Dynamically building RMarkdown - r

Question

I have a non-fixed List of *.Rmd files and want to dynamically render them into a single html File with RMarkdown.
Like this:
reportFiles <- list()
reportFiles[[1]] <- "F:\\report1.Rmd"
reportFiles[[2]] <- "F:\\report2.Rmd"
outputPath <- "F:\\report.html"
rmarkdown::render(input = reportFiles, output_file = outputPath)
But that doesnt work and i couldnt find a solution on how to do something like this. In all scenarios it either creates multiple files or you have to know what files yuo want to render beforehand or you have to create a temporary *.Rmd file.

Answer 1

One can combine multiple Rmd files into a single output document by modifying the code posted with the question.
First, the documents must be combined into a single Rmd before processing with rmarkdown::render(). Second, all files combined must take account of the following constraints.
Only the first Rmd file can contain document header information
Section labels must be unique across all Rmd files combined into a single Rmd for rendering.
The general approach is to read the files into a character vector, write the vector to a temporary Rmd file, then render the combined document.
library(rmarkdown)
# list of files to be combined
reports <- c("report1.Rmd","report2.Rmd")
# read the files & combine into a single character vector
theReports <- unlist(lapply(reports,readLines))
# use writeLines() to combine into single Rmd
tmpFile <- writeLines(theReports,"tmpReport.Rmd")
# render the combined document
render(input = "tmpReport.Rmd")
When rendered to an HTML document, the output looks like this:
Additional Considerations
We used a character vector instead of a list() to store the file names because the additional complexity of a list() was not needed to drive lapply() in this situation.
Use of a character vector allows the solution to be modified to potentially retrieve a list of files from a subdirectory with list.files(), as in:
reports <- list.files(path="./myReportDir/",
pattern="report[[:digit:]]+.Rmd",full.names=TRUE)
Also, one can segregate the header information into a file that contains only header info, such as report_header.Rmd.
Next, in order to automate retrieval of the files from a directory, one must ensure that the sort order of the file names matches the intended order of inclusion in the output document. H/T to Petr Kajzar for the regular expression to extract only files with a numbered report name from list.files().
Finally, also as suggested by Petr Kajzar in the comments, one can use a truly temporary file to drive rmarkdown::render() as follows.
tmpfile <- tempfile(fileext=".Rmd")
writeLines(theReports,tmpfile)
render(input = tmpfile)
Appendix
To make the example completely reproducible, we include the text of report1.Rmd and report2.Rmd. These files must be copied & saved to a local computer in order for the script above to read, write, and render them.
report1.Rmd
---
title: "report 1"
author: "lg"
date: "7/6/2020"
output: html_document
---
```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```
## R Markdown
This is an R Markdown document. Markdown is a simple formatting syntax for authoring HTML, PDF, and MS Word documents. For more details on using R Markdown see <http://rmarkdown.rstudio.com>.
When you click the **Knit** button a document will be generated that includes both content as well as the output of any embedded R code chunks within the document. You can embed an R code chunk like this:
```{r cars}
summary(cars)
```
## Including Plots
You can also embed plots, for example:
```{r pressure, echo=FALSE}
plot(pressure)
```
Note that the `echo = FALSE` parameter was added to the code chunk to prevent printing of the R code that generated the plot.
report2.Rmd
Notice that the content in the second report conforms to the two constraints listed above.
## Report number 2
This is some text for the second markdown document. Considerations to make concatenation of multiple Rmd files into a single output document work:
1. Files 2 thru N must not have Rmd header information
2. Files that are combined into a single Rmd must not have duplicate section labels
```{r cars2}
summary(cars)
```
## Including Plots
You can also embed plots, for example:
```{r pressure2, echo=FALSE}
plot(pressure)
```
Note that the `echo = FALSE` parameter was added to the code chunk to prevent printing of the R code that generated the plot.