r - How to get value of selectinput and show a variable data in another selectinput with shiny -

i want retrieve selected value user, put code in server.r inside finction selected_value <- input$valtext <-filter (catalog, gene_name == selected_value) b <-select (a,snp) valtext : id of selectizeinput

then want show snp correspondent of selected_value user (b) in selectizeinput in "choices", need retrieve variable b :

selectizeinput( 'snpvalue', 'choisir le snp visualiser :', choices = "" , multiple = true, options = list(maxitems = 2) )

my problem can not see b in data, seems nothing change !nothing execute, because if have variable b, can easly show in selectizeinput

regards.

in ui.r, replace second selectizeinput

uioutput('snpselect') 

in server.r, add

output$snpselect <- renderui({   selected_value <- input$valtext   <- filter (catalog, gene_name == selected_value)   b <- select (a,snp)   selectizeinput(      'snpvalue', 'choisir le snp visualiser :', choices = b, multiple = true, options = list(maxitems = 2)   ) })