Shiny Web Applications

Advanced 15 hrs 1 Concepts

Shiny Architecture

Concept 1

UI and Server Basics

A Shiny app has two parts:

UI (User Interface): defines layout and input/output widgets
Server: the R function that computes outputs from inputs

library(shiny)

ui <- fluidPage(
  titlePanel('Student Score Explorer'),
  sidebarLayout(
    sidebarPanel(
      selectInput('subject', 'Subject:',
                  choices = c('Math','Science','English')),
      sliderInput('min_score', 'Min Score:', min=0, max=100, value=70),
      downloadButton('dl', 'Download Data')
    ),
    mainPanel(
      plotOutput('hist', height='350px'),
      tableOutput('summary_tbl')
    )
  )
)

server <- function(input, output, session){
  # Reactive expression — recalculates when inputs change
  filtered <- reactive({
    students |>
      filter(subject == input$subject, score >= input$min_score)
  })

  output$hist <- renderPlot({
    ggplot(filtered(), aes(x=score)) +
      geom_histogram(fill='#2563eb', binwidth=5) +
      labs(title=paste('Scores —', input$subject)) +
      theme_minimal()
  })

  output$summary_tbl <- renderTable({
    filtered() |> summarise(n=n(), mean=mean(score), sd=sd(score))
  })

  output$dl <- downloadHandler(
    filename = function() paste0(input$subject,'_data.csv'),
    content  = function(file) write_csv(filtered(), file)
  )
}

shinyApp(ui, server)

Solved Examples

Example 1 Show a notification when the filtered data has fewer than 5 rows.

observe({
  if (nrow(filtered()) < 5){
    showNotification(
      paste('Only', nrow(filtered()), 'students match your filters.'),
      type = 'warning'
    )
  }
})
# Observers run whenever reactive dependencies change
# They're used for side effects (notifications, writing files)

Self-Assessment (2 questions)

Q1. What is the difference between reactive() and observe() in Shiny?

Q2. What does input$subject refer to?

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