knitr: conditionally evaluate normal chunks

Question

Here's what I'd like to do, in my Rhtml document:

<!--begin.rcode
if (errors==1) {
end.rcode-->
<p>You have an error!</p>
<!--begin.rcode

end.rcode-->

Basically, I'm trying to use knitr like PHP. The above doesn't work, but is there a right way to do it?

This question is different from Conditional `echo` (or eval or include) in rmarkdown chunks. That is about how to conditionally evaluate a knitr "chunk". This is about how to conditionally evaluate the "normal" HTML (or TeX, or Markdown) in between two knitr chunks.

If you do not insist using knitr, then you might give a try to pander with <% ... %> tags for conditionals and <%= ... %> for printing markdown: rapporter.github.io/pander/#brew-to-pandoc, otherwise you might want to brew::brew your document before passing to knitr. Or maybe there's an internal knitr support for this, I'm not sure :) — daroczig
Try putting your HTML (or TeX or md) in a cat statement in a conditionally evaluated chunk. — Thomas
@daroczig yes, brew is one way to go. But I would like to know if there is a knitr solution. — user3603486

arvi1000 arvi1000 · Accepted Answer · 2014-11-13T14:27:55

You can use a normal if block in the chunk. If you want an HTML formatted error message, you can use results='asis' in the chunk options. Like this:

<!--begin.rcode results='asis'
if (errors==1) {
  cat('<p>You have an error</p>')
} else {
  # r code to evaluate when errors!=1
}
end.rcode-->

knitr: conditionally evaluate normal chunks

1 Answers