Cross tab to complement stacked bar plot in Shiny app - r

Question

I am having trouble creating a shiny app that creates a dynamic cross tab where one of the variables can be changed.
That is, I would like to create an app that creates a cross tab that shows the percentage of students that passed against each characteristic, as well as displaying this in a bar plot.
The bar plot is no problem. I cannot figure out how to vary the input into the cross tab.
The output$my_table section in the server object is where the problem is.
Any assistance would be greatly appreciated
library(shiny)
library(ggplot2)
students <- data.frame(Passed = c("Y","Y","Y","Y","Y","Y","N","N"), Gender = rep(c("Male","Female"), each = 4),
Subject = rep(c("Maths","Science"), times = 4),Campus =c(rep(c("Victoria"), times = 7), "NSW"))
ui <- fluidPage(
titlePanel("Percentage Passed by Characteristic"),
sidebarLayout(sidebarPanel(
selectInput(
"Characteristic",
"Student characteristic",
choices = colnames(students)[!colnames(students) %in% c("Passed")],
selected = colnames(students)[2]
)
),
mainPanel(
tabPanel("Plot",
fluidRow(
plotOutput("barplot"),
verbatimTextOutput("summary"))
),
tabPanel("Summary",
tableOutput("my_table"))
)
))
server <- function(input, output) {
output$barplot <- renderPlot({
var <- input$Characteristic
gg <-
ggplot(students, aes_string(x = var, fill= "Passed"))
gg <- gg + geom_bar(position = "Stack")
gg
})
output$my_table <- renderTable({
var <- input$Characteristic
prop.table(table(students$Passed, students$var),1)
})
}
# Create Shiny object
shinyApp(ui = ui, server = server)

Answer 1

The problem is that students$var refers to a column literally named "var", whereas you want the column named by the value of var: students[[var]].
This is addressed the question "access data frame column using variable", and answered in the last sentence of the winning answer. access data frame column using variable
I diagnosed the problem by running your code in an interactive R session, setting var to Gender, then running the prop.table(... line, which gives the error:
Error in table(students$Passed, students$var) :
all arguments must have the same length
The next problem you'll face is formatting the resulting prop.table, but that deserves another question.