Common mistakes with Shiny

[Stéphane Laurent]

It could be useful to draw a list of common mistakes with Shiny programming.

1) Extra comma in ui.R

2) input/output id containing a point 

What else ? When someone requiring some help has made such a mistake in his code, we could refer him/her to the list of common mistakes.

[Stéphane Laurent]

3)  ggplot(...) + ... does not work in the output of renderPlot({}); use print(ggplot(...) + ... ).

[Stéphane Laurent]

We should also mention the typical error message generated by these mistakes

1) Extra comma in ui.R : 

"Error in tag("div", list(...)) : argument is missing, with no default"

2) input/output id containing a point

many possible error messages

3)  ggplot(...) + ... does not work in the output of renderPlot({}); use print(ggplot(...) + ... ). empty plot in the app

[David Ardestani]

I've seen on the codes here criticisms about using eval(parse(....)) and improper usage of reactive functions (e.g. - put in bad place, should use observe function, etc) .  That could be useful to have in an FYI section.

It would also be helpful to have a section on custom HTML and using the tag commands.  For example, I am trying to create a select input for different colors, and I want to the text to change color depending on the value.  I gather that I should use tags$option.  However, when I try to create my own selectInputColor function based on the code for selectInput, I get an error that it "could not find function choiceWithNames" even though it is used by selectInput.  Perhaps that should be a separate thread topic, but it that sort of general idea where a simple example or two would suffice.

[David Ardestani]

The concept of scoping with respect to working with data files would also be very helpful.

On Monday, December 16, 2013 12:30:23 PM UTC-8, Stéphane Laurent wrote:

[Floris Padt]

what about as.numeric(input$n) instead of just using input$n.

And the global.r file is really helpfull

[Patrick Toche]

What else ? When someone requiring some help has made such a mistake in his code, we could refer him/her to the list of common mistakes.

To quote from you, a common error:

"you cannot assign a value to an input object as if it was an ordinary R object (http://stla.github.io/ShinyPresentationIntroductory/#5) "

[Patrick Toche]

more mistakes seen and personally experienced recently:

1. trying to pass R code to conditionalPanel "condition" instead of javascript, e.g.

condition = "input$cond"  should be condition = "input.cond == true"

(and not TRUE)

another related mistake is something like 

condition = "input.cond1" || "input.cond2"  where the condition is incorrectly broken by closing " too early.

2. trying to pass reactiveValues, as in:

values <- reactiveValues(i=0)

condition = "values.i == 0"

[Patrick Toche]

another one is forgetting to install.packages("somepackage") on the remote server. And slightly more subtle is to have different versions of the packages locally and on the server.

[Joe Cheng]

Trying to access a reactive expression without using ().

foo <- reactive({...})

output$plot <- renderPlot({

  plot(foo)

})

On Thu, Jan 9, 2014 at 9:37 AM, Patrick Toche <patric...@usj.edu.mo> wrote:

another one is forgetting to install.packages("somepackage") on the remote server. And slightly more subtle is to have different versions of the packages locally and on the server.

--
You received this message because you are subscribed to the Google Groups "Shiny - Web Framework for R" group.
To unsubscribe from this group and stop receiving emails from it, send an email to shiny-discus...@googlegroups.com.
For more options, visit https://groups.google.com/groups/opt_out.

[Patrick Toche]

Trying to access a reactive expression without using ().

foo <- reactive({...})

output$plot <- renderPlot({

  plot(foo)

})

yes, a very common one for beginners, the mistake is calling the reactive values with foo instead of foo(), i.e. one should have:

output$plot <- renderPlot({

  plot(foo())

})

Also note that:

foo <- reactive({

  a <- something

  return (list(a = a))

})

is accessed with foo()$a rather than just foo().

Another mistake is to forget to return inside foo, i.e. to have say:

  foo <- reactive({

      a <- 1

  })

and in this context, Stéphane's remark about unbalanced and unclosed parentheses, here's one example I've seen:

  foo <- reactive({

      something

  )}

A mistake unrelated to the above is to forget to add session as an argument, when it is used, so to have 

  function(input, output)

when

  function(input, output, session)

is needed.

[Stéphane Laurent]

whenever I try to download, a new app window opens up : there's a typo in the name of downloadButton (it not correspond to the name of the downloadHandler)