How to include a header based on a condition in knitr

Question

I have a header followed by a code chunk in an Rmd file. I only want to include this header and the chunk followed by it, if a condition is met. I know how to do that with the chunk, since it's in the body of the code, but how do I do the former?

```{r}
 print_option <- TRUE
```

## My header
```{r}
 if(print_option==TRUE) {
 print (x)
 }
```

Answer 1

The chunk option eval and asis_output() provide a simple solution.

Assuming that print_option is a boolean indicating whether to show the header (and whether to execute other code like print(1:10) in chunk example1):

```{r setup}
library(knitr)
print_option <- TRUE
```

```{r, eval = print_option}
asis_output("## My header\\n") # Header that is only shown if print_option == TRUE
print(1:10) # Other stuff that is only executed if print_option == TRUE
```

Text that is shown regardless of `print_option`.

```{r setup2}
print_option <- FALSE
```

Now `print_option` is `FALSE`. Thus, the second header is not shown.

```{r, eval = print_option}
asis_out("## Second header\\n")
```

Output:

For longer conditional outputs (of text/markdown, without embedded R code) the engine asis can be helpful, see this answer (it's long, but the solution at the end is very concise).

Addendum

Why is ## `r Title` with Title set to "My header" or "" as suggested in this answer a bad idea? Because it creates an "empty header" in the second case. This header is not visible in the rendered HTML/markdown output, but it is still there. See the following example:

```{r, echo = FALSE}
title <- ""
```

## `r title`

This generates the following markdown ...

## 

... and HTML:

<h2></h2>

Besids being semantically nonsense it might lead to layout issues (depending on the style sheet) and disrupts the document outline.