using server outputs as input parameters in Shiny - r

Question

Below you will find some code for a very simple shiny dashboard.
It includes a global list called choice, which has three elements: date.1, date.2 and date.3 (each one is a different date sequence).
The UI includes just three inputs:
One select input which allows the user to select one of our global choices.
One start date input
One end date input.
The server function starts with a reactive expression which creates a vector of our choices selection. Then there are two text outputs which pull the min and max date values of our selection.
What i'm trying to do is use the start.date and end.date outputs to populate the value parameters of the date inputs in the ui, so depending on which choice you select the starting points of the date inputs will change.
library(shiny)
start.date1 <- as.Date("2017-01-01","%Y-%m-%d")
end.date1 <- as.Date("2017-01-31","%Y-%m-%d")
start.date2 <- as.Date("2017-02-01","%Y-%m-%d")
end.date2 <- as.Date("2017-02-28","%Y-%m-%d")
start.date3 <- as.Date("2017-03-01","%Y-%m-%d")
end.date3 <- as.Date("2017-03-31","%Y-%m-%d")
choice <- list()
choice$date.1 <- as.character(seq(start.date1,end.date1,by="week"))
choice$date.2 <- as.character(seq(start.date2,end.date2,by="week"))
choice$date.3 <- as.character(seq(start.date3,end.date3,by="week"))
ui <- fluidPage(
selectInput("select.scenario","select.scenario", choices = names(choice)),
dateInput("start.date","start.date",value = textOutput("start.date")),
dateInput("end.date","end.date",value = textOutput("end.date"))
)
server <- function(input, output) {
dates <- eventReactive(input$select.scenario,{
df <- choice[[as.character(input$select.scenario)]]
})
output$start.date <- renderText({
df <- dates()
start.date <- min(as.Date((df)))
start.date
})
output$end.date <- renderText({
df <- dates()
end.date <- max(as.Date((df)))
end.date
})
}
shinyApp(ui = ui, server = server)

Answer 1

You can't output into the ui definition. Either use uiOutput, or set the values/ranges using the update functions:
library(shiny)
start.date1 <- as.Date("2017-01-01","%Y-%m-%d")
end.date1 <- as.Date("2017-01-31","%Y-%m-%d")
start.date2 <- as.Date("2017-02-01","%Y-%m-%d")
end.date2 <- as.Date("2017-02-28","%Y-%m-%d")
start.date3 <- as.Date("2017-03-01","%Y-%m-%d")
end.date3 <- as.Date("2017-03-31","%Y-%m-%d")
choice <- list()
choice$date.1 <- as.character(seq(start.date1,end.date1,by="week"))
choice$date.2 <- as.character(seq(start.date2,end.date2,by="week"))
choice$date.3 <- as.character(seq(start.date3,end.date3,by="week"))
ui <- fluidPage(
selectInput("select.scenario","select.scenario", choices = names(choice)),
dateInput("start.date","start.date"),
dateInput("end.date","end.date")
)
server <- function(session, input, output) {
dates <- eventReactive(input$select.scenario,{
df <- choice[[as.character(input$select.scenario)]]
})
observeEvent(input$select.scenario, {
updateDateInput(session, "start.date", value=min(as.Date(dates())))
updateDateInput(session, "end.date", value=max(as.Date(dates())))
})
}
shinyApp(ui = ui, server = server)