Can knitr dynamically output narrative text based on R code results in each chunk?

Question

I'm thinking about developing a .rmd file that can dynamically write some chunks of narratives in the output file (.html,.pdf,...) based on the preceding R result. To put it simple below is how I want it works:

```{r,echo=F,include=FALSE}
x=1;

```
##if x=1 then output text below
text1

##if x=2 then output text below
text2

.....

Answer 1

When knitr processes a document, the document is split into two categories of input: ordinary text and code chunks. Ordinary text stays unchanged and is passed to the output file. Consequently, if plain text is supposed to be included dynamically, it must be inside a chunk.

Code chunks are evaluated according to their options. In the current scenario, to most important options are:

• eval
• echo
• results
• engine

eval determines whether a chunk is evaluated; eval = FALSE skips the chunk. echo determines whether to source code of the chunk is displayed. results determines how the output of a chunk is treated. By default (results = "markup) a output hook is used to apply document-type-specific markup to the output. results = "asis" means that all output is included in the output file "as-is", without any modifications.

With these three options, the following could be accomplished:

This is some text ...

```{r, echo = FALSE, eval = FALSE, results = "asis"}
cat("... with a secret inside ...")
```

```{r, echo = FALSE, eval = TRUE, results = "asis"}
cat("... that is dynamically generated.")
```

Output:

This is some text ...



... that is dynamically generated.

Note that the first chunk is not evaluated because eval = FALSE.

However, it is cumbersome to cat() lots of text from a R chunk. engine can be used to overcome this. Besides R, there are other engines that can be used to evaluate chunks, among them the (currently undocumented?) engine asis. This engine is very simple. From the knitr NEWS file:

added a new engine named asis to write the chunk content without processing it; it also respects the chunk options echo and eval -- when either one is FALSE, the chunk will be hidden; this makes it possible to write text conditionally

Combining the asis engine with the following syntactic sugar (source)

for language engines, the syntax of R Markdown code chunk headers can be ```{lang, option=value}` now (e.g. ```{python} and ```{Rcpp}), which is equivalent to ```{r, engine='lang', option=value}

the example from above becomes:

This is some text ...

```{asis, echo = FALSE}
... with a secret inside ...
```

```{asis, echo = TRUE}
... that is dynamically generated.
```