Shiny basics for interactive apps in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Shiny in R?

Shiny is a package in R that helps you build interactive web apps easily without needing to know web programming languages.

beginner

What are the two main parts of a Shiny app?

A Shiny app has two main parts: UI (user interface) which controls what the user sees, and server which controls how the app works behind the scenes.

beginner

What function is used to create the user interface in Shiny?

The function fluidPage() is commonly used to create a simple and responsive user interface in Shiny apps.

intermediate

How do you make a Shiny app respond to user input?

You use input widgets like sliderInput() or textInput() in the UI, and then use input$widgetId in the server to react to what the user selects or types.

beginner

What function runs a Shiny app?

The function shinyApp(ui, server) runs the app by connecting the UI and server parts together.

Which two components are essential to build a Shiny app?

A HTML and CSS

B Input and Output files

C Database and API

D UI and server

A Shiny app needs a UI to show the interface and a server to handle the logic.

What does the function sliderInput() do in Shiny?

A Creates a slider for user input

B Displays a plot

C Runs the app

D Saves data to a file

sliderInput() creates a slider widget so users can pick a number by sliding.

How do you access the value of an input widget named 'age' in the server function?

A age$input

B input$age

C output$age

D server$age

Input values are accessed using input$widgetId, so here it is input$age.

Which function is used to create a fluid and responsive UI layout in Shiny?

A fluidPage()

B fixedPage()

C runApp()

D renderUI()

fluidPage() creates a flexible layout that adjusts to screen size.

What does the server function in Shiny do?

A Starts the web server

B Defines the app's appearance

C Controls the app's logic and reactions to user input

D Stores user data permanently

The server function contains the code that reacts to user input and updates outputs.

Explain the roles of the UI and server parts in a Shiny app.

Think about what the user interacts with versus what happens behind the scenes.

Describe how you would create a simple Shiny app that takes a number input and shows it doubled.

Focus on input widgets, accessing input, and showing 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