Printing output based on dynamic user input in R Shiny - r

Question

I am quite new to R shiny and so have not been able to figure out the solution from similar questions posted on this site. I am trying to read and use the input that a user provides to R shiny to generate an output.
I am trying to create a simple GUI where a user selects the name of a person (from a drop-down menu) and then enters his/her weight. If the height is above a certain threshold the output recommendation is "Gain Weight", else it is "Loose Weight".
Everything seems to be working fine, except for the following error from the Server.R file:
Error in `$.shinyoutput`(output, value_weight) :
Reading objects from shinyoutput object not allowed
How can I read and use the variable 'value_weight' in an if-then-else condition?
Main.R
library(shiny)
runApp()
Server.R
function(input, output) {
# You can access the value of the widget with input$select, e.g.
output$value_name <- renderPrint({ input$select })
output$value_weight <- renderPrint({ input$num })
if(output$value_weight > 150)
{
output$value_recommendation <- "Loose Weight"
}
else{
output$value_recommendation <- "Gain Weight"
}
}
UI.R
names_list <- list("Adam", "Jenna","Peter")
fluidPage(
selectInput("select", label = h3("Select Name"), choices = names_list, selected = 1),
hr(),
fluidRow(column(3, verbatimTextOutput("value_name"))),
numericInput("num", label = h3("Enter Weight"), value = 0),
hr(),
fluidRow(column(3, verbatimTextOutput("value_weight"))),
hr(),
fluidRow(column(3, verbatimTextOutput("value_recommendation")))
)

Answer 1

The problem in your code is the line
if(output$value_weight > 150)
Generally speaking, outputs are write-only objects in the server, while inputs are readonly. If you replace output$value_weight with input$num, everything should work fine. You also need to use a render-function for outputs: in this case renderPrint or renderText (see the documentation for the difference between those two render functions).
## server.R
function(input, output) {
# You can access the value of the widget with input$select, e.g.
output$value_name <- renderPrint({ input$select })
output$value_weight <- renderPrint({ input$num })
output$value_recommendation <- renderPrint({
if(input$num > 150)
"Loose Weight"
else
"Gain weight"
})
}
Another way to do this is using a call to the function reactive
## server.R
function(input, output) {
# You can access the value of the widget with input$select, e.g.
output$value_name <- renderPrint({ input$select })
value_weight <- reactive({ input$num })
output$value_weight <- renderPrint({ value_weight() })
output$value_recommendation <- renderPrint({
if(value_weight() > 150)
"Loose Weight"
else
"Gain weight"
})
}

Answer 2

Using 'renderText' solved the issue!
Server.R
function(input, output)
{
output$value_market <- renderPrint({ input$select })
output$value_demand <- renderPrint({ input$num })
output$value_recommendation <- renderText({
if(input$num > 150)
{
print("Loose Weight")
}
else{
print("Gain Weight")
}
})
}