Shiny basics for interactive apps in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Shiny app's text output?

Consider this Shiny app code snippet. What will be displayed in the app's main panel when the slider is moved to 5?

R Programming

library(shiny)

ui <- fluidPage(
  sliderInput("num", "Choose a number", min = 1, max = 10, value = 1),
  textOutput("result")
)

server <- function(input, output) {
  output$result <- renderText({
    paste("You selected", input$num * 2)
  })
}

shinyApp(ui, server)

A You selected NULL

B You selected 1

C You selected 5

D You selected 10

💡 Hint

Remember the slider value is multiplied by 2 before displaying.

The slider input value is 5, and the server multiplies it by 2, so the text output shows "You selected 10".

🧠 Conceptual

intermediate

Which reactive function updates automatically when input changes?

In Shiny, which function is used to create a reactive expression that updates automatically when its inputs change?

A observeEvent()

B renderUI()

C reactive()

D isolate()

💡 Hint

Think about a function that returns a value and updates reactively.

The reactive() function creates reactive expressions that update automatically when their inputs change. observeEvent() is for side effects, renderUI() creates UI outputs, and isolate() prevents reactive updates.

🔧 Debug

advanced

Why does this Shiny app not update the plot when slider changes?

Look at this server code snippet. The plot does not update when the slider input changes. What is the cause?

R Programming

server <- function(input, output) {
  output$plot <- renderPlot({
    x <- 1:10
    y <- x * 2
    plot(x, y)
  })
}

A The plot does not use input$num, so it never reacts to slider changes.

B renderPlot() is missing parentheses after it.

C The plot function is incorrect and causes an error.

D The server function is missing a return statement.

💡 Hint

Check if the reactive input is used inside renderPlot.

The plot code does not use input$num or any reactive input, so Shiny does not know to update the plot when the slider changes.

📝 Syntax

advanced

Which option causes a syntax error in Shiny UI definition?

Which of these UI code snippets will cause a syntax error?

A

fluidPage(
  titlePanel("App Title"),
  sidebarLayout(
    sidebarPanel(
      sliderInput("num", "Number", 1, 10, 5),
    ),
    mainPanel(
      textOutput("text")
    )
  )
)

B

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,)    
)5 ,01 ,1 ,"rebmuN" ,"mun"(tupnIredils      
(lenaPrabedis    
(tuoyaLrabedis  
,)"eltiT ppA"(lenaPeltit  
(egaPdiulf

C

luidPage(
  titlePanel("App Title"),
  sidebarLayout(
    sidebarPanel(
      sliderInput("num", "Number", 1, 10, 5)
    ),
    mainPanel(
      textOutput("text")
    )
  )
)

D

fluidPage(
  titlePanel("App Title"),
  sidebarLayout(
    sidebarPanel(
      sliderInput("num", "Number", 1, 10, 5)
    ),
    mainPanel(
      textOutput("text")
    )
  )
)

💡 Hint

Look for misplaced commas inside function calls.

Option A has an extra comma after the sliderInput call inside sidebarPanel, causing a syntax error.

🚀 Application

expert

How many items are in the reactiveValues list after this code runs?

Given this server code, how many items does vals contain after the app starts?

R Programming

server <- function(input, output, session) {
  vals <- reactiveValues()
  vals$a <- 10
  vals$b <- 20
  observe({
    vals$c <- vals$a + vals$b
  })
}

A 2

B 3

C 1

D 0

💡 Hint

Consider when the observe block runs and what it adds to vals.

Initially, vals has 'a' and 'b'. The observe block runs immediately and adds 'c', so vals has 3 items.