Create dynamic input fields for a shiny application

Question

Im using the following dataframe to build a shiny application:

listIDs <- c(100,100,100,100,200,200,200,200),
values <- c(2.12, 2.43, 2.12, 4.45, 3.23, 4.23, 3.23, 4.23),  
horses <- c(2.1, 3.2, 4.1, 4.2, 5.4, 4.7, 2.8, 2.0),  
month <- c("JAN", "FEB", "JAN", "FEB","MAY","APRIL","MAY", "APRIL"),
df <- data.frame(listIDs, values, horses, month),

Im building a shiny application with the following tabs:

ui.R

shinyUI(fluidPage(

#outlinen title
titlePanel(title = "This is the title"),
sidebarLayout(
 sidebarPanel(
  
  selectInput("var1", "Select the eventID", choices = listIDs),
  selectInput("var2", "Select the eventID", choices = month),
  br()
 ),

 mainPanel(("Personal information"),
          plotOutput("myhist"))
)
)) 

server.R

library(shiny)
library(ggplot2)

shinyServer(function(input, output){

output$myhist <- renderPlot({

df_graph <- df[df$listIDs == input$var1,]
df_graph <- df_graph[df_graph$month == input$var2,]

ggplot(data=df_graph, aes(x=month, y=values, group = horses)) +
  geom_line() + geom_point() + theme(axis.text.x = element_text(angle = 90, hjust = 1))

})
})

It all works but the thing is that when I select 100 in my first selection box I stil get the options "JAN", "FEB", "MRT", "APRIL" (while I only should get JAN and FEB). Any thoughts on how I can make this dynamic?

Answer 1

Your selectInput element corresponding to the month selection should be rendered dynamically based on the value of input$var1. Here's a simple example:

shinyApp(
    ui = fluidPage(

        titlePanel(title = "This is the title"),
        sidebarLayout(
            sidebarPanel(
                selectInput("var1", "Select the eventID", 
                            choices = listIDs
                ),
                uiOutput("select_month_ui"),
                br()
            ),
            mainPanel(
                "Personal information",
                plotOutput("myhist")
            )
        )
    ),
    server = function(input, output) {
        output$select_month_ui <- renderUI({
            selectInput("var2", "Select the eventID", 
                        choices = df[df$listIDs %in% input$var1,"month"]
            )
        })

        output$myhist <- renderPlot({
            df_graph <- df[df$listIDs == input$var1,]
            df_graph <- df_graph[df_graph$month == input$var2,]

            ggplot(data = df_graph, 
                    aes(x = month, y = values, group = horses)) +
                geom_line() + geom_point() + 
                theme(axis.text.x = element_text(angle = 90, hjust = 1))
        })
    }
)

---

The selectInput object is moved from your UI code into the server code as

    output$select_month_ui <- renderUI({
        selectInput("var2", "Select the eventID", 
                    choices = df[df$listIDs %in% input$var1,"month"]
        )
    }) 

and replaced with uiOutput("select_month_ui").