# Josh Errickson

## Creating Variants of a File with R + Knitr + Brew + LaTeX

The concept for this is based upon http://botthoughts.wordpress.com/2012/05/17/generating-reports-for-different-data-sets-using-brew-and-knitr/.

I had the need to create personalized exams for students - There were three sets of size seven, three and one respectively, of which each student essentially received one randomly. This setup could be useful in other settings involving the need for multiple files based on a template, and some variable.

First, you need in R some way to differentiate the variants. A data frame where each row is a variant works perfectly. Considering the following example

        > data
X1   X2
1  Alice ver1
2    Bob ver2 

Here, X1 is a name I want to include in the output, and X2 refers to some larger change I want to make. In my personalized exam setup, that was two different questions.

The R code to handle this is as follows.

        library(brew)
library(tools)
library(knitr)

genReports <- function(x, prepend) {
rnw.file <- paste0(prepend, x[1], ".Rnw")
brew("template.Rnw", rnw.file)
knit(rnw.file)
latex.file <- paste0(prepend, x[1], ".tex")
texi2pdf(latex.file, clean = TRUE, quiet = TRUE)
out.file <- paste0(prepend, x[1], ".pdf")
return(out.file)
}

apply(data, 1, genReports, prepend="report_") 

brew and knit handle replacing any R code, and then texi2pdf generates the actual PDF.

That code references a template.Rnw file. That can be just a regular LaTeX file, with a small change. A minimal example is below.

        \documentclass{article}
\begin{document}
Name: <%= x[1] %>

\input{<%= x[2] %>
\end{document}

(Note that you can also put Knitr code, which is similar to Sweave, directly in the file to run R code. For example, you could add

        <<printingchunk>>=
plot(rnorm(100))
@
which would be replaced with that plot in the PDF.)

The <%= x[1] %> notation is what does the magic - that will be replaced by brew + knit. Using the \input command allows means that ver1.tex and ver2.tex have to exist, but they should be LaTeX chunks, not a complete file (e.g. no header.)

So our end result, assuming we have ver1.tex and ver2.tex as two different questions, is that we have two PDF files, named report_Alice.pdf and report_Bob.pdf, where both have the individuals' names in the first line, and then contain two different questions.

As an additional note, there are two easy ways to print a lot of these simultaneously. First, you could call something like

        for f in report_*.pdf; do lp  \$f; done
and passing any additional arguments to lp. Secondly, the command pdfunite in the poppler package can be used to merge the files. (Just ensure that each file has an even number of pages if you plan to print double-sided, padding each file with a \newpage if needed.)