Shiny basics for interactive apps in R Programming - Mini Project: Build & Apply

Shiny basics for interactive apps

📖 Scenario: You want to create a simple interactive web app using Shiny in R. This app will let users enter a number and see its square displayed immediately.

🎯 Goal: Build a Shiny app that takes a number input from the user and shows the squared value as output.

📋 What You'll Learn

Create a UI with a numeric input called num

Create a server function that calculates the square of num

Display the squared value in the UI using textOutput

Use renderText in the server to update the output dynamically

💡 Why This Matters

🌍 Real World

Shiny apps let you create interactive dashboards and tools for data analysis that anyone can use in a web browser.

💼 Career

Many data science and analytics jobs require building Shiny apps to share insights and reports interactively.

1

Set up the UI with a numeric input

Write code to create a Shiny UI object called ui using fluidPage() that contains a numeric input with inputId num, label Enter a number:, and default value 1.

R Programming

library(shiny)

# Create UI with numeric input
# Your code here

Hint

Use numericInput() inside fluidPage() to create the input.

2

Add a text output to the UI

Add a textOutput with outputId result inside the existing fluidPage() UI object called ui.

R Programming

library(shiny)

ui <- fluidPage(
  numericInput(inputId = "num", label = "Enter a number:", value = 1),
  # Add textOutput here
  
  # Your code here
)

Hint

Use textOutput() to show text output in the UI.

3

Create the server function to calculate square

Write a server function called server with arguments input and output. Inside it, assign output$result to renderText() that returns the square of input$num.

R Programming

library(shiny)

ui <- fluidPage(
  numericInput(inputId = "num", label = "Enter a number:", value = 1),
  textOutput(outputId = "result")
)

# Create server function

# Your code here

Hint

Use renderText() to create reactive text output that depends on input$num.

4

Run the Shiny app

Write code to run the Shiny app by calling shinyApp() with the ui and server objects.

R Programming

library(shiny)

ui <- fluidPage(
  numericInput(inputId = "num", label = "Enter a number:", value = 1),
  textOutput(outputId = "result")
)

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

# Run the app
# Your code here

Hint

Use shinyApp() to launch the app with your UI and server.

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