如何在R中的Shiny中上传文件并命名列？

Question

Goal

To upload a file in a Shiny app that reads the data, name the variables (columns) and do some data analysis before presenting plot output

Reference Shiny App

I am using this app from Shiny gallery as a reference: enter link description here

What I have tried:

I want to use the uploaded data in many outputs after doing different analyses. So, instead of reading file inside renderTable or renderPlot , I read it in server function:

server <- function(input, output) {
     inFile <- reactive({input$file1})
     sdf <- reactive({read.csv(inFile()$datapath, header=F)})     

colnames(sdf()) <- c('Vehicle.ID', 'Time', 'Vehicle.class.no', 'Vehicle.type2', 
                         'Vehicle.Length', 'Lane', 'Preceding.Vehicle.ID', 'Spacing','Spacing2', 'State',
                         'svel.mps', 'deltaV.mps', 'sacc', 'lane.change')  }

Error

But when I run this app I get:

shiny::runApp('app-CC')

Listening on http://127.0.0.1:7484
Error in colnames(sdf()) <- c("Vehicle.ID", "Time", "Vehicle.class.no",  : 
  invalid (NULL) left side of assignment

Question

How can I fix this error? I don't want to read the same file again in every render* function. Are there any online examples of shiny apps where a new file is read, column names are defined and then some analysis is done before using the render* functions?

Answer 1

You'd be better off assigning the column names during the read.csv

server <- function(input, output) {
     inFile <- reactive({input$file1})
     sdf <- reactive({
         read.csv(inFile()$datapath, header=F, col.names = c('Vehicle.ID', 'Time', 'Vehicle.class.no', 'Vehicle.type2', 
            'Vehicle.Length', 'Lane', 'Preceding.Vehicle.ID', 'Spacing','Spacing2', 'State',
            'svel.mps', 'deltaV.mps', 'sacc', 'lane.change')  
         )
     }) 
}

Alternatively I believe you can perform multiple operations in the reactive block as long as you return the final object

server <- function(input, output) {
     inFile <- reactive({input$file1})
     sdf <- reactive({
         dd<-read.csv(inFile()$datapath, header=F)
         colnames(dd) <- c('Vehicle.ID', 'Time', 'Vehicle.class.no', 'Vehicle.type2', 
            'Vehicle.Length', 'Lane', 'Preceding.Vehicle.ID', 'Spacing','Spacing2', 'State',
            'svel.mps', 'deltaV.mps', 'sacc', 'lane.change')  
         )
         dd
     }) 
}

Answer 2

An alternative is to use an actionButton and then to check for the validity of the files when you upload them. Lots of observeEvent is probably appropriate for firing off when something is "triggered," like a file being uploaded or a button being pressed. In addition, to make a chain of changes, it's probably best to have an "update" reactiveValue() thing to fire based on flags (which, I admit, is kind of messy, but works with Shiny since it doesn't have a proper callback framework like JS.)

Answer 3

There is a tutorial on RStudio that teaches how to solve problems such as these. Hope it helps.

https://shiny.rstudio.com/tutorial/written-tutorial/lesson6/