Invalid (NULL) left side of assignment error in Shiny

Question

I know this question has been posted a few times but this is my first time developing something i Shiny and I am getting confused with a couple different things. One of them is inputting the data frame correctly and using it in the output functions.

My only goals right now is to:

1. Display the head or complete dataframe depending on user choice

2. I have a binary column called status (status being Pass or Fail). I want to group by dates to count the status (any one would do) and plot it.

#
# This is a Shiny web application. You can run the application by clicking
# the 'Run App' button above.
#
# Find out more about building applications with Shiny here:
#
#    http://shiny.rstudio.com/
#

library(readxl)
library(shiny)

# Define UI for application that draws a histogram
ui <- fluidPage(

   # Application title
   titlePanel("Data Quality Result Monitoring"),

   # Sidebar with a slider input for number of bins 
   sidebarLayout(
      sidebarPanel(
        fileInput('file1', 'Choose xlsx file',
                  accept = c(".xlsx")
        ),

        sliderInput("bins",
                     "Number of bins:",
                     min = 1,
                     max = 50,
                     value = 30)
      ),

        radioButtons("disp", "Display",
                choices = c(Head = "head",
                            All = "all"),
                selected = "head")

      ),
      # Show a plot of the generated distribution
      mainPanel(
         #plotOutput("linechart"),

         h4("Observations"),
         tableOutput("contents")
      )
)    


# Define server logic required to draw a histogram'
library(ggplot2)
server <- function(input, output) {

   df <- reactive({

     inFile <- input$file1

     if (is.null(inFile))
     return(NULL)

     df <- read_xlsx(inFile$datapath, sheet =  1)

    return(inFile)})

  output$linechart <- renderPlot({
   ndf() <- group_by(df,Execution_Date) %>% summarize( count = n() )

   ggplot(ndf()) + geom_bar(aes(x=week,y=count),stat="identity")
   })

   output$contents <- renderTable({

     # input$file1 will be NULL initially. After the user selects
     # and uploads a file, head of that data file by default,
     # or all rows if selected, will be shown.

     dataset() <- df
     if(input$disp == "head") {
       return(head(dataset()))
     }
     else {
       return(dataset())
     }

   })

}

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

Answer 1

dataset() <- df

This is where you get the error:

 "Error in <-: invalid (NULL) left side of assignment"

You can not assign a value to a reactive expression. It works the other way round:

dataset <- df()

Play around with this by using the print function.

Another error in your code is this:

 df <- read_xlsx(inFile$datapath, sheet =  1)

return(inFile)

You return the wrong variable, you want to return the df.

Here is the code which should work for you:

#
# This is a Shiny web application. You can run the application by clicking
# the 'Run App' button above.
#
# Find out more about building applications with Shiny here:
#
#    http://shiny.rstudio.com/
#

library(readxl)
library(shiny)

# Define UI for application that draws a histogram
ui <- fluidPage(

  # Application title
  titlePanel("Data Quality Result Monitoring"),

  # Sidebar with a slider input for number of bins 
  sidebarLayout(
    sidebarPanel(
      fileInput('file1', 'Choose xlsx file',
                accept = c(".xlsx")
      ),

      sliderInput("bins",
                  "Number of bins:",
                  min = 1,
                  max = 50,
                  value = 30)
    ),

    radioButtons("disp", "Display",
                 choices = c(Head = "head",
                             All = "all"),
                 selected = "head")

  ),
  # Show a plot of the generated distribution
  mainPanel(
    #plotOutput("linechart"),

    h4("Observations"),
    tableOutput("contents")
  )
)    


# Define server logic required to draw a histogram'
library(ggplot2)
server <- function(input, output) {

  df <- reactive({

    inFile <- input$file1

    if (is.null(inFile))
      return(NULL)

    df <- read_xlsx(inFile$datapath, sheet =  1)

    df

    })

  output$linechart <- renderPlot({
    ndf <- group_by(df(),Execution_Date) %>% summarize( count = n() )

    ggplot(ndf + geom_bar(aes(x=week,y=count),stat="identity"))
  })

  output$contents <- renderTable({

    # input$file1 will be NULL initially. After the user selects
    # and uploads a file, head of that data file by default,
    # or all rows if selected, will be shown.

    dataset <- df()
    if(input$disp == "head") {
      return(head(dataset))
    }
    else {
      return(dataset)
    }

  })

}

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

I also recommend that you implement a check for structure and names in your code.

Answer 2

This is due to ndf() <- group_by(df,Execution_Date) %>% summarize( count = n() )

ndf() is NULL function that does not exist.

df is a reactive and you use it with df() instead of df, meaning that code is evaluated each time the reactivity changes.