[R] conditionally disable evaluation of chunks in Rmarkdown...

[Henrik Bengtsson]

On Tue, Nov 10, 2015 at 9:00 AM, Yihui Xie <xie at yihui.name> wrote:
> The short answer is you cannot. Inline R code is always evaluated.
> When it is not evaluated, I doubt if your output still makes sense,
> e.g. "The value of x is `r x`." becomes "The value of x is ." That
> sounds odd to me.
>
> If you want to disable the evaluate of inline code anyway, you may use
> a custom function to do it. e.g.
>
> cond_eval = function(x) {
>   if (isTRUE(knitr::opts_chunk$get('eval'))) x
> }
>
> Then `r cond_eval(x)` instead of `r x`.
>
> Regards,
> Yihui
> --
> Yihui Xie <xieyihui at gmail.com>
> Web: http://yihui.name
>
>
> On Tue, Nov 10, 2015 at 4:40 AM, Witold E Wolski <wewolski at gmail.com> wrote:
>> I do have an Rmd where I would like to conditionally evaluate the second part.
>>
>> So far I am working with :
>>
>> ```{r}
>> if(length(specLibrary at ionlibrary) ==0){
>>   library(knitr)
>>   opts_chunk$set(eval=FALSE, message=FALSE, echo=FALSE)
>> }
>> ```
>>
>> Which disables the evaluation of subsequent chunks.
>>
>> However my RMD file contains also these kind of snippets : `r `
>>
>> How do I disable them?

Just a FYI and maybe/maybe not a option for you; this is one of the
use cases where RSP (https://cran.r-project.org/package=R.rsp) is
handy because it does not require that code snippets (aka "code
chunks" as originally defined by weave/tangle literate programming) to
contain complete expressions.  With RSP-embedded documents, you can do
things such

    <% if (length(specLibrary at ionlibrary) > 0) { %>

    [... code and text blocks to conditionally include ...]

    <% } # if (length(specLibrary at ionlibrary) > 0) %>

or include from a separate file, e.g.

    <% if (length(specLibrary at ionlibrary) > 0) { %> <%@include
file="extras.md.rsp"%> <% } %>

You can also use loops over a mixture of code and text blocks etc.

Depending on when 'specLibrary at ionlibrary' gets assigned, you could
preprocess you R Markdown file with RSP, but for this to work out of
the box you basically need to know the value
length(specLibrary at ionlibrary) before your R Markdown code is
evaluated, i.e. before you compile the Rmd file.  Your build pipeline
would then look something like:

    rmd <- R.rsp::rfile("report.rmd.rsp")
    rmarkdown::render(rmd)

/Henrik
(author of R.rsp)

>>
>> regards
>>
>>
>>
>> --
>> Witold Eryk Wolski
>
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