r shiny - reduce uploaded dataset through selection of values

Question

I am trying to write a script to upload a .csv file and to throw a subset option (in the shape of checkboxes) back in the UI. After uploading, previously invisible checkboxes are displayed, showing actual data values. At the same time, an action button also appears, to be clicked after (potential) alteration of the selected data values (via the checkboxes).

So what do I have?
1) Upload - OK.
2) Selection and display of checkboxes - OK.
3) Display of action button - OK.
4) Proper execution of the desired action (= subsetting the data) - not OK; subsetting returns an empty or NULL dataframe, apparently.

Any help would be greatly appreciated.

Here's the code.

I will add that the Dte field I am basing the subsetting on is a date field, which is originally in the csv as dd/mm/yyyy format and without quotes.

library(shiny)
library(lubridate)

ui = fluidPage(sidebarPanel(width = 2,
                            fileInput('files', '', accept = c('text/csv','text/comma-separated-values','text/tab-separated-values','text/plain','.csv','.tsv')),
                            checkboxInput('header', 'Header', TRUE),
                            radioButtons('sep', 'Separator', c(Comma=',',Semicolon=';',Tab='\t'), selected=';'),
                            radioButtons('quote', 'Quote', c(None='','Double Quote'='"','Single Quote'="'"), selected='"'),
                            uiOutput('apply')),
               mainPanel(width = 10, uiOutput('years'), dataTableOutput("tabel4")
               )
)

server = function(input, output, session) {
  ### if a csv is selected (through fileInput()), it is read
  vis0 <- eventReactive(input$files, {
    vis <- read.csv(input$files$datapath, header = input$header, sep = input$sep, quote = input$quote,
                           stringsAsFactors =FALSE)
    vis$Dte <- as.Date(as.character(vis$Dte), "%d/%m/%Y")
    vis$year <- factor(year(vis$Dte))
    vis
    })
  ### button appearing only after 
  output$apply <- renderUI({
    if (is.null(input$files)) return()
    actionButton("Load", "APPLY DATA")
    })
  ### get checkbox values from data and throw them back to UI
  output$years <- renderUI({
    df <- vis0()
    df$year <- as.character(df$year)
    if (is.null(df)) return(NULL)
    items= sort(unique(df$year))
    names(items)=items
    checkboxGroupInput("dependent","Check or uncheck years to be included or excluded",
                       choices=items, selected = items, inline = TRUE)
    })
  ### use selected checkbox values to reduce initial dataset
  vis1 <- reactive({
    if (is.null(input$Load)) return()
    if (input$Load==0) return()
    data <- vis0()
    data <- subset(data, as.character(year) %in% input$years)
    data
    })
  ### using the checkbox-reduced data to create a table  
  output$tabel4 <- renderDataTable({
    vis1()
    })
  }

shinyApp(ui = ui, server = server, options = list(launch.browser=TRUE))

Answer 1

I found the solution!

One silly error (at least) was that dependent had to be years. Maybe there were more (especially in the if checks) that I fixed and didn't track properly, but the below code works.

library(shiny)
library(lubridate)

ui = fluidPage(sidebarPanel(width = 2,
                            fileInput('files', '', accept = c('text/csv','text/comma-separated-values','text/tab-separated-values','text/plain','.csv','.tsv')),
                            checkboxInput('header', 'Header', TRUE),
                            radioButtons('sep', 'Separator', c(Comma=',',Semicolon=';',Tab='\t'), selected=';'),
                            radioButtons('quote', 'Quote', c(None='','Double Quote'='"','Single Quote'="'"), selected='"'),
                            uiOutput('apply')),
               mainPanel(width = 10, uiOutput('years'),tags$br(),tags$br(), 
                         textOutput('showyrs'),tags$br(),tags$br(),
                         textOutput('yrz'),tags$br(),tags$br(),
                         dataTableOutput("tabel4")
               )
)


######################################################################

server = function(input, output, session) {
  ### if a csv is selected (through fileInput()), it is read
  vis0 <- reactive({
    req(input$files)
    vis <- read.csv(input$files$datapath, header = input$header, sep = input$sep, quote = input$quote,
                           stringsAsFactors =FALSE)
    vis$Dte <- as.Date(as.character(vis$Dte), "%d/%m/%Y")
    vis$year <- factor(year(vis$Dte))
    vis
    })

  ### button appearing only after 
  output$apply <- renderUI({
    if (is.null(input$files)) return()
    actionButton("Load", "APPLY DATA")
    })

  ### get checkbox values from data and throw them back to UI
  output$years <- renderUI({
    req(vis0)
    df <- vis0()
    df$year <- as.character(df$year)
    if (is.null(df)) return(NULL)
    items= sort(unique(df$year))
    names(items)=items
    checkboxGroupInput("years","Check or uncheck years to be included or excluded",
                       choices=items, selected = items, inline = TRUE)
    })

  ### show input$years
  output$showyrs <- renderText({
    req(input$files)
    if (is.null(input$years)) {
      "Nothing..."
    } else if (input$years==0) {
      "Nothing..."
    } else {input$years}
  })

  ### use selected checkbox values to reduce initial dataset
  vis1 <- reactive({
    req(input$years)
    if (is.null(input$Load)) return()
    if (input$Load==0) return()
    data <- vis0()
    data <- subset(data, as.character(year) %in% input$years)
    data
    })

  ### using the checkbox-reduced data to create a table  
  output$tabel4 <- renderDataTable({
    vis1()
    })
  }

shinyApp(ui = ui, server = server, options = list(launch.browser=TRUE))