Newline (\\) suddenly doesn't work

[yarnabrina]

If I use \\ inside $$, a new line is not created anymore. Consider the following example:

---
title: "Example"
output: pdf_document
---

some text

$$
just \ some \ more \ text \\
again \ some \ more \ text \\
once \ again \ more \ text
$$

some text

This generates the following file: try.pdf

Screenshot:

As you can find there, the three sentences in the .Rmd file separated by \\ are in same line in the rendered document. However, the preview displays correctly, and if I render as HTML, it works there too.

Here's my sessioninfo:

xfun::session_info('rmarkdown')
#> R version 3.6.1 (2019-07-05)
#> Platform: x86_64-w64-mingw32/x64 (64-bit)
#> Running under: Windows 10 x64 (build 18362)
#> 
#> Locale:
#>   LC_COLLATE=English_India.1252  LC_CTYPE=English_India.1252   
#>   LC_MONETARY=English_India.1252 LC_NUMERIC=C                  
#>   LC_TIME=English_India.1252    
#> 
#> Package version:
#>   base64enc_0.1.3  digest_0.6.20    evaluate_0.14    glue_1.3.1      
#>   graphics_3.6.1   grDevices_3.6.1  highr_0.8        htmltools_0.3.6 
#>   jsonlite_1.6     knitr_1.23.3     magrittr_1.5     markdown_1.0    
#>   methods_3.6.1    mime_0.7         Rcpp_1.0.1       rmarkdown_1.14.1
#>   stats_3.6.1      stringi_1.4.3    stringr_1.4.0    tinytex_0.14    
#>   tools_3.6.1      utils_3.6.1      xfun_0.8         yaml_2.2.0      
#> 
#> Pandoc version: 2.6

---

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[yihui]

By "suddenly", how sudden was it? When did it work before?

I doubt if it ever worked at all. I'm not a LaTeX expert, but a quick Google search of "latex linebreak in math mode" led to several StackExchange posts, and all of them said \\ wouldn't work in the display math mode, e.g.

• https://tex.stackexchange.com/q/46189/9128
• https://tex.stackexchange.com/a/336762/9128
• https://stackoverflow.com/a/1956479/559676

[yarnabrina]

By "suddenly", how sudden was it? When did it work before?

I was seriously taken aback while I couldn't find any .Rmd file rendered to PDF where I used only \\ inside $$. Now, this issue seems stupid.

However, \\ does work if I use it within split inside $$. If you substitute the math mode part in my question with the following, it'll be rendered with line breaks.

$$
\begin{split}
just \ some \ & more \ text \\
again \ some \ & more \ text \\
once \ again \ & more \ text
\end{split}
$$

Here's the screnshot:

\\ wouldn't work in the display math mode

Also, \\ do work when I render as HTML. Why does it work there? Why should same LaTeX expression have different behaviour while rendered to HTML and PDF?

[yihui]

Why should same LaTeX expression have different behaviour while rendered to HTML and PDF?

Because LaTeX math is rendered by MathJax (a JS library) in HTML.

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