r - tryCatch error handling within Shiny

Question

Background

I am using a shiny app to cut and table some data.

The dataset is included in the shiny code below, but the head is:

> head(df_in)
  Report_Year Position   Target
1        2014      CEO 29.27644
2        2014      CEO 29.27644
3        2014      CFO 17.56586
4        2014       CE 17.56586
5        2014      COO 17.56586
6        2014      CEO 46.84231

I am using the following statement to cut and table the data

  df <- df_in %>%
    filter(Report_Year == input$v_year,
           Position == "CEO") %>%
    select(Target) %>%
    filter(!is.na(Target)) %>%
    mutate(bins = cut(Target, breaks=seq(0, (max(Target)+25), 25))) %>%
    select(bins) %>%
    table %>%
    as.data.frame

>
         . Freq
1   (0,25]    0
2  (25,50]    6
3  (50,75]    2
4 (75,100]    1

In the data there are no entries for Report_Year == 2012, so when the user selects 2012 I want it to either display a message like 'no data' or at the moment I'd be happy with an empty data frame.

I have tried a tryCatch() statement but I'm obviously not doing it correctly as the app crashes when the user selects 2012.

Question

How should I be writing the tryCatch?

Shiny App

library(shiny)
library(dplyr)

ui <- shinyUI(fluidPage(
  sidebarLayout(
    sidebarPanel(
      selectInput("v_year", label = "select year", choices = c(2012, 2014), selected = 2014)
    ),
    mainPanel(
      dataTableOutput("dt")
    )
  )
))

server <- shinyServer(function(input, output) {


df <- reactive({

  ## data
  df_in <- structure(list(Report_Year = c(2014L, 2014L, 2014L, 2014L, 2014L, 
  2014L, 2014L, 2014L, 2014L, 2014L, 2014L, 2014L, 2014L, 2014L, 
  2014L, 2014L, 2014L, 2014L, 2014L, 2014L, 2014L, 2014L, 2014L, 
  2014L, 2014L, 2014L, 2014L, 2014L, 2014L, 2014L), Position = c("CEO", 
  "CEO", "CFO", "CE", "COO", "CEO", "CFO", "BUE", "CE", "CFO", 
  "CEO", "COO", "CE", "BUE", "COO", "CFO", "CE", "GC", "CEO", "CEO", 
  "CE", "BUE", "CEO", "CFO", "CE", "GC", "CFO", "CEO", "CEO", "CE"
  ), Target = c(29.2764408921928, 29.2764408921928, 17.5658645353157, 
  17.5658645353157, 17.5658645353157, 46.8423054275084, 38.6449019776945, 
  38.6449019776945, 38.6449019776945, 35.1317290706313, 35.1317290706313, 
  46.8423054275084, 35.1317290706313, 35.1317290706313, 43.9146613382892, 
  43.9146613382892, 35.1317290706313, 35.1317290706313, 29.2764408921928, 
  87.8293226765783, 11.7105763568771, 11.7105763568771, 29.2764408921928, 
  17.5658645353157, 35.1317290706313, 40.9870172490699, 40.9870172490699, 
  73.1911022304819, 70.2634581412627, 46.8423054275084)), class = "data.frame", .Names = c("Report_Year", 
  "Position", "Target"), row.names = c(NA, -30L))

    tryCatch({
      df <- df_in %>%
        filter(Report_Year == input$v_year,
               Position == "CEO") %>%
        select(Target) %>%
        filter(!is.na(Target)) %>%
        mutate(bins = cut(Target, breaks=seq(0, (max(Target)+25), 25))) %>%
        select(bins) %>%
        table %>%
        as.data.frame
    }, 
    warning=function(w) { 
      print("Warning")
      df <- data.frame()
      return(NA)
    }, 
    error=function(e) {
      print("Error")
      df <- data.frame()
      return(NULL)
    }
    )
})

output$dt <- renderDataTable({
  df  <- df()
})

})

shinyApp(ui = ui, server = server)

Session Info

> sessionInfo()
R version 3.2.0 (2015-04-16)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 14.04.2 LTS

locale:
 [1] LC_CTYPE=en_AU.UTF-8       LC_NUMERIC=C               LC_TIME=en_AU.UTF-8        LC_COLLATE=en_AU.UTF-8    
 [5] LC_MONETARY=en_AU.UTF-8    LC_MESSAGES=en_AU.UTF-8    LC_PAPER=en_AU.UTF-8       LC_NAME=C                 
 [9] LC_ADDRESS=C               LC_TELEPHONE=C             LC_MEASUREMENT=en_AU.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
 [1] knitr_1.10.5   rmarkdown_0.7  stringr_1.0.0  extrafont_0.17 scales_0.2.5   tidyr_0.2.0    ggplot2_1.0.1  reshape2_1.4.1
 [9] dplyr_0.4.2    shiny_0.12.1 

Error Message

Answer 1

How about something like this?

server <- shinyServer(function(input, output) {

    df_in <- structure(...) # As before

    # Filter data according to input$v_year
    df <- reactive({
        df_in %>%
            filter(Report_Year == input$v_year, Position == "CEO") %>%
            select(Target) %>%
            filter(!is.na(Target))
    })

    # Prepare bins or return if df is empty
    bins <- reactive({
        if(nrow(df()) == 0) return()
        df() %>%
            mutate(bins = cut(Target, breaks=seq(0, (max(Target)+25), 25))) %>%
            select(bins) %>%
            table %>%
            as.data.frame
    })

    output$dt <- renderDataTable({
        bins()
    })        
})

Since we check if df() is empty as long as your input is correct seq will never receive an empty argument.