ifelse action depending on document type in rmarkd

问题:

When preparing reports using rmarkdown: http://rmarkdown.rstudio.com/ one may want the document to render differently depending upon the document type. For instance I may want to embed a youtube video if the document being rendered is an html file where as if it's pdf or MS Word I would want the hyper-linked URL instead.

Is there a way to tell rmarkdown something like this:

if (html) {
    <iframe width="640" height="390" src="https://www.youtube.com/embed/FnblmZdTbYs?    feature=player_detailpage" frameborder="0" allowfullscreen></iframe>
} else {
    https://www.youtube.com/watch?v=ekBJgsfKnlw
}

code

devtools::install_github("rstudio/rmarkdown")
library(rmarkdown)
render("foo.Rmd", "all")

foo.Rmd

---
title: "For Fun"
date: "`r format(Sys.time(), '%d %B, %Y')`"
output:
  html_document:
    toc: true
    theme: journal
    number_sections: true
  pdf_document:
    toc: true
    number_sections: true
  word_document:
    fig_width: 5
    fig_height: 5
    fig_caption: true
---

## Good times

<iframe width="640" height="390" src="https://www.youtube.com/embed/FnblmZdTbYs?feature=player_detailpage" frameborder="0" allowfullscreen></iframe>

回答1:

Yes, you can access the output format via knitr::opts_knit$get("rmarkdown.pandoc.to"). This will return a string with the target output format. Here's an example:

---
title: "Untitled"
output: html_document
---

```{r}
library(knitr)
opts_knit$get("rmarkdown.pandoc.to")
```

This returns "html" for html_document, "docx" for word_document, and "latex" for pdf_document. So to answer your question you can do something like:

html <- knitr::opts_knit$get("rmarkdown.pandoc.to") == "html"

回答2:

As pointed out in an answer to a related question, knitr 1.18 introduced the following functions

knitr::is_html_output()
knitr::is_latex_output()

As the name suggests, is_html_output() checks if the output is HTML. You would add something like this to foo.Rmd:

```{r results='asis'}
if (knitr::is_html_output()) {
    cat('<iframe width="640" height="390" src="https://www.youtube.com/embed/FnblmZdTbYs?        feature=player_detailpage" frameborder="0" allowfullscreen></iframe>')
} else {
    cat("https://www.youtube.com/watch?v=ekBJgsfKnlw")
}
```