Example to help https://stackoverflow.com/questions/48652185/creating-new-column-for-reactive-dataframe-in-shiny-shiny-dashboard-r/

shinyReprex.r

library(shiny)
library(BatchGetSymbols)

ui <- fluidPage(
  textInput('stock','stock',"GE"),
  sliderInput('length', 'length', min = 1, max = 100, value = 50),
  dataTableOutput('my_table')
)


server <- function(input, output, session) {
  l.out <- reactive({
    BatchGetSymbols(tickers = input$stock, 
                    first.date = Sys.Date() - as.integer(input$length),
                    last.date = Sys.Date())
  })
  stock_info <- reactive({
    l.out()$df.tickers
  })
  stock_info()$return <- reactive({
    rep(0, length(stock_info()$ref.date))
  })
  
  stock_info()$return <- reactive({
    for (i in 2:length(stock_info()$ref.date)){
      stock_info()$return[i] <- ((stock_info()$price.close[i] - 
                                    stock_info()$price.close[i - 1]) / stock_info$price.close[i - 1])
    }
  })
  
  output$my_table <- renderDataTable({
    stock_info()
  })
}

shinyApp(ui, server)