Datatable is not printed in combination with cat command in Rmd / RStudio

Question

Could someone please explain why in the Rmd code (to generate a HTML report using RStudio) below only the cat command is being displayed? When I move the cat command outside the if clause or comment it out the table is printed. I believe the same thing happens when using library(printr), but I haven't confirmed this with a minimal sample.

It seems that the code inside the if clause is somehow interpreted together and that the cat doesn't go well with the datatable.

If you could give me some clues on how to debug this, it would be helpful, too. As there are no warnings/error messages anywere.

---
title: "test"
output: 
  html_document
---


```{r}
if(TRUE){
  DT::datatable(iris)
  cat("I am here with my cat")
}

```

Implicit printing does not work inside for loops and if blocks. In these situations, you need to print objects explicitly using `print (object)` — Benjamin, Feb 03 '16 at 12:20
sorry, I don't understand.... print(DT::datatable(iris)) is not working — user224637, Feb 03 '16 at 12:22
That surprises me. I'll have to look at this when I get to a computer. It may be related to your chunk options. Currently, your code returns console output, and `datatable` may need some help to render correctly — Benjamin, Feb 03 '16 at 12:25
Extremly interesting. The question can be generalized further: Why does a `asis` chunk with implict printing `DT::datatable(iris)` work, but `print(DT::datatable(iris))` doesn't? (Note that the code in the question works if `cat` is placed before `DT::datatable(iris)` because `if` returns it's last statement, resulting in implicitly printing the datatable.) — CL., Feb 03 '16 at 14:14
I don't think it is limited to "asis". And for me it doesn't matter if cat is before or after datatable... Also, if I just append "i=1", there is no change either, i.e. the cat still works but not the datatable — user224637, Feb 03 '16 at 14:28
when I store the `DT::datatable` in a variable and print it after the last bracket the table is being printed: `if(TRUE){ cat("I am here with my cat.") mytable=DT::datatable(iris) } mytable` — user224637, Feb 09 '16 at 13:57

Yihui Xie · Accepted Answer · 2016-02-04T17:59:41.867

This is essentially the same issue as knitr#1137. HTML widgets, including DT/DataTables, only work when they are generated from top-level R expressions. This is because only top-level expressions are actually printed. Expressions that are not at the top level are only evaluated. There is a big difference between the two cases. Printing involves calling a printing function. In most cases, this function is, not surprisingly, print() (or show() for S4 objects). In the knitr world, it is more complicated than that: the default printing function is knitr::knit_print, which is very similar to print(), but it does one more thing besides generating the text output, which is collecting the metadata (e.g. HTML dependencies) of the objects being printed. After knitting is done, rmarkdown will resolve the meta data into appropriate HTML code (e.g. <script src="dataTables.js"></script> in <head>).

To sum up, if the expression is not at the top level, neither printing nor metadata collection is done, so there is no way for the widget to be actually rendered.

A simple example to illustrate this:

if (TRUE) {

  1:10  # not printed

  11:20 # printed because this is the visible value returned by if()

}

Now I'm curious – would you mind adding a short explanation (or a reference) why this happens? — CL., Feb 04 '16 at 07:34
Thank you for elaborating on this. I'm still digesting your answer … — CL., Feb 04 '16 at 21:53

Datatable is not printed in combination with cat command in Rmd / RStudio

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