R shiny: run code if req not satisfied (equivalent to "req else"?)

user_5143653 0
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In a shiny app, I check the validity of an input with req and update an output if the requirement on the input is TRUE (i.e. not empty, not FALSE, etc.).

I would like to run some code if the req is FALSE, i.e. use a req() else {}-like structure.

I could use an if() {} else {} conditional structure, but the req tests many things that I would prefer to avoid implementing myself.

Here is a MWE:

  • If the input text_in is valid, a reactive value local$text is updated.
  • The content of the reactive value is then printed to the output text_out.
  • When the input text_in is not valid, I would like to execute some code, e.g. local$text <- "no text"

Note: I could run local$text <- "no text" before using the req but the app I am developing (not this example) uses multiple req and I cannot define a "default" value before calling these req.

library(shiny)

ui <- fluidPage(
    textInput("text_in", label = "write", value = "default"),
    verbatimTextOutput("text_out", placeholder = TRUE)
)

server <- function(input, output) {
    
    local <- reactiveValues(text = NULL)
    
    observe({
        req(input$text_in)
        local$text <- input$text_in
        # else {
        #     local$text <- "no text"
        # }
        
    })
    
    output$text_out <- renderText({
        local$text
    })
}
shinyApp(ui = ui, server = server)

Solution proposed by @mr-putter using shiny::isTruthy:

observe({
    if(isTruthy(input$text_in)) {
        local$text <- input$text_in
    } else {
        local$text <- "no text"
    }
})
shiny r
1 Answers

req returns "the first value that was passed in". So, you should be able to do something like this:

observe({
    if(isTruthy(input$text_in))
       local$text <- input$text_in
    else {
        local$text <- "no text"
    }
})
user_132415450
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