebarPanel(
                      selectInput('selectfile','Select File',choice = list.files(fpath, pattern = ".csv")),
                      mainPanel("Main Panel",dataTableOutput("ftxtout"),style = "font-size:50%") # mainPanel
                    ), #sidebarPanel
                  ), #tabPanel
                  tabPanel("Subset Data",
                           sidebarPanel(
                             dropdown(
                               label = "Please Select Columns to Display",
                               icon = icon("sliders"),
                               status = "primary",
                               pickerInput(
                                 inputId = "columns",
                                 #       label = "Select Columns",
                                 choices = NULL,
                                 multiple = TRUE
                               )#pickerInput
                             ), #dropdown
                             selectInput("v_attribute1", "First Attribute to Filter Data", choices = NULL),
                             selectInput("v_attribute2", "Second Attribute to Filter Data", choices = NULL),
                             selectInput("v_filter1", "First Filter", choices = NULL),
                             selectInput("v_filter2", "Second Filter", choices = NULL),

                           ), #sidebarPanel

                           mainPanel(tags$br(),tags$br(),
                                     h4("Data Selection"),
                                     dataTableOutput("txtout"),style = "font-size:70%"
                           ) # mainPanel

                  ), # Navbar 1, tabPanel
                  tabPanel("Create Label", "This panel is intentionally left blank")

                ) # navbarPage
) # fluidPage


# Define server function
server <- function(input, output, session) {
  
  output$fileselected<-renderText({
    paste0('You have selected: ', input$selectfile)
  })

  info <- eventReactive(input$selectfile, {
    fullpath <- file.path(fpath,input$selectfile)
    read.csv(fullpath, header = TRUE,  sep = ",")
  })

  observeEvent(info(), {
    df <- info()
    vars <- names(df)
    # Update select input immediately after clicking on the action button.
    updatePickerInput(session, "columns","Select Columns", choices = vars, selected=vars[1:2])
  })
  
  observeEvent(input$columns, {
    vars <- input$columns
    updateSelectInput(session, "v_attribute1","First Attribute to Filter Data", choices = vars)
    updateSelectInput(session, "v_attribute2","Second Attribute to Filter Data", choices = vars, selected=vars[2]) #everything seems to work until here...
  })

  observeEvent(input$v_attribute1, {
    choicesvar1=unique(info()[[input$v_attribute1]])
    req(choicesvar1)
    updateSelectInput(session, "v_filter1","First Filter", choices = choicesvar1)
  })

  observeEvent(input$v_attribute2, {
    choicesvar2=unique(info()[[input$v_attribute2]])
    req(choicesvar2)
    updateSelectInput(session, "v_filter2","Second Filter", choices = choicesvar2)
  })

  output$ftxtout <- renderDataTable({
    head(info())
  }, options =list(pageLength = 5))

  output$txtout <- renderDataTable({
    f <- info() %>% subset(select = input$columns) 
    f$var1 <- f[[input$v_attribute1]]
    f$var2 <- f[[input$v_attribute2]]
    ff <- f %>% dplyr::filter(var1 == input$v_filter1 & var2 == input$v_filter2) 
    fff <- ff %>% subset(select=-c(var1,var2))
    head(fff)
  }, options =list(pageLength = 5)
  )

} # server

# Create Shiny object
shinyApp(ui = ui, server = server)