req: Check for required values
In sheikhbarabas/shiny: Web Application Framework for R

Description Usage Arguments Details Value Examples

Ensure that values are available ("truthy"–see Details) before proceeding with a calculation or action. If any of the given values is not truthy, the operation is stopped by raising a "silent" exception (not logged by Shiny, nor displayed in the Shiny app's UI).

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req(..., cancelOutput = FALSE)

isTruthy(x)

`...`	Values to check for truthiness.
`cancelOutput`	If `TRUE` and an output is being evaluated, stop processing as usual but instead of clearing the output, leave it in whatever state it happens to be in.
`x`	An expression whose truthiness value we want to determine

The req function was designed to be used in one of two ways. The first is to call it like a statement (ignoring its return value) before attempting operations using the required values:

rv <- reactiveValues(state = FALSE)
r <- reactive({
  req(input$a, input$b, rv$state)
  # Code that uses input$a, input$b, and/or rv$state...
})

In this example, if r() is called and any of input$a, input$b, and rv$state are NULL, FALSE, "", etc., then the req call will trigger an error that propagates all the way up to whatever render block or observer is executing.

The second is to use it to wrap an expression that must be truthy:

output$plot <- renderPlot({
  if (req(input$plotType) == "histogram") {
    hist(dataset())
  } else if (input$plotType == "scatter") {
    qplot(dataset(), aes(x = x, y = y))
  }
})

In this example, req(input$plotType) first checks that input$plotType is truthy, and if so, returns it. This is a convenient way to check for a value "inline" with its first use.

Truthy and falsy values

The terms "truthy" and "falsy" generally indicate whether a value, when coerced to a logical, is TRUE or FALSE. We use the term a little loosely here; our usage tries to match the intuitive notions of "Is this value missing or available?", or "Has the user provided an answer?", or in the case of action buttons, "Has the button been clicked?".

For example, a textInput that has not been filled out by the user has a value of "", so that is considered a falsy value.

To be precise, req considers a value truthy unless it is one of:

FALSE
NULL
""
An empty atomic vector
An atomic vector that contains only missing values
A logical vector that contains all FALSE or missing values
An object of class "try-error"
A value that represents an unclicked actionButton

Note in particular that the value 0 is considered truthy, even though as.logical(0) is FALSE.

If the built-in rules for truthiness do not match your requirements, you can always work around them. Since FALSE is falsy, you can simply provide the results of your own checks to req:

req(input$a != 0)

Using req(FALSE)

You can use req(FALSE) (i.e. no condition) if you've already performed all the checks you needed to by that point and just want to stop the reactive chain now. There is no advantange to this, except perhaps ease of readibility if you have a complicated condition to check for (or perhaps if you'd like to divide your condition into nested if statements).

Using cancelOutput = TRUE

When req(..., cancelOutput = TRUE) is used, the "silent" exception is also raised, but it is treated slightly differently if one or more outputs are currently being evaluated. In those cases, the reactive chain does not proceed or update, but the output(s) are left is whatever state they happen to be in (whatever was their last valid state).

Note that this is always going to be the case if this is used inside an output context (e.g. output$txt <- ...). It may or may not be the case if it is used inside a non-output context (e.g. reactive, observe or observeEvent) – depending on whether or not there is an output$... that is triggered as a result of those calls. See the examples below for concrete scenarios.

The first value that was passed in.

## Only run examples in interactive R sessions
if (interactive()) {
  ui <- fluidPage(
    textInput('data', 'Enter a dataset from the "datasets" package', 'cars'),
    p('(E.g. "cars", "mtcars", "pressure", "faithful")'), hr(),
    tableOutput('tbl')
  )

  server <- function(input, output) {
    output$tbl <- renderTable({

      ## to require that the user types something, use: `req(input$data)`
      ## but better: require that input$data is valid and leave the last
      ## valid table up
      req(exists(input$data, "package:datasets", inherits = FALSE),
          cancelOutput = TRUE)

      head(get(input$data, "package:datasets", inherits = FALSE))
    })
  }

  shinyApp(ui, server)
}