R Shiny: How can I reference reactive elements in renderUI through uiOutput() in server function?

Question

I am trying to get one UI element to react with another, and then test to see if the first element is equal to one particular value. I seem to be able to print out the value of the reactive element, but I cannot use it in an if statement to check its value. Code is below. I have commented out the troublesome part around line 23. It runs with the lines commented out, and verifies that the reactive element input$paramRes1 does have an character value of an integer between 1 - 5, as expected, but when I un-comment out the lines to check if that integer value is 1, it tells me that input$paramRes1 is null. The exact error is:

Warning:
Error in if: argument is of length zero
Stack trace (innermost first):
59: observerFunc [#16]
 4: <Anonymous>
 3: do.call
 2: print.shiny.appobj
 1: <Promise>

Why would it tell me that input$paramRes1 is null, when it prints out as an char? Or am I misreading the error? Thanks for your help!

suppressWarnings(library(shiny))
suppressWarnings(library(shinyFiles))
    ui <- function(request) {
    fluidPage(
        uiOutput("param1"),
        uiOutput("param2"),
        fluidRow(column(12, verbatimTextOutput("value", placeholder = T)))
    )
}

server <- function(input, output, session) {
    state = reactiveValues()

    observe({
          state$choice <- setNames(as.list(c(1, 2, 3, 4, 5)), c("First", "Second",
            "Third", "Fourth", "Fifth"))
    })
    output$param1 <- renderUI({
        selectInput("paramRes1", "Select Param 1", state$choice, selected = 1)
    })
    output$param2 <- renderUI({
        textInput("paramNm", "Param", value = input$paramRes1, width = '150px')
    })

    observe({
      #  if (input$paramRes1 %in% 1) {
            state$val <- input$paramRes1
      #  } else {
      #      state$val <- 0
      #  }

    })

    output$value <- renderText({ str(state$val) })
}
shinyApp(ui = ui, server = server)

Answer 1

Here is a (slightly) simplified version of your app with the fix I suggested in the comments

library(shiny)

ui <- fluidPage(
  uiOutput("param1"),
  uiOutput("param2"),
  fluidRow(column(12, verbatimTextOutput("value", placeholder = T)))
)

server <- function(input, output, session) {
  state = reactiveValues(choice = list(
    first = 1, second = 2, third = 3, fourth = 4, fifth = 5
  ))
  output$param1 <- renderUI({
    selectInput("paramRes1", "Select Param 1", state$choice, selected = 1)
  })
  output$param2 <- renderUI({
    textInput("paramNm", "Param", value = input$paramRes1, width = '150px')
  })
  observeEvent(input$paramRes1,{
    if (input$paramRes1 %in% 1)
      state$val <- "It's 1"
    else
      state$val <- "It's not 1"
  })
  output$value <- renderText({ state$val })
}

shinyApp(ui = ui, server = server)

Answer 2

Would the following work? I'm only changing the server portion here. The difference is highlighted in between ###### .

server <- function(input, output, session) {
  state = reactiveValues()

  observe({
    state$choice <- setNames(as.list(c(1, 2, 3, 4, 5)), c("First", "Second",
                                                      "Third", "Fourth", "Fifth"))
   })
  output$param1 <- renderUI({
    selectInput("paramRes1", "Select Param 1", state$choice, selected = 1)
  })
  output$param2 <- renderUI({
    textInput("paramNm", "Param", value = input$paramRes1, width = '150px')
  })

#####
  observeEvent(input$paramRes1, {
     if (input$paramRes1 == 1) {
     state$val <- input$paramRes1
      } else {
     state$val <- 0
      }
  })
###### 

   output$value <- renderText({ str(state$val) })
 }