Input widgets

What does the server receive?

Every input widget has the same first two arguments:

1someInput(inputId = "id",
2          label   = "Label",
3          ...)

1

How the server calls it: input$id

2

What the user reads

3

Widget-specific options (choices, min, max, starting value…)

What changes from widget to widget is what arrives in input$id: a number, a text, a vector, a date, a data frame…

In all the apps on this page, the server prints the class and the structure (str()) of each input, so you can see exactly what it receives. Play with the widgets and watch the boxes on the right.

Choosing from a list

#| '!! shinylive warning !!': |
#|   shinylive does not work in self-contained HTML documents.
#|   Please set `embed-resources: false` in your metadata.
#| standalone: true
#| components: [editor, viewer]
#| layout: vertical
#| viewerHeight: 560
## file: ui.R
library(shiny)

ui <- fluidPage(
  fluidRow(
    column(6, selectInput("sel", "selectInput()",
                          choices = c("rock", "pressure", "cars"))),
    column(6, verbatimTextOutput("sel_out"))
  ),
  fluidRow(
    column(6, selectInput("sel_named", "selectInput() with named choices",
                          choices = c("rock" = 1, "pressure" = 2, "cars" = 3))),
    column(6, verbatimTextOutput("sel_named_out"))
  ),
  fluidRow(
    column(6, selectInput("sel_multi", "selectInput(multiple = TRUE)",
                          choices = c("A", "B", "C"), multiple = TRUE)),
    column(6, verbatimTextOutput("sel_multi_out"))
  ),
  fluidRow(
    column(6, radioButtons("radio", "radioButtons()",
                           choices = c("Low" = "low", "High" = "high"))),
    column(6, verbatimTextOutput("radio_out"))
  ),
  fluidRow(
    column(6, checkboxGroupInput("group", "checkboxGroupInput()",
                                 choices = c("A", "B", "C"))),
    column(6, verbatimTextOutput("group_out"))
  )
)
## file: server.R
library(shiny)

# helper: print the class and the structure of an object
inspect <- function(x) {
  cat("class:", class(x), "\n")
  str(x)
}

server <- function(input, output, session) {
  output$sel_out       <- renderPrint(inspect(input$sel))
  output$sel_named_out <- renderPrint(inspect(input$sel_named))
  output$sel_multi_out <- renderPrint(inspect(input$sel_multi))
  output$radio_out     <- renderPrint(inspect(input$radio))
  output$group_out     <- renderPrint(inspect(input$group))
}

Things to notice

• With named choices ("rock" = 1) the user sees the name, the server receives the value, and the value is always text: "1", not 1
• multiple = TRUE and checkboxGroupInput() return a vector, and NULL when nothing is selected

Numbers and text

#| '!! shinylive warning !!': |
#|   shinylive does not work in self-contained HTML documents.
#|   Please set `embed-resources: false` in your metadata.
#| standalone: true
#| components: [editor, viewer]
#| layout: vertical
#| viewerHeight: 480
## file: ui.R
library(shiny)

ui <- fluidPage(
  fluidRow(
    column(6, sliderInput("slider", "sliderInput()",
                          min = 0, max = 100, value = 50)),
    column(6, verbatimTextOutput("slider_out"))
  ),
  fluidRow(
    column(6, sliderInput("range", "sliderInput() with a range",
                          min = 0, max = 100, value = c(20, 80))),
    column(6, verbatimTextOutput("range_out"))
  ),
  fluidRow(
    column(6, numericInput("num", "numericInput()",
                           value = 10, min = 0, max = 100, step = 0.5)),
    column(6, verbatimTextOutput("num_out"))
  ),
  fluidRow(
    column(6, textInput("text", "textInput()", value = "hello")),
    column(6, verbatimTextOutput("text_out"))
  )
)
## file: server.R
library(shiny)

inspect <- function(x) {
  cat("class:", class(x), "\n")
  str(x)
}

server <- function(input, output, session) {
  output$slider_out <- renderPrint(inspect(input$slider))
  output$range_out  <- renderPrint(inspect(input$range))
  output$num_out    <- renderPrint(inspect(input$num))
  output$text_out   <- renderPrint(inspect(input$text))
}

Things to notice

• If you give sliderInput() two starting values, you get a range: a vector of length 2
• Delete the number in numericInput(): the server receives NA
• textInput() always returns text, also when the user types 42

Yes/no and dates

#| '!! shinylive warning !!': |
#|   shinylive does not work in self-contained HTML documents.
#|   Please set `embed-resources: false` in your metadata.
#| standalone: true
#| components: [editor, viewer]
#| layout: vertical
#| viewerHeight: 380
## file: ui.R
library(shiny)

ui <- fluidPage(
  fluidRow(
    column(6, checkboxInput("check", "checkboxInput()", value = FALSE)),
    column(6, verbatimTextOutput("check_out"))
  ),
  fluidRow(
    column(6, dateInput("date", "dateInput()")),
    column(6, verbatimTextOutput("date_out"))
  ),
  fluidRow(
    column(6, dateRangeInput("dates", "dateRangeInput()")),
    column(6, verbatimTextOutput("dates_out"))
  )
)
## file: server.R
library(shiny)

inspect <- function(x) {
  cat("class:", class(x), "\n")
  str(x)
}

server <- function(input, output, session) {
  output$check_out <- renderPrint(inspect(input$check))
  output$date_out  <- renderPrint(inspect(input$date))
  output$dates_out <- renderPrint(inspect(input$dates))
}

Buttons

#| '!! shinylive warning !!': |
#|   shinylive does not work in self-contained HTML documents.
#|   Please set `embed-resources: false` in your metadata.
#| standalone: true
#| components: [editor, viewer]
#| layout: vertical
#| viewerHeight: 200
## file: ui.R
library(shiny)

ui <- fluidPage(
  fluidRow(
    column(6, actionButton("go", "actionButton(): click me!")),
    column(6, verbatimTextOutput("go_out"))
  )
)
## file: server.R
library(shiny)

inspect <- function(x) {
  cat("class:", class(x), "\n")
  str(x)
}

server <- function(input, output, session) {
  output$go_out <- renderPrint(inspect(input$go))
}

An actionButton() is a counter: it starts at 0 and increases by 1 at every click. The value itself is rarely useful: what matters is that it changes. In Wait for it: eventReactive() we will use this to decide when the app does something.

Files

#| '!! shinylive warning !!': |
#|   shinylive does not work in self-contained HTML documents.
#|   Please set `embed-resources: false` in your metadata.
#| standalone: true
#| components: [editor, viewer]
#| layout: vertical
#| viewerHeight: 330
## file: ui.R
library(shiny)

ui <- fluidPage(
  fileInput("file", "fileInput(): upload any CSV file", accept = ".csv"),
  verbatimTextOutput("file_out")
)
## file: server.R
library(shiny)

inspect <- function(x) {
  cat("class:", class(x), "\n")
  str(x)
}

server <- function(input, output, session) {
  output$file_out <- renderPrint(inspect(input$file))
}

No CSV at hand? Use example.csv.

input$file is not the data: it is a small data frame describing the file (name, size, type) and, most importantly, datapath, the place where Shiny saved a copy of it. To read the data you use it like any path:

read.csv(input$file$datapath)

Before the upload, input$file is NULL.

Cheat sheet

Widget	input$id contains	Class
selectInput(), radioButtons()	the value of the selected choice	character (always)
selectInput(multiple = TRUE), checkboxGroupInput()	the selected values, NULL if none	character vector
sliderInput()	a number (two numbers for a range)	numeric/integer
numericInput()	a number, NA if empty	numeric/integer
textInput()	the text, "" if empty	character
checkboxInput()	TRUE/FALSE	logical
dateInput(), dateRangeInput()	one / two dates	Date
actionButton()	how many times it was clicked	counter (integer)
fileInput()	NULL, then name, size and datapath of the file	data.frame

The complete list, with code, is in the Shiny widget gallery.

Your turn

Exercise 1 · Predict, then check

Without running anything, what does the server receive in these cases?

1. selectInput("n", "Rows", choices = c("five" = 5, "ten" = 10)), the user picks ten
2. checkboxGroupInput("v", "Variables", choices = c("x", "y")), the user picks nothing
3. sliderInput("s", "Range", min = 1, max = 10, value = c(2, 5))

Check your answers by pasting the widgets in one of the apps above.

Solution 1

1. "10": a text, not a number. To use it as a number: as.numeric(input$n)
2. NULL
3. c(2, 5): input$s[1] is the minimum, input$s[2] the maximum

Exercise 2 · In your project

In my-shiny-app, add a sliderInput() with ID "n" (from 1 to 10) to ui.R, and a second verbatimTextOutput() with ID "n_out". In server.R, print input$n * 2 in it. Run the app.

Solution 2

# ui.R
ui <- fluidPage(
  selectInput(inputId = "mySelection", label = h3("Select box"),
              choices = list("Choice 1" = 1, "Choice 2" = 2, "Choice 3" = 3),
              selected = 1),
  verbatimTextOutput(outputId = "myOutput"),
  sliderInput(inputId = "n", label = "Pick a number", min = 1, max = 10, value = 5),
  verbatimTextOutput(outputId = "n_out")
)

# server.R
server <- function(input, output, session) {
  output$myOutput <- renderPrint({
    paste("This is my choice:", input$mySelection)
  })
  output$n_out <- renderPrint({
    input$n * 2
  })
}