Shiny basics for interactive apps in R Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a basic Shiny app UI with a title.

R Programming

ui <- fluidPage(
  [1]("My Shiny App")
)

Drag options to blanks, or click blank then click option'

AtitlePanel

BsidebarLayout

CmainPanel

DfluidRow

Attempts:

3 left

💡 Hint

Common Mistakes

Using layout functions like sidebarLayout instead of titlePanel for the title.

Confusing mainPanel with titlePanel.

The titlePanel function adds a title to the Shiny app UI.

2fill in blank

medium

Complete the code to define the server function that outputs text.

R Programming

server <- function(input, output) {
  output$text <- renderText({
    [1]("Hello, Shiny!")
  })
}

Drag options to blanks, or click blank then click option'

Aprint

BtextOutput

CrenderText

Dcat

Attempts:

3 left

💡 Hint

Common Mistakes

Using print or cat which do not work as reactive render functions.

Using textOutput which is a UI function, not server.

The renderText function is used inside the server to create text output.

3fill in blank

hard

Fix the error in the code to correctly display a numeric input in the UI.

R Programming

ui <- fluidPage(
  numericInput([1], "Enter a number:", value = 10)
)

Drag options to blanks, or click blank then click option'

AnumericInput

Bnum

Cinput$num

D"num"

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting quotes around the input ID.

Using input$num which is not valid here.

The first argument of numericInput must be a string ID in quotes.

4fill in blank

hard

Fill both blanks to create a UI with a slider input and display its value as text.

R Programming

ui <- fluidPage(
  sliderInput([1], "Choose a value:", min = 1, max = 100, value = 50),
  textOutput([2])
)

Drag options to blanks, or click blank then click option'

A"slider1"

B"sliderValue"

D"outputText"

Attempts:

3 left

💡 Hint

Common Mistakes

Using different IDs for sliderInput and textOutput.

Not quoting the IDs.

The slider input and the text output must share the same ID string to link input and output.

5fill in blank

hard

Fill all three blanks to create a Shiny app that shows the square of a numeric input.

R Programming

ui <- fluidPage(
  numericInput([1], "Number:", value = 2),
  textOutput([2])
)

server <- function(input, output) {
  output[3] <- renderText({
    input$num ^ 2
  })
}

shinyApp(ui, server)

Drag options to blanks, or click blank then click option'

A"num"

B"result"

C$result

D$num

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatching IDs between UI and server.

Using incorrect syntax for output assignment.

The numeric input ID is "num". The text output ID is "result". The server output uses output$result to match the text output.

Practice

1. In a Shiny app, what is the main role of the ui part?

easy

A. To perform calculations and data processing

B. To define what the user sees and interacts with

C. To store data permanently

D. To connect to external databases

Solution

1. Step 1: Understand the purpose of ui in Shiny

   The ui defines the layout and elements the user interacts with, like buttons and plots.

2. Step 2: Differentiate from server role

   The server handles logic and calculations, not the ui.

3. Final Answer:

   To define what the user sees and interacts with -> Option B

4. Quick Check:

   ui = user interface [OK]

Hint: Remember UI means User Interface, what users see [OK]

Common Mistakes:

• Confusing ui with server
• Thinking ui does data processing
• Assuming ui stores data

2. Which of the following is the correct way to create a numeric input in Shiny's UI?

easy

A. numericInput(inputId = "num", label = "Number", value = 0)

B. numericInput("Number", inputId = "num", value = 0)

C. numericInput(0, "Number", 0)

D. numericInput(label = "Number", value = 0)

Solution

1. Step 1: Recall numericInput syntax

   The correct order is numericInput(inputId, label, value).

2. Step 2: Check each option

   numericInput(inputId = "num", label = "Number", value = 0) matches the correct syntax exactly; others have wrong argument order or missing arguments.

3. Final Answer:

   numericInput(inputId = "num", label = "Number", value = 0) -> Option A

4. Quick Check:

   Correct argument order = numericInput(inputId = "num", label = "Number", value = 0) [OK]

Hint: Remember inputId first, then label, then value [OK]

Common Mistakes:

• Mixing argument order
• Omitting inputId
• Using unnamed arguments incorrectly

3. What will be the output shown in the Shiny app if the server code is:

server <- function(input, output) {
  output$text <- renderText({
    paste("Hello", input$name)
  })
}

and the UI has a textInput with inputId = "name" where the user types "Alice"?

medium

A. Error: object 'name' not found

B. input$name

C. Hello input$name

D. Hello Alice

Solution

1. Step 1: Understand renderText and input usage

   The renderText function returns text that updates automatically using input$name.

2. Step 2: Substitute input$name with user input

   User typed "Alice", so paste("Hello", input$name) becomes "Hello Alice".

3. Final Answer:

   Hello Alice -> Option D

4. Quick Check:

   Dynamic text output = "Hello Alice" [OK]

Hint: renderText shows text with input values replaced [OK]

Common Mistakes:

• Thinking input$name prints literally
• Confusing renderText with renderPrint
• Expecting error due to input usage

4. Identify the error in this Shiny server code snippet:

server <- function(input, output) {
  output$plot <- renderPlot({
    plot(data)
  })
}

Assuming data is not defined anywhere else.

medium

A. Missing definition of 'data' causes an error

B. renderPlot should be renderPlotly

C. output$plot should be output$plotOutput

D. plot() cannot be used inside renderPlot

Solution

1. Step 1: Check for variable definitions

   The code uses data inside plot() but data is not defined anywhere.

2. Step 2: Understand consequences of missing data

   This will cause an error because R cannot find the object data.

3. Final Answer:

   Missing definition of 'data' causes an error -> Option A

4. Quick Check:

   Undefined variable = error [OK]

Hint: Always define variables before using in render functions [OK]

Common Mistakes:

• Assuming plot() is invalid in renderPlot
• Confusing output variable names
• Thinking renderPlotly is required

5. You want to create a Shiny app that shows a histogram of a numeric variable chosen by the user from a dropdown menu. Which approach correctly connects the input and output?

hard

A. Use sliderInput to select variable, then plot histogram of entire data

B. Use numericInput for variable choice, then plot fixed variable in server

C. Use selectInput in UI for variable choice, then in server use renderPlot to plot histogram of data[[input$var]]

D. Use textInput for variable name, then plot histogram of input$var directly

Solution

1. Step 1: Choose correct input control for variable selection

   selectInput lets user pick a variable name from choices.

2. Step 2: Use input to select data column in server

   Access the chosen variable as data[[input$var]] and plot histogram with renderPlot.

3. Final Answer:

   Use selectInput in UI for variable choice, then in server use renderPlot to plot histogram of data[[input$var]] -> Option C

4. Quick Check:

   Dropdown input + dynamic plot = Use selectInput in UI for variable choice, then in server use renderPlot to plot histogram of data[[input$var]] [OK]

Hint: Use selectInput for choices, access with data[[input$var]] [OK]

Common Mistakes:

• Using numericInput instead of selectInput
• Trying to plot input$var directly without data
• Using sliderInput for variable selection