Reference slide number (question)

[giabaio]

Hi all,
I have a set of slides arranged in a folder with the following structure

slides/
   lecture1/
      index.Rmd
      ...
   lecture2/
      index.Rmd
      ...
   ...

(with a view of publishing the whole lot under the static/slides folder of a course website).

I can cross-ref to another lecture (eg using the code

See [Lecture 1](../lecture1/index.html)

I know I can point the link to the #name-of-the-slide tag, say

See [here](../lecture1/#some-slide)

using the code

name: some-slide

in the relevant .Rmd file.

But is there a way for me to have as output something like

See slide 5 in Lecture 1

where the "5" is automatically mapped from the link I'm giving (say, ../lecture1/#some-slide)?

I hope this is clear enough and I'm using the right format to ask the question! :-)

Thanks
Gianluca

[yihui]

That's a great question, but I don't have an answer. It might be possible to implement an R function to calculate the page number based on a name attribute, but I don't have time for that. If anyone wants to submit a PR, you may take a look at the function xaringan:::slide_context, in which I have a rough implementation of finding the slide page number corresponding to the cursor position in the editor. If you treat the name: id as the "cursor position", you can get the page number similarly.

ref_slide = function(input, name) {
  ctx = slide_context(......)  # you'll need to extend this function
  n = ctx$n
  sprintf('[Page %d](%s#%d)', n, with_ext(input, '.html'), n)
}

Then you can write an inline R expression in the Rmd source:

See `r xaringan::ref_slide('../lecture1/index.Rmd', 'some-slide')`

[giabaio]

Thanks, @yihui. In the end, I made an R function to create the "syllabus" (with the list of lectures) and then use that to reference the actual lecture and, possibly, the slide.

syllabus=tibble(
  title=c(
    "intro_bayes",
    "bugs",
    "mcmc",
    "intro_he",
    "ild",
    "survival",
    "ald",
    "nma",
    "mm",
    "missing",
    "voi"
  ),
  number=c(
    1,2,3,4,5,6,7,8,9,10,11
  ),
  path=c(
    "01_Intro",
    "02_BUGS",
    "03_MCMC",
    "04_Intro_HE",
    "05_ILD",
    "06_Survival",
    "07_ALD",
    "08_NMA",
    "09_MM",
    "10_Missing",
    "11_VoI"
  )
)

#' Then this function formats the link-out to the lecture file
#' @param lecture The title of the lecture (as given in the table 'syllabus' in the column 'title')
#' @param name The name of a given slide (if given in the preamble to the slide, eg 'name: a-given-name')
ref_lecture=function(lecture,name=NULL) {
  if(!is.null(name)) {
    url=paste0("/#",name)
  } else {
    url="/index.html"
  }
  paste0('<a href="../',syllabus %>% filter(title==lecture) %>% pull(path),url,'">Lecture ',syllabus %>% filter(title==lecture) %>% pull(number),'</a>')
}

That's a bit hacky and, more importantly, I think very specialised to what I need to do, rather than a general function that people may use... But perhaps it may give inspiration for something better?