Dynamic select input not working with loaded data - r

Question

Using R shiny, I am developing a simple app that allows user to input data from a Rdata file. I want the app to load the data, show the names of numeric variables in a select input field, and after the user selected one of variables do some analysis. But I can not get it working. In the code provided I obtain two outputs: summary, which works fine, and the MEAN of the selected variable which I can not get work.
server.R
library(shiny)
library(shinydashboard)
library(data.table)
library(DT)
shinyServer(function(input, output) {
#### DATA LOAD
df <- reactive({
df <- input$datafile
if (is.null(df)) {
# User has not uploaded a file yet
return(NULL)
}
objectsLoaded <- load(input$datafile$name)
# the above returns a char vector with names of objects loaded
df <- eval(parse(text=objectsLoaded[1]))
# the above finds the first object and returns it
df<-data.table(df)
})
#### SELECTS
num <- reactive({
num <- sapply(df(),is.numeric)
num <- names(num)
})
output$var_num <- renderUI({
vector.num <- as.vector(num())
selectInput("var_num", "Select Variables :", as.list(vector.num), multiple = FALSE)
})
#### OUTPUTS
### SUMMARY
output$summary_num <-renderDataTable({
x<-t(sapply(df(), summary))
x<-as.data.frame(x)
x<-setDT(x, keep.rownames = TRUE)[]
colnames(x) <- c("Variable","Mínimo","1er Quartil", "Mediana", "Media", "3er Quartil","Máximo")
datatable(x)
})
### MEAN OF SELECTED VAR
output$test <-renderPrint ({
if(is.null(df()))
return()
dat<- df()
dat <- dat[,num(), drop = FALSE]
mean(dat[,input$var_num])
})
})
UI.R
dashboardPage(
dashboardHeader(title = "TITLE", titleWidth = 500),
dashboardSidebar(disable = TRUE), #---> fin SIDEBAR
dashboardBody(
fluidRow(
box(width=12, status = "primary",
tabsetPanel(
tabPanel("Test",
fileInput("datafile", label = h3("File input")),
uiOutput("var_num"),
br(),hr(),br(),
fluidRow(column(width=4, uiOutput("var_caracter"),textOutput("test"))),
br(),hr(),br(),
fluidRow(column(width=8, "Variables Numericas", dataTableOutput("summary_num")))
)
) # fin tabsetPanel
) # fin box
)# fin fluidRow
)# fin dashboardBody
)# fin dashboardPage
When I run the app everything goes fine (select input, summary, etc) except the calculation and printing of the MEAN of the selected variable. I guess for some reason the subsetted dataframe is empty, but I do not know why...
Any help will be great! Thanks in advance.

Answer 1

I get it working.
The solution was to define the dataset I used as.data.frame:
### MEAN OF SELECTED VAR
output$test <-renderPrint ({
if(is.null(df()))
return()
dat<- as.data.frame(df()) ## THIS IS THE CORRECTION
dat <- dat[,num(), drop = FALSE]
mean(dat[,input$var_num])
})
I do not really understand why... The reactive file df() was defined as data.table and dat shoul inherit that, but for some reason it was necesary an explicit definition as dataframe.