Interactive Apps with Shiny

Shiny is an R package for building interactive web applications, pairing a UI that defines inputs and outputs with a reactive server that recomputes results automatically whenever a user changes an input.

Learn Interactive Apps with Shiny in our free R course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free R course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

By the end of this lesson you'll understand the ui/server split, the reactive model, how inputs and outputs connect, and how to build and launch a small interactive app with shinyApp().

What You'll Learn in This Lesson

1️⃣ The Three Parts of an App

Every Shiny app is a ui (the layout the user sees), a server function (the R logic), and a call to shinyApp() that launches them together. Inputs in the UI carry an inputId ; the server reads them via input$id and writes results to output$id .

2️⃣ Inputs, Outputs, and Reactivity

Inputs such as sliderInput() send values to the server; render*() functions like renderPlot() compute results, which a matching *Output() placeholder displays. Because Shiny is reactive , any output that mentions input$bins automatically re-runs when the slider moves — no manual refresh.

3️⃣ Sharing Work with reactive()

When several outputs need the same intermediate value, wrap it in reactive() . The expression runs once per change and caches its result, so a plot and a summary can share one data set instead of computing it twice. You access a reactive by calling it like a function: sample_data() .

Your turn. Fill in the # TODO lines, then run the app in RStudio.

Mini-Challenge: BMI App

Build a complete small app from the outline: two numeric inputs, a shared reactive() , and a text output. It ties together everything in this lesson.

Common Errors (and the fix)

• Mismatched IDs — ❌ plotOutput("graph") but output$plot . ✅ The output ID and the server slot must match exactly.
• Forgetting () on a reactive — ❌ mean(sample_data) . ✅ Call it: mean(sample_data()) .
• Reading input outside a render/reactive — ❌ touching input$x at the top of server. ✅ Read inputs inside reactive contexts only.
• Computing in the UI — ❌ putting logic in fluidPage() . ✅ UI builds layout; all logic lives in the server.

Pro Tips

• 💡 Delay heavy work with eventReactive() and an actionButton() so it runs only on click.
• 💡 Deploy free on shinyapps.io , or run apps fully in-browser with shinylive .
• 💡 Debug reactivity with reactlog , which visualizes the dependency graph.

📋 Quick Reference — Shiny

Task

R Syntax

App page

fluidPage(...)

Slider input

sliderInput("bins", ...)

Plot output slot

plotOutput("hist")

Render a plot

output$hist <- renderPlot({ ... })

Shared reactive

d <- reactive({ ... }); d()

Launch app

shinyApp(ui, server)

Frequently Asked Questions

🎉 Lesson Complete!

• ✅ Every app has a ui , a server , and shinyApp()
• ✅ Inputs flow in via input$id ; outputs flow out via output$id
• ✅ Shiny is reactive : outputs re-run when their inputs change
• ✅ reactive() shares and caches work across outputs
• ✅ Run apps locally in RStudio; deploy to shinyapps.io
• ✅ Next lesson: Reports with R Markdown

Practice quiz

Continue this course

• Previous: Fast Data with data.table
• Next: Reports with R Markdown