How to create dependent prompts in Shiny App in R - r

Question

I am tying to create an R shiny app and I would like to have two selectInput i.e. data set name and column name. Right now, I am able to get data set names in the first Input but I am not able to create dependent column selectIput (whose list will depend upon data set selected). Please guide.
require(shiny)
require(MASS)
a <- data(package = "MASS")[3]
b <- a$results[,3]
ui <- fluidPage(
sidebarPanel(width = 2,
selectInput(inputId = "dsname",label = "Select Dataset:",choices = c(b)),
colnames <- names("dsname"),
selectInput(inputId = "columns", label = "Choose columns",
choices = c(colnames))
)
)
server <- function(input,output) {}
shinyApp(ui <- ui, server <- server)

Answer 1

In order to have "responsive" elements in Shiny, you need to wrap your expressions for computing the responsive elements in reactive({...}).
You could use renderUI in your server() and uiOutput in your ui() with something like this. Here is an example I had built for using some of R's data sets (iris, mtcars, and diamonds):
library(shinythemes)
library(shiny)
library(ggplot2)
ui <- fluidPage(theme = shinytheme("superhero"),
titlePanel("Welcome to Responisve Shiny"),
sidebarLayout(
sidebarPanel(
selectInput("data", "Dataset:",
choices = c("mtcars", "iris", "diamonds")
),
uiOutput("x_axis"),
uiOutput("y_axis"),
uiOutput("color")
),
mainPanel(
plotOutput("distPlot")
)
)
)
server <- function(input, output) {
output$x_axis <- renderUI({
col_opts <- get(input$data)
selectInput("x_axis2", "X-Axis:",
choices = names(col_opts))
})
output$y_axis <- renderUI({
cols2 <- reactive({
col_opts2 <- get(input$data)
names(col_opts2)[!grepl(input$x_axis2, names(col_opts2))]
})
selectInput("y_axis2", "Y-Axis:",
choices = cols2(),
selected = "hp")
})
output$color <- renderUI({
col_opts <- get(input$data)
selectInput("color", "Color:",
choices = names(col_opts),
selected = "cyl")
})
output$distPlot <- renderPlot({
if(input$data == "mtcars"){
p <- ggplot(mtcars, aes_string(input$x_axis2, input$y_axis2, color = input$color)) +
geom_point()
}
if(input$data == "iris"){
p <- ggplot(iris, aes_string(input$x_axis2, input$y_axis2, color = input$color)) +
geom_point()
}
if(input$data == "diamonds"){
p <- ggplot(diamonds, aes_string(input$x_axis2, input$y_axis2, color = input$color)) +
geom_point()
}
print(p)
})
}
shinyApp(ui = ui, server = server)