Specifying parameters in yml file for Quarto - rstudio

Question

I am creating a quarto book project in RStudio to render an html document.
I need to specify some parameters in the yml file but the qmd file returns
"object 'params' not found". Using knitR.
I use the default yml file where I have added params under the book tag
project:
type: book
book:
title: "Params_TEst"
author: "Jane Doe"
date: "15/07/2022"
params:
pcn: 0.1
chapters:
- index.qmd
- intro.qmd
- summary.qmd
- references.qmd
bibliography: references.bib
format:
html:
theme: cosmo
pdf:
documentclass: scrreprt
editor: visual
and the qmd file looks like this
# Preface {.unnumbered}
This is a Quarto book.
To learn more about Quarto books visit <https://quarto.org/docs/books>.
```{r}
1 + 1
params$pcn
When I render the book, or preview the book in Rstudio the error I receive is:
Quitting from lines 8-10 (index.qmd)
Error in eval(expr, envir, enclos) : object 'params' not found
Calls: .main ... withVisible -> eval_with_user_handlers -> eval -> eval
I have experimented placing the params line in the yml in different places but nothing works so far.
Could anybody help?

Answer 1

For multi-page renders, e.g. quarto books, you need to add the YAML to each page, not in the _quarto.yml file
So in your case, each of the chapters that calls a parameter needs a YAML header, like index.qmd, intro.qmd, and summary.qmd, but perhaps not references.qmd.
The YAML header should look just like it does in a standard Rmd. So for example, your index.qmd would look like this:
---
params:
pcn: 0.1
---
# Preface {.unnumbered}
This is a Quarto book.
To learn more about Quarto books visit <https://quarto.org/docs/books>.
```{r}
1 + 1
params$pcn
But, what if you need to change the parameter and re-render?
Then simply pass new parameters to the quarto_render function
quarto::quarto_render(input = here::here("quarto"), #expecting a dir to render
output_format = "html", #output dir is set in _quarto.yml
cache_refresh = TRUE,
execute_params = list(pcn = 0.2))

Answer 2

For now, this only seems to work if you add the parameters to each individual page front-matter YAML.
If you have a large number of pages and need to keep parameters centralized, a workaround is to run a preprocessing script that replaces the parameters in all pages. To add a preprocessing script, add the key pre-render to your _quarto.yml file. The Quarto website has detailed instructions.
For example, if you have N pages named index<N>.qmd, you could have a placeholder in the YML of each page:
---
title: This is chapter N
yourparamplaceholder
---
Your pre-render script could replace yourparamplaceholder with the desired parameters. Here's an example Python script:
for filename in os.listdir(dir):
if filename.endswith(".qmd"):
with open(filename, "r") as f:
txt = f.read()
f.replace('yourparamplaceholder', 'params:\n\tpcn: 0.1\n\tother:20\n')
with open(filename, "w") as ff:
ff.write(txt)
I agree with you that being able to set parameters centrally would be a good idea.