How to adjust numericinput in a app shiny

Question

I have an app with two numericinput . Both presented values between 0 and 1. What I wanted to do is the following: As the sum of the two weights must equal 1, so when I select the first weight, for example, 0.2, the second will be 0.8. Got the idea?

Executable code below

library(shiny)

ui <- fluidPage(
  
  numericInput("weight1", label = h4("Weight 1"),
               min = 0, max = 1, value = 0.5),
  
  numericInput("weight2", label = h4("Weight 2"),
               min = 0, max = 1, value = 0.5),
  
  helpText("The sum of weights should be equal to 1"),
  
  hr(),
  
  fluidRow(column(3, verbatimTextOutput("value1"))),
  fluidRow(column(3, verbatimTextOutput("value2")))
  
)

server <- function(input, output,session) {
  
  output$value1 <- renderPrint({ input$weight1 })
  
  output$value2 <- renderPrint({ input$weight2 })
  
}

shinyApp(ui = ui, server = server)

Answer 1

You can do it by using observeEvent and updateNumericInput . Here's what the code will look like:

server <- function(input, output,session) {
  
  observeEvent(input$weight1, {
    updateNumericInput(session, 'weight2',
                       value = 1 - input$weight1)
  })
  
  output$value1 <- renderPrint({ input$weight1 })
  
  output$value2 <- renderPrint({ input$weight2 })
  
}

Note : You don't need updateNumericInput if you are dealing with only two numbers and every time you need the sum to be equal to 1.