Allow Shiny numericInput to depend on another numericInput - shiny

Question

I am creating a Shiny app where I would like to have the default of a numericInput be dependent on another default to a previously defined numericInput.
e.g.,
Here I would like the numericInput elements of (2) to be the reciprocal of (1), without having to specify values for value,min,max, and step beforehand:
(1) numericInput("obs1", "Label1", value = 10, min = 10, max = 20, step = 1)
(2) numericInput("obs2", "Label2", value = 1/10, min = 1/10, max = 1/20, step = 1)
Above (1) is the previously-defined numericInput.
Is there a simple way to do this?

Answer 1

If you want an input object to have dynamic parameters you need to use uiOutput, so that you can generate them in runtime (in server.R).
Example: In the first column you can set min, max and value. Modifying any of them renders obs1 and obs2 with new parameter values.
library(shiny)
ui <- fluidPage(
column(6,
tags$h2("Set parameters"),
numericInput("value", "Value", value = 20, min = 10, max = 60, step = 10),
numericInput("min", "Min", value = 10, min = 0, max = 30, step = 10),
numericInput("max", "Max", value = 40, min = 40, max = 60, step = 10)
),
column(6,
uiOutput("ui")
)
)
server <- function(input, output, session) {
output$ui <- renderUI( {
tagList(
tags$h2("Numeric inputs that depend on reactive data"),
numericInput("obs1", "Label1", value = input$value, min = input$min, max = input$max, step = 1),
numericInput("obs2", "Label2", value = input$value + 5, min = input$min - 5, max = input$max + 5, step = 1)
)
})
}
shinyApp(ui, server)
Please note that you need to wrap elements in tagList, when you want to pass more than one input element to renderUI.