Basic R shiny local scoping object - r

Question

I want a local variable in each session that can be updated by an input, which can be used by all other functions in the server. See the simple example below, I want the object to be updated when the user changes value but it doesn't?
library(shiny)
# Define UI for application
ui = shinyUI(pageWithSidebar(
# Application title
headerPanel("Hello Shiny!"),
# Sidebar with a slider input for data type
sidebarPanel(
selectInput("data",
"Pick letter to us in complex app?", choices = c("A","B"),
selected = "A")
),
# Print letter
mainPanel(
textOutput("Print")
)
))
server =shinyServer(function(input, output) {
MYLetter = "A";
updateData = reactive({
if (input$data == "A") {
MYLetter <<- "A"
} else {
MYLetter <<- "B"
}
})
output$Print <- renderText({
print(MYLetter)
})
})
shinyApp(ui, server)
I feel a solution will be global variables, but if two people are on the app at the same time. Will one person assigning a new value to a global variable change the variable for the other user?

Answer 1

There's a couple problems with your code. Here is the code that you want, I tried making very minimal changes to your code in order to make it work:
ui = shinyUI(pageWithSidebar(
# Application title
headerPanel("Hello Shiny!"),
# Sidebar with a slider input for data type
sidebarPanel(
selectInput("data",
"Pick letter to us in complex app?", choices = c("A","B"),
selected = "A")
),
# Print letter
mainPanel(
textOutput("Print")
)
))
server =shinyServer(function(input, output) {
MYLetter = reactiveVal("A");
observe({
if (input$data == "A") {
MYLetter("A")
} else {
MYLetter("B")
}
})
output$Print <- renderText({
print(MYLetter())
})
})
shinyApp(ui, server)
Essentially the two problems were:
What you are looking for is creating a reactive value with reactiveVal() or reactiveValues(). You're absolutely correct that creating a global variable is not the correct solution, because then it would be shared among all the users. It also is not reactive that way.
I changed the reactive({...}) to an observe({...}). It's very important to understand the difference between a reactive and an observer. I suggest reading online about it. I changed it to an observe because you weren't returning a value that was being used - rather, you were making an assignment within it.