insert portions of a markdown document inside another markdown document using knitr

Question

I know this can be done with php and other languages, but was wondering whether the following could be accomplished using knitr:

Let's say I have an Rmarkdown (.rmd) document with two heading 1 sections:

# This is the first heading for the first document
Lorem ipsum dolor sit amet

# This is the second heading for the first document
plot(object)

1. Question 1: if open another .rmd document, how can i create a link so that when parsed this document would present its content as well as the whole content from the first document. For example:

   # This is the first heading for the second document
   Lorem ipsum dolor sit amet
   
   [command goes here to insert the first document]
   

   result would be:

   # This is the first heading for the second document
   Lorem ipsum dolor sit amet
   
   # This is the first heading for the first document
   Lorem ipsum dolor sit amet
   
   # This is the second heading for the first document
   [plot shows up here]
   

2. Question 2: would knitr allow me to select and insert only selected portions of document 1 into document 2? For example, only heading 1 and the content below it, or only heading 2 and its plot

Answer 1

1. that is what the chunk option child is for, e.g. in second.Rmd, you can

   ```{r child='first.Rmd'}
   ```
   

2. that is a little bit trickier, but you can call knit_child() manually, e.g.

   ```{r echo=FALSE, results='asis'}
   # knit the first three lines of first.Rmd
   cat(knit_child(text = readLines('first.Rmd')[1:3]), sep = '\n')
   ```