R Shiny - Operation not allowed without an active reactive context - r

Question

This is my first time creating a shiny application, and I am getting this error message:
Error in .getReactiveEnvironment()$currentContext() :
Operation not allowed without an active reactive context. (You tried to do something that can only be done from inside a reactive expression or observer.)
I suspect it has something to do with using the slider's value to do my calculation. Please assist, and thanks in advance!
UI Start
header = dashboardHeader(title = "Fifa player valuation")
sidebar = dashboardSidebar(
sidebarMenu(
menuSubItem("Player Valuation - Striker", tabName = "StrikerTab"),
menuSubItem("Player Valuation - Midfielder", tabName = "MidfielderTab"),
menuSubItem("Player valuation - Goalkeeper", tabName = "GkTab")
)
)
ageSlider = sliderInput('age', 'Enter your age in years', min = 0, max = 150, value = 25)
dribblingSlider = sliderInput('dribbling_skills', 'Enter your dribbling skills', min = 0, max = 100, value = 70)
tacklingSlider = sliderInput('tackling_skills', 'Enter your tackling skills', min = 0, max = 100, value = 70)
shootingSlider= sliderInput("shooting_skills", "Enter your shooting skills", min = 0, max = 100, value = 70)
body = dashboardBody(
tabItems(
tabItem(tabName = "StrikerTab",ageSlider,dribblingSlider, tacklingSlider, shootingSlider,verbatimTextOutput("strikerValue")),
tabItem(tabName = "MidfielderTab",ageSlider,dribblingSlider, tacklingSlider),
tabItem(tabName = "GkTab", ageSlider, gkSlider)
),
verbatimTextOutput("playerValueTxt")
)
ui = dashboardPage(header, sidebar, body, skin = "green")
UI end
Server Start
strikerProjectedValue = function(age, dribbling, tackling = 0, shooting = 0){
return (-250.19*age) + (1987.33*dribbling) + (4439.32*tackling) + (3232.44*shooting)
}
server = function(input, output) {
predictedStrikerValue = strikerProjectedValue(input$age, 150)
output$strikerValue = renderPrint(predictedStrikerValue)
}

Answer 1

Probably someone with more programming, and especially shiny programming, can give you a better answer. Your question can use quite some clarification in my opinion, it is a bit unclear to me what the exact results is that you want / what your rational is behind this code. Despite that I will try to give some answer. Maybe some of the things I mention are 'personal' as I would not program a shiny like this.
Personally, I would not use a function in Shiny, rather use the reactive({}) statement. I do not know why you would prefer the function over reactive, especially as you have a lot of sliders that you do not use anywhere. So what is the point of having all these expressions in your function?
In addition I do not really understand why you want every step to be written into a separate vector. Specifically, I mean the part of
predictedStrikerValue = strikerProjectedValue(input$age, 150)
output$strikerValue = renderPrint(predictedStrikerValue)
Maybe you have some plans to use them later but again that is not clear from your description. Why not putting renderPrint(strikerProjectedValue(input$age, 150))? (this is why I stated in the beginning your question might need some extra explanation ;), because there could be a good rational to do this).
In order to get your code working I replaced everything in your code of server with
output$strikerValue = renderText((-250.19*input$age) +
(1987.33*input$dribbling_skills) +
(4439.32*input$tackling_skills) +
(3232.44*input$shooting_skills))
In order to get exactly the same as in your function you will have to put the sliders initially on 0.
Another way to do it, which is more like your initial function:
predictedStrikerValue<- reactive({(-250.19*input$age) + (1987.33*input$dribbling_skills)
})
output$strikerValue = renderText(predictedStrikerValue())
Similar to the code above you can add all the other parts of the function to get it completely reactive.