Insert non-breaking space in [R]Markdown math expression for HTML output

Question

I am writing scientific reports in bookdown and I would like to use non-breaking spaces as thousands separators follwoing the SI/ISO 31-0 standard.

Actually, I would prefer the non-breaking thin space (U+202F/ ) but for simplicty let's consider U+00A0/  for here.

I setup a knitr hook to do this on the fly:

knitr::knit_hooks$set(inline=function(output)
                               ifelse(is.numeric(output),
                                      prettyNum(round(output, 1),
                                                big.mark=' '),
                                      output))

This works as intended as long as I don't use any inline R-expressions returning numerical output > 999 within math expressions.

The following bookdown MWE illustrates the problem:

---
output:
  bookdown::html_document2: default
---
```{r set-output-hook, include=FALSE}
knitr::knit_hooks$set(inline=function(output)
                               ifelse(is.numeric(output),
                                      prettyNum(round(output, 1),
                                                big.mark=' '),
                                      output))
```

This works:
The product of $\pi$ and `r 1000` is `r pi*1000`.

This fails to render: 
$\pi\cdot`r 1000`=`r pi*1000`$

This renders but is cumbersome as it requires me to know *a priori* which
values might exceed 999:
$\pi\cdot1000=`r as.character(round(pi*1000, 1))`$

I tried to track it down and came up with the following rmarkdown MWE:

---
output:
  rmarkdown::html_document:
    keep_md: true
---

| Rmarkdown    | Render     | HTML                                                | Markdown     |
|--------------|------------|-----------------------------------------------------|--------------|
| `1000`       | 1000       |`1000`                                               | `1000`       |
|`$1000$`      |$1000$      |`<span class="math inline">\(1000\)</span>`          |`$1000$`      |
|              |            |                                                     |              |
|  `100,0`     | 100,0      |`100,0`                                              | `100,0`      |
|`$100,0$`     |$100,0$     |`<span class="math inline">\(100,0\)</span>`         |`$100,0$`     |
|              |            |                                                     |              |
|  `100 0`     | 100 0      |`100 0`                                              | `100 0`      |
|`$100 0$`     |$100 0$     |`<span class="math inline">\(100 0\)</span>`         |`$100 0$`     |
|              |            |                                                     |              |
|  `100&nbsp;0`| 100&nbsp;0 |`100 0`                                              | `100&nbsp;0` |
|`$100&nbsp;0$`|$100&nbsp;0$|`<span class="math inline">\(100&amp;nbsp;0\)</span>`|`$100&nbsp;0$`|

The first two columns of the table are sufficient to see the problem: Each pair of rows shows the number 1000 1 000) in text and math context; without any space, with a comma, with a simple space, and with a non-breaking space as thousands separator. The latter fails to render in math context.

To track down the problem, I inspected the resulting HTML and Markdown (keep_md: true) output and added the corresponding code as columns three and four for a better overview what's going on.

For clarity, here is an adjusted version of the above rmarkdown MWE replacing simple spaces by _ and non-breaking spaces by - in the HTML and Markdown output columns:

---
output:
  rmarkdown::html_document:
    keep_md: true
---

| Rmarkdown    | Render     | HTML                                                | Markdown     |
|--------------|------------|-----------------------------------------------------|--------------|
| `1000`       | 1000       |`1000`                                               | `1000`       |
|`$1000$`      |$1000$      |`<span_class="math_inline">\(1000\)</span>`          |`$1000$`      |
|              |            |                                                     |              |
|  `100,0`     | 100,0      |`100,0`                                              | `100,0`      |
|`$100,0$`     |$100,0$     |`<span_class="math_inline">\(100,0\)</span>`         |`$100,0$`     |
|              |            |                                                     |              |
|  `100 0`     | 100 0      |`100_0`                                              | `100_0`      |
|`$100 0$`     |$100 0$     |`<span_class="math_inline">\(100_0\)</span>`         |`$100_0$`     |
|              |            |                                                     |              |
|  `100&nbsp;0`| 100&nbsp;0 |`100-0`                                              | `100&nbsp;0` |
|`$100&nbsp;0$`|$100&nbsp;0$|`<span_class="math_inline">\(100&amp;nbsp;0\)</span>`|`$100&nbsp;0$`|

So from what I can tell

1. This is not a bookdown issue as it can be reproduced by plain rmarkdown.
   • I'm just mentioning bookdown as I would be happy with a bookdown-specific work-around.
2. This is not an rmarkdown issue, as the generated Markdown looks exactly as I would expect it to look like.
   • I'm just mentioning rmarkdown as I would be happy with an rmarkdown-specific work-around.
3. This is not a MathJax issue, as the HTML code has the plain & replaced by & and I would not expect that to render properly.
   • Anyways,I would be happy with an MathJax-related work-around.
4. I suspect it's pandoc that replaces & by & in code and math context but not in text context.
   • I'm sure if there is a way to convince pandoc not to do this, it will be easy to configure this through the rmarkdown YAML header.

Any idea on how to get the   transferred literally from Markdown to HTML in math context would probably help me to figure out the rest.

---

Addendum:

As pointed out by @tarleb, $100 0$ is not valid Latex. However, modifiying the HTML manually to contain \(100 0\) works just fine as MathJax treats non-breaking spaces as spaces. As I am not concerned about PDF output via LaTex, this means simply not converting $100 0$ to \(100 0\) but to \(100 0\) (just as 100 0 is not converted to 100 0 either) when converting the Markdown to HTML would be all that I need.

Answer 1

Pandoc expects math environments to contain LaTeX math markup, not HTML. Conversion fails as pandoc tries to output $100 000$ as LaTeX, but that gives \(100 000\) instead of what you intended.

As a solution, you could try to use the literal narrow no-break space unicode character " " in your hook.

Alternatively, one could use a pandoc lua filter (or possibly a R pandoc-filter) to force pandoc to pass-through math content unaltered:

-- filename: force plain math
function Math (el)
  if el.mathtype == 'DisplayMath' then
    return pandoc.RawInline('html', '\\[' .. el.text .. '\\]')
  else -- InlineMath
    return pandoc.RawInline('html', '\\(' .. el.text .. '\\)')
  end
end

Save to a file and use it by adding

output:
  bookdown::html_document2:
    pandoc_args: --lua-filter=force-plain-math.lua

to your document.