Subtracting columns from different dataframes inside reactive expression

Question

Newbie to RShiny here, hoping for some guidance.

I am trying to build a simple shiny app that allows the user to upload two csv files, subtract specific columns of one dataframe (B) from another (A) [both with the same column structures], and incorporate these results into the corresponding columns for df A. It should then display the results for this updated dataframe in the UI.

When I try to run the app, it comes up with the following error:

Error in .getReactiveEnvironment()$currentContext() : 
  Operation not allowed without an active reactive context. (You tried to do something that can only be done from inside a reactive expression or observer.)

I had thought the use of reactive statements when importing files A and B would mean that the remainder of the function is done inside a reactive expression... maybe I've got the wrong end of the stick. Grateful for any suggestions?

The server function is as follows:

server <- function(input, output) {
    #fileA import
    A <- reactive(
        read.csv(
            input$fileA$datapath,
            header = input$header,
            sep = input$sep,
            quote = input$quote
        )
    )
    #FileB import
    B <- reactive(
        read.csv(
            input$fileB$datapath,
            header = input$header,
            sep = input$sep,
            quote = input$quote
        )
    )
    
    ##subtract B from A (columns 2 and 4)
    A()[,2] <- as.numeric(A()[,2])
    B()[,2] <- as.numeric(B()[,2])
    A()[,4] <- as.numeric(A()[,4])
    B()[,4] <- as.numeric(B()[,4])
    
    Finalcosts_AB2 <- A()[,2]-B()[,2]
    Finalcosts_AB4 <- A()[,4]-B()[,4]
    
    A()[,2] <- Finalcosts_AB2
    A()[,4] <- Finalcosts_AB4
    
    output$ABdiff <- renderTable({
        req(input$filaA)
        req(input$fileB)
        return(A())})

}

#Run the application 
shinyApp(ui = ui, server = server)

I've been using the following approach in the UI:

    ui <- fluidPage(

    titlePanel("testfile"),
    
    sidebarPanel(
        #Input: select file A----
        fileInput(
            "fileA",
            "Upload fileA results (.CSV format)",
            multiple = FALSE,
            accept = c("text/csv", "text/comma-separated-values,
                                    text/plain", ".csv")
        ),
        #Horizontal line
        tags$hr(),
        #Input: select file B----
        fileInput(
            "fileB",
            "Upload fileB results (.CSV format)",
            multiple = FALSE,
            accept = c("text/csv", "text/comma-separated-values,
                                    text/plain", ".csv")
        ),
        #Horizontal line
        tags$hr(),
        #settings
        # Input: Checkbox if file has header ----
        checkboxInput("header", "Header", TRUE),
        
        # Input: Select separator ----
        radioButtons(
            "sep",
            "Separator",
            choices = c(
                Comma = ",",
                Semicolon = ";",
                Tab = "\t"
            ),
            selected = ","
        ),
        
        # Input: Select quotes ----
        radioButtons(
            "quote",
            "Quote",
            choices = c(
                None = "",
                "Double Quote" = '"',
                "Single Quote" = "'"
            ),
            selected = '"'
        ),
        
        # Horizontal line ----
        tags$hr(),
        
        # Input: Select number of rows to display ----
        radioButtons(
            "disp",
            "Display",
            choices = c(Head = "head",
                        All = "all"),
            selected = "head"
        )
    ),
    mainPanel(
        #header
        h3("Results"),
        #Output: HTML table of results----
        tableOutput("ABdiff")
    )
    
    )

Answer 1

Please try to avoid modifying reactive data frames. Once you modify your server code as shown below, it works fine.

server <- function(input, output) {
  #fileA import
  A <- reactive({
    da <- read.csv(
      req(input$fileA$datapath),
      header = input$header,
      sep = input$sep,
      quote = input$quote
    )
    da[,2] <- as.numeric(da[,2])
    da[,4] <- as.numeric(da[,4])
    da
  })
  
  #FileB import
  B <- reactive({
    db <- read.csv(
      req(input$fileB$datapath),
      header = input$header,
      sep = input$sep,
      quote = input$quote
    )
    db[,2] <- as.numeric(db[,2])
    db[,4] <- as.numeric(db[,4])
    db
  })
  
  output$ABdiff <- renderTable({
    req(A(),B())
    Finalcosts_AB2 <- A()[,2]-B()[,2]
    Finalcosts_AB4 <- A()[,4]-B()[,4]
    AA <- A()
    AA[,2] <- Finalcosts_AB2
    AA[,4] <- Finalcosts_AB4
    return(AA)
  })
  
}