R Quick Tip: parameter re-use within rmarkdown YAML

Ever wondered how to make an rmarkdown title dynamic? Maybe, wanted to use a parameter in multiple locations? Maybe wanted to pass through a publication date? Advanced use of YAML headers can help!

Normally, when we write rmarkdown, we might use something like the basic YAML header that the rmarkdown template gives us.

---
title: "My report"
date: "18th April, 2017"
output: pdf_document
---

You may already know the trick about making the date dynamic to whatever date the report gets rendered on by using the inline R execution mode of rmarkdown to insert a value.

---
title: "My report"
date: "`r Sys.Date()`"
output: pdf_document
---

What you may not already know is that YAML fields get evaluated sequentially so you can use a value created further up in the params section, to use it later in the block.

---
params:
  dynamictitle: "My report"
  reportdate: !r Sys.Date()
output: pdf_document
title: "`r params$dynamictitle`"
date: "`r params$reportdate`"
---

By doing this, you can then pass parameter values into the render() function when you want to generate a report.

rmarkdown::render("MWE.Rmd", 
    params=list(dynamictitle="New",
                reportdate=Sys.Date()-1)
                )

Parameter re-use within rmarkdown enables you to dynamically generate vital metadata for the report and use values in multiple places within the document. Very handy!

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