Page ranges to print multiple pages on both sides of a sheet -- an R script using shiny

print both sides

# Page ranges to print on both sides of a sheet ----
# author       : Srikanth KS (talegari)
# contact      : gmail me at sri dot teach
# version      : 1
# last updated : 31st Dec 2015
# purpose      : If your printer prints on one side(you manually
#                turn and feed the papers) and you need to print
#                multiple pages per sheet

# load libraries ----
library('shiny')
# ui ----
ui <-
  pageWithSidebar(
  headerPanel("Page ranges to print on both sides of a sheet")
  ,sidebarPanel(
    numericInput(inputId = 'number_of_pages'
                 , label = 'Number of Pages'
                 , value = 50
                 , min = 2
                 , max = 10000),
    numericInput(inputId = 'number_of_pages_per_sheet'
                 , label = 'Number of Pages per sheet'
                 , value = 2
                 , min = 1
                 , max = 100),
    p('----'),
    a('code on github gist',href = 'https://gist.github.com/talegari/165a689f8b6f8b9dd35f')
  )
  ,mainPanel(
    p('If your printer prints on one side(you manually turn and feed the papers) and you need to print multiple pages per sheet')
    ,h4('First print these,')
    ,verbatimTextOutput('odd_pages')
    ,h4('Turn and insert the printed pages and print these')
    ,verbatimTextOutput('even_pages')
  )
) # end of pageWithSidebar
# define print_both_sides ----
print_both_sides <- function(n_pages
                             ,n_pages_per_sheet){
  stopifnot(n_pages_per_sheet < n_pages)
  # left/right ends of pages that go into one sheet
  left  <- seq(from = 1
               ,to = n_pages
               ,by = n_pages_per_sheet)
  right <- seq(from = n_pages_per_sheet
               ,to = n_pages
               ,by = n_pages_per_sheet)
  # handle oddity
  ell <- length(left)
  if (ell > length(right))
    right[ell] <- tail(left,1)
  # combine left right ends
  print_vector <- vapply(1:ell
                         ,function(x){
                           paste(left[x],right[x],sep = '-')
                         }
                         ,character(1)
  )
  # split into front/back of sheet
  ell2              <- length(print_vector)
  odd_pages_vector  <- print_vector[seq(from = 1,to = ell2,by = 2)]
  even_pages_vector <- print_vector[seq(from = 2,to = ell2,by = 2)]
  odd_pages  <- do.call('paste'
                        ,as.list(c(odd_pages_vector,sep = ',')))
  even_pages <- do.call('paste'
                        ,as.list(c(even_pages_vector,sep = ',')))
  # screen printing
  list(odd_pages = odd_pages
       ,even_pages = even_pages)
}
# server ----
server <-
  function(input,output){
    np  <- reactive({
      input$number_of_pages
    })
    nps <- reactive({
      input$number_of_pages_per_sheet
    })
    os  <- reactive({
      print_both_sides(np(),nps())
    })
    output$odd_pages  <- renderText({
      os()$odd_pages
    })
    output$even_pages <- renderText({
      os()$even_pages
    })
  }
# shinyall call ----
shinyApp(ui = ui, server = server)