Use a specific slide number for slideNumberFormat in xaringan - r

Question

In a xaringan presentation, the slideNumberFormat argument is often set to %current% / %total%. It is possible to use a specific slide number in this argument?
For instance, if I have a "final" slide like this:
---
name: mylastslide
# Thanks
with appendix slides behind, I want to display slide numbers like %current% / %mylastslide% with %mylastslide% the number of my slide called mylastslide.
Thanks.
[Edit after #user2554330 suggestion]
For this code, with an incremental slide,
---
title: "Presentation Ninja"
subtitle: "⚔<br/>with xaringan"
author: "Yihui Xie"
institute: "RStudio, PBC"
date: "2016/12/12 (updated: `r Sys.Date()`)"
output:
xaringan::moon_reader:
lib_dir: libs
---
background-image: url(https://upload.wikimedia.org/wikipedia/commons/b/be/Sharingan_triple.svg)
```{r setup, include=FALSE}
options(htmltools.dir.version = FALSE)
```
<script>
var slideshow = remark.create({slideNumberFormat : function (current, total) {
return 'Slide ' + current + ' of ' + (this.getSlideByName("mylastslide").getSlideIndex() + 1); },
highlightStyle: "github",
highlightLines: true,
countIncrementalSlides: false});
</script>
Image credit: [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Sharingan_triple.svg)
---
class: center, middle
# xaringan
---
hello
--
world
--
thanks
--
really
--
byebye
---
name: mylastslide
# Final slide
Install the **xaringan** package from [Github](https://github.com/yihui/xaringan):
```{r eval=FALSE, tidy=FALSE}
devtools::install_github("yihui/xaringan")
```
---
# Appendix
The last slide (ie appendix) is numbered as Slide 6 of 9 (and not Slide 6 of 5) and 9 is the url index of mylastslide. (I used + 1 in the slideNumberFormat function because indexes start at 0.)
Thanks.

Answer 1

You could certainly set the format to a fixed value for the last slide, e.g. %current% / 34. But you can also set it to a Javascript function. (Edited to add:) The tricky part is that you need to include all the options that would normally appear in the nature argument as well. So you want something like
<script>
var slideshow = remark.create({slideNumberFormat : function (current, total) {
return 'Slide ' + current + ' of ' + this.getSlideByName("mylastslide").getSlideIndex(); },
highlightStyle: "github",
highlightLines: true,
countIncrementalSlides: false});
</script>
You name a slide by putting the text
name: mylastslide
at the bottom of the slide after --- marks. So here's a complete example, based on the first few slides of the xaringan template:
---
title: "Presentation Ninja"
subtitle: "⚔<br/>with xaringan"
author: "Yihui Xie"
institute: "RStudio, PBC"
date: "2016/12/12 (updated: `r Sys.Date()`)"
output:
xaringan::moon_reader:
lib_dir: libs
---
background-image: url(https://upload.wikimedia.org/wikipedia/commons/b/be/Sharingan_triple.svg)
```{r setup, include=FALSE}
options(htmltools.dir.version = FALSE)
```
<script>
var slideshow = remark.create({slideNumberFormat : function (current, total) {
return 'Slide ' + current + ' of ' + this.getSlideByName("mylastslide").getSlideIndex(); },
highlightStyle: "github",
highlightLines: true,
countIncrementalSlides: false});
</script>
???
Image credit: [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Sharingan_triple.svg)
---
class: center, middle
# xaringan
### /ʃaː.'riŋ.ɡan/
---
class: inverse, center, middle
# Get Started
---
name: mylastslide
# Hello World
Install the **xaringan** package from [Github](https://github.com/yihui/xaringan):
```{r eval=FALSE, tidy=FALSE}
devtools::install_github("yihui/xaringan")
```
This has 5 slides, numbered "1 of 4" to "5 of 4".
Edited to add: As discussed in the comments, this doesn't handle incremental slides properly: getSlideIndex() counts incremental slides separately. We want to use getSlideNumber(), which stays the same on all of them when we use the option countIncrementalSlides: false. However, the online version of remark-latest.min.js doesn't have getSlideNumber() in it, you need to ask for remark-0.15.0.min.js.
You do this with the following YAML:
xaringan::moon_reader:
lib_dir: libs
chakra: https://remarkjs.com/downloads/remark-0.15.0.min.js
After this the code below worked fine:
<script>
var slideshow = remark.create({slideNumberFormat : function (current, total) {
return 'Slide ' + current + ' of ' + this.getSlideByName("mylastslide").getSlideNumber(); },
highlightStyle: "github",
highlightLines: true,
countIncrementalSlides: false});
</script>