R shiny error when using reactive to plot

@Suzie - as mentioned above, it would help if you edited your question with your complete code as you currently have it.

A few things that would help:

• Salary should be numeric in your df (or be converted with as.numeric before trying to take the mean
• Your reactive expression can use !!as.symbol with input$selection to filter by the string name from df
• The plot can use aes_string for the variable names.

Edit:

For further explanation of !!as.symbol, first consider what the result of input$selection. If you use browser() in your shiny code, and inspect what input$selection returns, you will see something like "Area" (it returns a string). But a string would not be appropriate in your filter - it is expecting a symbol that represents a column in your data frame. (A symbol is the name of an object like df or mtcars, etc.)

First, you want to convert a string to a symbol. You can do that either by using as.symbol() or rlang::sym(). You can try this out in your console. If you do as.symbol("df") it would return the symbol df. If you entered eval(as.symbol("df")) it would be the same as just entering df itself (and it would show the contents of your data frame).

The other issue is that tidyverse functions evaluate code expressions in a special context (searching for names within a data frame, for example). In this case dplyr knows that the name Area is in the context of df (one of the column names). This is a complicating factor since arguments are quoted. To address this, you need to unquote (replace a name with its value) with the bang-bang !! operator.

Putting both together you get !!as.symbol().

Of note, varSelectInput is a newer shiny alternative to selectInput that can be considered for use in situations like these.

For more information:

shinymeta special topics

advanced R

library(tidyverse)
library(shiny)

Area <- c("Mexico", "USA", "USA", "Canada")
Type_of_participants <- c("Doctor", "Doctor", "Engineer", "Dancer")
Salary <- c(4000, 6000, 8000, 5000)

df <- data.frame(Area, Type_of_participants, Salary)

ui <- fluidPage(
  titlePanel("Survey Results"),
  sidebarLayout(
    sidebarPanel(strong("Overview Plot"),
                 br(),
                 ###1a.Area input
                 selectInput("selection","Var",
                             choices = c("Area","Type_of_participants"),
                             selected = "Area"),
                 uiOutput("choice_selection")      
    ),
    mainPanel(    
      plotOutput("Overview")
    )
  )
)

server <- function(input, output) {

  output$choice_selection <- renderUI({
    checkboxGroupInput("baseinput", "Detail", unique(df[,input$selection]))
  })

  dt1 <- reactive({
    df %>%
      group_by(Area, Type_of_participants) %>%
      filter(!!as.symbol(input$selection) %in% input$baseinput) %>%
      summarise(avg_salary_by_area = mean(Salary, na.rm = TRUE))
  })

  output$Overview <- renderPlot({
    ggplot(data = dt1(), aes_string(x = input$selection, y = "avg_salary_by_area", fill = "Type_of_participants")) +
      geom_bar(stat="identity", position = position_dodge())
  })
}

shinyApp(ui, server)