How to link a variable number of files from rmarkdown - r-markdown

Question

I have a system consisting of parameters in Access, which are read by an R script, which then starts an Rmarkdown report. In Rmarkdown, a Stata script is built, which reads a data file and creates a graph specified by the Access parameters. To get the Stata graph into the report, I have to store it as a PNG file and link to this file in the Rmarkdown code. Finally, the report is rendered as a Word file (using knitr and Pandoc).
In the present setup, I have several places in the report where a graph can be called for. I can create a single PNG file for each of these places, I know the filenames (controlled by the Access parameters), and I link to each file using the standard command ![](path/to/filename.png. This works properly.
The next development step is that in each place, I need to create an unknown and varying number of PNG files (up to ca. 20 files). I will do this in Stata. The problem is to link to a varying number of files in the Rmd code. I haven't found a way to do this, and need advice on how.
I have some ideas for a solution, but I cannot find the commands or syntax to implement them. I have read the Introduction to Rmarkdown from Rstudio.com, and the Rmarkdown Reference Guide (5 pages) from the same source. I am rather new to both R and Rmarkdown, so I might have overlooked or not understood that there is a solution.
Is it possible to set up a loop or branch (e.g. "if", "for" or "while") in Rmarkdown? Then I could loop over the current number of files, or branch around unused file links.
Can I fetch all files in a certain directory, e.g. by making a link containing wildcards in the filename? Or is there another way of achieving this?
Is there a way of having links to files that do not exist in the present run, without crashing the program? Then I could set up enough links to cover all foreseeable cases.
Or, does anyone have other suggestions?

Answer 1

Sure, you could use a loop like
```{r, results="asis"}
files <- list.files(path = '/path/to/your/pngdirectory/',
pattern = '\\.png', full.names = T)
for(f in files) cat(paste0('![](',f,')\n'))
```
If you want to filter for certain png files you can extend the pattern argument to some more sophisticated regular expression. For exampele, If I only want png files containing '2017-07-11' in their name I would do
list.files(path = '/Users/martin/Dropbox/Screenshots',
pattern = '.*2017-07-11.*\\.png', full.names = T)
where .* matches any character.