tweak demo page for #92

_posts/2012-01-18-child.md

@@ -5,8 +5,12 @@ subtitle: Input child files into the main document
 category: demo
 ---
 
+_Warning: this page is under construction and should not be considered to be complete. The feature described here relies on the development version of **knitr** on GitHub. General users can start using this feature after this warning message is removed._
+
 Like the command `\SweaveInput{}` in Sweave, **knitr** also has the similar functionality, which enables us to insert child documents into a main document. The idea is like the command `\input{}` or `\include{}` in LaTeX to manage a large document in smaller parts.
 
+## Using `knit_child()` with inline R code
+
 Sweave searches for `\SweaveInput{}` in the main document, and weaves the child documents, whereas **knitr** does not search for such character strings, instead, you can directly use `\Sexpr{}` with the function `knit_child()` to input a child document.
 
 For example, we have a main document named [`knitr-input.Rnw`](https://github.com/yihui/knitr/blob/master/inst/examples/knitr-input.Rnw) here:
@@ -51,4 +55,17 @@ library(knitr)
 knit('knitr-input.Rnw')
 {% endhighlight %}
 
-You can freely nest one Rnw document in the other, as long as the nested structure is clear in your mind. This sounds like the movie [Inception](http://en.wikipedia.org/wiki/Inception).
+You can freely nest one Rnw document in the other, as long as the nested structure is clear in your mind. This sounds like the movie [Inception](http://en.wikipedia.org/wiki/Inception).
+
+Be sure to read `?knit_child` to know more about the control of evaluation (argument `eval`), which is similar to what is described below in the second approach.
+
+## Using chunk option `child`
+
+The chunk option `child` supports the inclusion of child documents. The above example can be equivalently done with:
+
+{% highlight r %}
+<<child-demo, child=knitr-input-child.Rnw>>=
+@
+{% endhighlight %}
+
+You can control the evaluation of such child chunks with the usual chunk option `eval=TRUE/FALSE`, therefore the feature of conditional evaluation can also be used here (i.e. you can evaluate the chunks according to a global variable; see the main manual). It is fairly flexible to control which child chunks to include into the main document. This is useful when a large report consists of smaller pieces, and different pieces may generate different reports.