Conceptually I want to do something like this:
---
title: Fun With Params
output: html_document
params:
x: 1
y: !r params$x + 1
---
```{r}
params
```
That is I want to use the value of one parameter in the definition of another one.
This does not work. It renders though with an Error:
Error in eval(expression) : object 'params' not found Calls: <Anonymous> -> eval -> eval Warning message: In yaml::yaml.load(yaml, handlers = knit_params_handlers(evaluate = evaluate), : an error occurred when handling type 'r'; using default handler
And the document looks like this:
Basically, this tells me that in the moment the YAML header is parsed params does not yet exist and parms$x cannot be evaluated, hence it returns simply the string.
I could assign a default value (NULL say) and push the logic to my first chunk. This, however, has the annoying drawback that in RStudio I should not forget to run this very chunk to get the proper value of params$y. This is of course not an issue when rendering the full document, but annoying if I want (for whatever reason) to run some chunks (not necessarily in order) interactively.
Thus, is there a way how I could use another param in the definition of another one?
>Solution :
I don’t think this is wholly satisfactory, but one approach would be to use dplyr::lst() which allows the sequential building of objects. This would result in a nested parameter object however.
---
title: Fun With Params
output: html_document
params:
myparams: !r dplyr::lst(x = 1, y = x + 1)
---
```{r}
params
```
Alternatively, you could put some placeholders in your YAML header and replace these with a render() call:
---
title: Fun With Params
output: html_document
params:
x: NA
y: NA
---
```{r}
params
```
And use:
rmarkdown::render("funwithparams.rmd", params = dplyr::lst(x = 1, y = x + 1))
Giving:
