Successive calculations in Shiny - r

Question

I want to make a shiny app that can make successive calculations based on user input. Something like this:
a <- input$inputa
b <- a+2
c <- b-3
d <- c*4
e <- d/5
So the user would choose input a, and the shiny app would do the rest and show values a, b, c, d, e.
I managed to do it if the app always makes the entire calculations based on a. But if I try to create value b and call it, it breaks.
The following code works and shows the final result as it should, but I'm sure it can be improved upon, removing repetitions:
# UI
ui <- fluidPage(
# Application title
titlePanel("Doing algebra"),
# Sidebar with numeric input
sidebarLayout(
sidebarPanel(
numericInput("inputa",
"Input a:",
min = 0,
max = 100,
value = 20,
step=1)
),
# Show results of successive calculations
mainPanel(
verbatimTextOutput("output1"),
h4("+2"),
verbatimTextOutput("output2"),
h4("-3"),
verbatimTextOutput("output3"),
h4("*4"),
verbatimTextOutput("output4"),
h4("/5"),
verbatimTextOutput("output5")
)
)
)
# server
server <- function(input, output) {
output$output1 <- renderText({ input$inputa })
output$output2 <- renderText({ input$inputa+2 })
output$output3 <- renderText({ ( input$inputa+2)-3 })
output$output4 <- renderText({ (( input$inputa+2)-3)*4 })
output$output5 <- renderText({ ((( input$inputa+2)-3)*4)/5 })
}
shinyApp(ui, server)
The last bit, (((input$inputa+2)-3)*4)/5, looks terrible and is terrible. Can I make a shiny app that creates a value in one equation and uses that value in the next equation?
Thanks!

Answer 1

You can store the data in a reactive expression.
ui <- fluidPage(
# Application title
titlePanel("Doing algebra"),
# Sidebar with numeric input
sidebarLayout(
sidebarPanel(
numericInput("inputa",
"Input a:",
min = 0,
max = 100,
value = 20,
step=1)
),
# Show results of successive calculations
mainPanel(
verbatimTextOutput("output1"),
h4("+2"),
verbatimTextOutput("output2"),
h4("-3"),
verbatimTextOutput("output3"),
h4("*4"),
verbatimTextOutput("output4"),
h4("/5"),
verbatimTextOutput("output5")
)
)
)
# server
server <- function(input, output) {
rv <- reactive({
tibble::tibble(a = input$inputa, b = a + 2, c = b-3, d = c*4, e = d/5)
})
output$output1 <- renderText({rv()$a})
output$output2 <- renderText({rv()$b})
output$output3 <- renderText({rv()$c})
output$output4 <- renderText({rv()$d})
output$output5 <- renderText({rv()$e})
}
shinyApp(ui, server)