Passing user specifications as arguments to dplyr within Shiny

Question

I\'m writing a new Shiny app, and I would like to stay within the Hadleyverse by using dplyr commands for data manipulation. I want Shiny to print a table showing only the t

Answer 1

You may also want to look at arrange_ (see vignette). Really nice for in Shiny apps.

The 1st option does what you want. The 2nd is cleaner but not exactly what you are looking for. I guess the 3rd option would be ideal but desc_ is not a function. Might be a nice addition :)

input <- list()
input$top3 <- "mpg"

mtcars %>%
  arrange_(paste0("desc(",input$top3,")"))  %>%
  head(n=3)

mtcars %>%
  arrange_(input$top3)  %>%
  tail(n=3)

# desc_ is not a function (yet)
mtcars %>%
  arrange(desc_(input$top3)) %>%
  head(n=3)

Answer 2

The problem is that arrange() function expects your argument as a symbol. However, your input$top3 is a string.

The trick is:

output$t3 <- renderTable({ 
    sorted_stats <- eval(substitute(employeestats %>% arrange(desc(col)), 
                         list(col=as.symbol(input$top3))))
    head(sorted_stats,n=3)  
  },include.rownames=TRUE)

You can use ?substitute to see how it works.

Short version, it parse the expression:

employeestats %>% arrange(desc(col))

into a parse tree consisting of call's (functions) and name's (symbols, constants, objects), allowing us to substitute the components to form a new expression.

To this point, the expression is not evaluated (meaning, the employeestats is not arranged yet).

Here col doesn't really mean anything. It simply serves as a placeholder. And by passing list(col=as.symbol(input$top3)) to substitute, we replace the dummy symbol col by the actual symbol we would like to arrange by, as.symbol(input$top3). Depending on the current input, this would either be TotalAwarded or NumberOfAwards (symbols, not strings anymore).

Finally, the eval() function evaluated the expression (with col replaced by the actual symbol), and returns the sorted data.frame.