Conditionally display a block of text in R Markdown

Question

I am using knitr to parse an R Markdown document . Is there a way to conditionally display a block of text in R Markdown depending on a variable in the environment I pass into knitr?

For instance, something like:

`r if(show.text) {`
  la la la
`r }`

Would print "la la la" in the resulting doc if show.text is true.

Answer 1

You need a complete R expression, so you cannot break it into multiple blocks like you show, but if the results of a block are a text string then it will be included as is (without quotes), so you should be able to do something like:

`r if(show.text){"la la la"}`

and it will include the text if and only if show.text is TRUE.

Answer 2

You can do this using the "eval" chunk option. See http://yihui.name/knitr/options/.

```{r setup, echo=FALSE}
show_text <- FALSE
````

```{r conditional_block, eval=show_text}
print("this will only print when show.text is TRUE")
```

I've been using YAML config files to parameterize my markdown reports which makes them more reusable.

```{r load_config}
library(yaml)
config <- yaml.load_file("config.yaml")
```

...

```{r conditional_print, eval=config$show_text}
print("la la la")
````

Answer 3

I find it easiest to do this by putting all of my text into a separate file and then include it from the main file with:

```{r conditional_print, child='text.Rmd', eval = show_text}
```

This has the advantage that you can still put inline R statements or other chunks into the child file, so that if you change your mind about what counts as optional text, you don't have to refactor your project.

Answer 4

Here's a tweak to Paul Boardman's approach that gives proper markup in the output.

```{r setup, echo=FALSE}
show_text <- FALSE
```

```{r conditional_block, echo=FALSE, results='asis', eval=show_text}
cat("## Hey look, a heading!

lorem ipsum dolor emet...")
```

Even better, if we invoke the python engine to generate our output, we can use triple quoting to easily handle text that contains single or double quotes without needing to do anything fancy:

```{python, conditional_block_py, echo=FALSE, results='asis', eval=show_cond_text}
print("""
## Still a heading

Block of text with 'single quotes' and "double quotes"
""")
```