Avoid re-loading datasets within a reactive in shiny - input

Question

I have a shiny app that requires the input from one of several files. A simplified example would be:
library(shiny)
x <- matrix(rnorm(20), ncol=2)
y <- matrix(rnorm(10), ncol=4)
write.csv(x, 'test_x.csv')
write.csv(y, 'test_y.csv')
runApp(list(ui = fluidPage(
titlePanel("Choose dataset"),
sidebarLayout(
sidebarPanel(
selectInput("data", "Dataset", c("x", "y"), selected="x")
),
mainPanel(
tableOutput('contents')
)
)
)
, server = function(input, output, session){
myData <- reactive({
inFile <- paste0("test_", input$data, ".csv")
data <- read.csv(inFile, header=FALSE)
data
})
output$contents <- renderTable({
myData()
})
}))
In reality, the files I read in are much large, so I would like to avoid reading them in each time input$data changes, if it has already been done once. For example, by making the matrices mat_x and mat_y available within the environment, and then within myData testing:
if (!exists(paste0("mat_", input$data))) {
inFile <- paste0("test_", input$data, ".csv")
data <- read.csv(inFile, header=FALSE)
assign(paste0("mat_", input$data), data)
}
Is there a way to do this, or do I have to create a separate reactive for mat_x and mat_y and using that within myData? I actually have 9 possible input files, but each user may only want to use one or two.

Answer 1

You could do something like
myData <- reactive({
data <- fetch_data(input$data)
data
)}
fetch_data <- function(input) {
if (!exists(paste0("mat_", input))) {
inFile <- paste0("test_", input, ".csv")
data <- read.csv(inFile, header=FALSE)
assign(paste0("mat_", input), data)
} else {
data <- paste0("mat_", input)
}
return (data)
}