Strange behavior in shiny RMarkdown - r

Question

I'm having some very strange behavior when knitting a shiny RMarkdown HTML document. In the MWE below I simply (1) include a slider input, (2) include a column in the df as the value of the input and then (3) return the df. The MWE below returns the error Error in : Column "b" must be length 1 (the number of rows), not 0. If I take out the multiplication of the input value when I assign a value to b, the document knits with no problem. MWE:
---
output: html_document
runtime: shiny
---
```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
library(dplyr)
```
Testing
```{r eruptions, echo=FALSE}
# Input slider
sliderInput("test", "Test", value = 0.5, min = 0, max = 1, width = "100%")
# Pre-define df
output <- reactiveValues(data = data.frame(a = 20))
observe({
output$data <- output$data %>%
# Manipulate df
mutate(b = input$test * 2)
})
# Return df
renderDataTable({output$data})
```
Does anyone know what could be causing this strange behavior?

Answer 1

Here's what I suspect is going on ---
I think that the first time your observe is run through, that input$test is NULL.
If your mutate sets a column to NULL, it would just remove that column.
If you use input$test * 2 that will return an empty vector numeric(0). Trying to add a column with an empty vector would give the error.
If you add req(input$test) at the beginning of your observe function, does it work?