Conditional `echo` (or eval or include) in rmarkdown chunks

Question

I would like to create an Rmarkdown document (pdf or html) that has some chunks \"executed\" conditionally. The particular case I have in mind is that I might want a more v

Answer 1

opts_chunk$set() is what you are after. Anything "set" will be the default for subsequent chunks (unless overwritten on a chunk-by-chunk basis)

```{r setup}
library(knitr)
opts_chunk$set(eval = TRUE, include= TRUE)
````

You can then alter as you see fit.

Answer 2

knitr options can be stated as R expressions. Per the "output" documentation on the knitr webpage:

Note all options in knitr can take values from R expressions, which brings the feature of conditional evaluation introduced in the main manual. In short, eval=dothis means the real value of eval is taken from a variable named dothis in the global environment; by manipulating this variable, we can turn on/off the evaluation of a batch of chunks.

In other words if you write some chunks like:

```{r label}
doNextChunk <- as.logical(rbinom(1,1,.5))
```

```{r conditional, eval = doNextChunk}
"hello world!"
```