With R markdown, is it possible to change the format of the first reference to a display item?

Question

Some journals requires that the first reference to a display item should be bold, but from the second one onward the normal format would be used. Basically:

Figure 1 showed our amazing results. Figure 2 is also important. However, it should be noted that unlike Figure 1, the method used in Figure 2 had changed...

Is it possible to realize in R markdown? More specifically, is it possible with bookdown::word_document2?

Answer 1

I doubt there is any built in functionality for this. The idea being that you would manage it yourself by denoting each initial reference with bold markdown (i.e., **Figure 1**).

But you might accomplish this automatically with something like a custom knitr hook or maybe even a lua filter to process the document during document render.

Here is an example of a custom knitr output hook:

---
title: "Custom Knitr Document Output Hook"
output: bookdown::word_document2
date: "2024-04-25"
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)

local({
  hook_old <- knitr::knit_hooks$get("document") 
  knitr::knit_hooks$set(document = function(x) {
    # previously observed figure references
    obs_fig_refs <- character(0)
    
    # seq along the knitr outputs consisting of the yaml, chunks, and text
    for(i in seq_along(x)) {
      # if current output is text
      if(stringr::str_detect(x[i], "(---|```)", negate = TRUE)) {
        # extract the unique figure references
        fig_refs <- unique(stringr::str_extract_all(x[i], "Figure \\d+")[[1]])
        
        # seq along the unique figure references
        for(j in seq_along(fig_refs)) {
          # if this reference has not been previously observed
          if(!(fig_refs[j] %in% obs_fig_refs)) {
            # remember it has been observed
            obs_fig_refs <- c(obs_fig_refs, fig_refs[j])
            # and make the first reference bold
            x[i] <- stringr::str_replace(
              x[i], 
              fig_refs[j], 
              sprintf("**%s**", fig_refs[j])
            )
          }
        }
      }
    }
    hook_old(x)
  })
})
```

Figure 1 showed our amazing results. Figure 2 is also important. However, it should be noted that unlike Figure 1, the method used in Figure 2 had changed...