Understanding the Issue with Multiple Player Selection in a Shiny App

As a developer, we’ve all been there - staring at our code, scratching our heads, trying to figure out why something isn’t working as expected. In this blog post, we’ll delve into the world of Shiny apps and explore the issue you’re facing with multiple player selection.

Introduction to Shiny Apps

Shiny is an R package that allows us to create web-based interactive applications using R. It’s built on top of the Shiny Server, which enables real-time rendering of dynamic UI components. With Shiny, we can create complex user interfaces and interactivity, making it an ideal tool for data analysis, visualization, and more.

The Code Behind the App

Let’s take a look at the provided ui.R and server.R files, which contain the essential code for our app.

# ui.R
library(shiny)
library(ggplot2)

dataset <- data.frame(
  player = c("Joel", "Marty", "Tinus", "Jelle", "Gio", "Tom",
            "Tim", "Peter", "Marco", "Martin"),
  pass = c(1, 10, 30, 50, 80, 200, 250, 300, 500, 2000),
  goals = c(12, 10, 8, 1, 3, 5, 9, 7, 2, 17)
)

fluidPage(
  titlePanel("Player"),
  sidebarPanel(
    selectInput('y', 'Y', names(dataset), names(dataset)[[3]]),
    selectInput('z','Player', levels(dataset$player), multiple=TRUE)
  ),
  mainPanel(
    plotOutput('plot')
  )
)

# server.R
library(shiny)
library(ggplot2)
library(dplyr)

function(input, output) {
  dataset <- data.frame(
    player = c("Joel", "Marty", "Tinus", "Jelle", "Gio", "Tom",
              "Tim", "Peter", "Marco", "Martin"),
    pass = c(1, 10, 30, 50, 80, 200, 250, 300, 500, 2000),
    goals = c(12, 10, 8, 1, 3, 5, 9, 7, 2, 17)
  )

  dataset1 <- reactive({
    y <- input$y
    data <- dataset

    if (!is.null(input$z) && input$z != "") {
      data <- filter(data, player %in% input$z)
    }

    data
  })

  output$plot <- renderPlot({
    ggplot(dataset1(), aes_string(x = "player", y = input$y)) +
      geom_bar(stat = "identity")

    }, height=700) 
}

The Problem with Multiple Player Selection

As you’ve noticed, the issue arises when you select multiple players. The filter function doesn’t work as expected because it’s trying to match a single value (input$z) against a vector of values (dataset$player). This is where we need to introduce the %in% operator.

Using the `%in%` Operator

The %in% operator in R allows you to check if an element belongs to a set of elements. In our case, we want to filter the dataset based on whether the selected player(s) are present in the player column.

# Before using %in%
if (!is.null(input$z) && input$z != "") {
  data <- filter(data, player == input$z)
}

# Using %in% operator
data <- filter(data, player %in% input$z)

The Issue with `aes_string`

Another issue lies in the aes_string function. When you pass a column name as a string ("player"), R expects it to be a character vector containing only characters (e.g., "a", "bc", etc.). However, when we use %in%, the resulting data frame might contain multiple values for each player. This causes an error because aes_string can’t handle multiple values in the column name.

To fix this, we need to pass a single value for the x aesthetic, which is obtained by taking the first element of the player vector.

# Using aes()
ggplot(dataset1(), aes(x = player[1], y = input$y)) +
  geom_bar(stat = "identity")

The Corrected Code

Let’s combine the fixes and update our code:

library(shiny)
library(ggplot2)
library(dplyr)

dataset <- data.frame(
  player = c("Joel", "Marty", "Tinus", "Jelle", "Gio", "Tom",
            "Tim", "Peter", "Marco", "Martin"),
  pass = c(1, 10, 30, 50, 80, 200, 250, 300, 500, 2000),
  goals = c(12, 10, 8, 1, 3, 5, 9, 7, 2, 17)
)

fluidPage(
  titlePanel("Player"),
  sidebarPanel(
    selectInput('y', 'Y', names(dataset), names(dataset)[[3]]),
    selectInput('z','Player', levels(dataset$player), multiple=TRUE)
  ),
  mainPanel(
    plotOutput('plot')
  )
)

library(shiny)
library(ggplot2)
library(dplyr)

function(input, output) {
  dataset <- data.frame(
    player = c("Joel", "Marty", "Tinus", "Jelle", "Gio", "Tom",
              "Tim", "Peter", "Marco", "Martin"),
    pass = c(1, 10, 30, 50, 80, 200, 250, 300, 500, 2000),
    goals = c(12, 10, 8, 1, 3, 5, 9, 7, 2, 17)
  )

  dataset1 <- reactive({
    y <- input$y
    data <- dataset

    if (!is.null(input$z) && length(input$z) > 0) {
      data <- filter(data, player %in% input$z)
    }

    data
  })

  output$plot <- renderPlot({
    ggplot(dataset1(), aes_string(x = "player", y = input$y)) +
      geom_bar(stat = "identity")

    }, height=700) 
}

Conclusion

By understanding the issues with multiple player selection and using the %in% operator, we’ve successfully resolved the problem. With this updated code, our Shiny app should now work as expected when selecting one or more players.