Use shiny input to select variable for dplyr group_by

Question

I want to group a data.frame based on a column the user can choose as an input. Therefore I use the group_by function in the dplyr package.

However, input$variable is a string, so this won't work in group_by, which expects the variable. I am not sure how to change the code to make this work:

library(shiny)
library(dplyr)

df <- data.frame(
         a = c("a1", "a2", "a1"), 
         d = c("d1", "d1", "d2"), 
         b = c(2, 4, 5)
)
ui <- fluidPage(
  selectInput("variable", "Select a variable", choices = c("a", "d"), selected = "a"),
  tableOutput("table")
)

server <- function(input, output) {
  output$table <- renderTable({
    df %>% group_by(input$variable) %>% summarise(the_sum = sum(b))
    # this would work: df %>% group_by(a) %>% summarise(the_sum = sum(b))
  })
}

shinyApp(ui, server)

Any ideas on how to solve this?