如何使用selectInput的输入来切片数据

Question

我与R Shiny selectInput有类似的问题，我想使用input像这样对数据进行切片：

selectInput("category", "Choose a Category:",
                            choices = c('Any', levels(as.factor(unique(BD$DX_01_Cat))))), 

uiOutput("secondSelection")

if (input$category != "Any"){
       subsetSubsTab <<- subsetSubsTab[subsetSubsTab$DX_01_Cat==input$category];

output$secondSelection <- renderUI({
       selectInput("subdiagnosis", "Choose a Subdiagnosis:", choices = c("Any", as.character(subsetSubsTab[subsetSubsTab$DX_01_Cat==input$category, DX_01_Sub])) , selected = "Any")
     })

if (input$subdiagnosis != "Any"){
       subsetSubsTab <<- subsetSubsTab[subsetSubsTab$DX_01_Sub==input$subdiagnosis];
     }

但最后一个if statement无效。

我收到警告Warning: Error in if: argument is of length zero， subsetSubsTab实现一秒钟然后返回。有人可以帮忙吗？