why seq() function in shinyApps does not work? - r

Question

I tried to create a shinyApp with seq() function within the Apps.
header <- dashboardHeader(title = 'Testing' ,titleWidth = 300)
sidebar <- dashboardSidebar(uiOutput("sidebarpanel"), width = 300)
body <- dashboardBody(uiOutput("body"))
uix <- dashboardPage(header, sidebar, body)
serverx <- function(input, output, session){
output$sidebarpanel <- renderUI({
div(
sidebarMenu(id="tabs",
menuItem("Tes 1", tabName = "tes1", icon = icon("dashboard"), selected = TRUE)
)
)
})
output$body <- renderUI({
tabItems(tabItem(tabName = "tes1",
fluidRow(column(2, textInput("s1", "From :", value = 1))
,column(2, textInput("s2", "To", value = 7))
),
textOutput("result")
)
)
})
segment_low <- reactiveValues(ba=NULL)
segment_high <- reactiveValues(ba=NULL)
results <- reactiveValues(ba=NULL)
toListen <- reactive({
list(input$s1, input$s2)
})
observeEvent(toListen(),{
segment_low$ba <- input$s1 %>% as.numeric()
segment_high$ba <- input$s2 %>% as.numeric()
})
observe({
results$ba <- seq(segment_low$ba,segment_high$ba, 1)
})
output$result <- renderText({
results$ba
})
}
shinyApp(uix, serverx)
In that syntax, I would create a variable called results$ba because I want to escalate that value in the next time. But it turns out an error :
Warning: Error in seq.default: 'from' must be of length 1
[No stack trace available]
Could someone help me how to solve this problem? Since this error just happened if I put the reactiveValues to the seq() function, while I input a static input, for example seq(2,5,1) it will not return a error. And I already put the initial value for each input in textInput() function also.
Kindle need your help, developers!
Many Thanks.

Answer 1

The issue is that you're rendering the s1 and s2 inputs server-side. Because of this, the server at the beginning renders them as NULL, and your seq function errors when it gets the NULL value.
The simplest thing to do is to add a req function to prevent your code from evaluating unless it's getting some non-NULL values.
observe({
req(segment_low$ba, segment_high$ba)
results$ba <- seq(segment_low$ba,segment_high$ba, 1)
})
Basically, since you're using observe, which is very eager, you are telling the seq function to evaluate right away. By using the req function, you're stopping the chain of evaluation from happening unless the segment_low$ba and segment_high$ba have non-NULL values.