Can markdown expressions and results be interleaved in the same block?

Question

A simple chunk in R markdown:

```{r}
1 + 2
3 + 4
```

Would produce the following when knitr converts to html:

<pre><code class="r">1 + 2</code></pre>
<pre><code>## 3</code></pre>

<pre><code class="r">3 + 4</code></pre>
<pre><code>## 7</code></pre>

I'm trying to output the expressions and results in one block

<pre><code class="r">
1 + 2
## 3
3 + 4
## 7
</code></pre>

I've tried tinkering around with the chunk parameters (e.g. results and echo) to no avail. Is there any way to accomplish this?

Note: I could probably hack at CSS with ::first and ::last selectors, but I'm curious as to whether there's a built-in option.

Answer 1

This can be done using hooks. Add the following code chunk right at the top of your Rmd document. It uses the document hook which is run on the md file at the last stage of knitting. The hook defined below identifies subsequent code chunks without any text chunk in between and collapses it into one.

```{r setup, cache = F, echo = F}
knitr::knit_hooks$set(document = function(x){ 
  gsub("```\n*```r*\n*", "", x) 
})
```

NOTE. It is important to set cache = F in this chunk so that this code is always run.