How to write complex captions with evaluated R code?

[svraka]

One nice feature of the new YAML-style chunk options is that we can write complex text for e.g. captions without the need for escaping everything in an R character vector (by the way this is not Quarto-specific, it also works with plain knitr).

```{r plot1}
#| fig.cap: A caption with some math, $\gamma = 0.5$
boxplot(Sepal.Length ~ Species, data = iris)
```

Is there a way to include R code in such a caption? Obviously, we can't use the standard way for evaluating R code by prefixing the value with !expr, as the value in this case is not an R expression.

In bookdown text references can be used for such a purpose:

```{r plot2}
#| fig.cap = "(ref:plot-cap)"
boxplot(Sepal.Length ~ Species, data = iris)
```

(ref:plot-cap) A caption with some math, $\gamma = 0.5$, $N = `r nrow(iris)`$.

Is something like this possible with Quarto without resorting to character vectors?

#| fig-cap: !expr sprintf("A caption with some math, $\\gamma = 0.5$, ($N = %s$)", nrow(iris))
boxplot(Sepal.Length ~ Species, data = iris)

[mcanouil]

I am not sure to understand the question or issue.
The following works just fine and is "complex":

#| fig-cap: !expr sprintf("A caption with some math, $\\gamma = 0.5$, ($N = %s$)", nrow(iris))
boxplot(Sepal.Length ~ Species, data = iris)

Are you asking for a new feature to be implemented in Quarto, which is the "text reference" thing from bookdown?

[mcanouil]

I use text references for captions that just have a lot of random R code and formatting involved, nothing to do with equations, so this question is more general than any single format.

Perhaps this should be converted into a bug report/feature request?

My point was not about the format but about the engine.

At least a feature request, one can create it from this discussion using the link on the menu on the right of the discussion.

[dmkaplan2000]

Oops, I went ahead and filed a feature request before I saw this message...

[mcanouil]

so you did not see the "create issue from discussion" button ^^

This automatically links the discussion to the issue, for example from another discussion:

[dmkaplan2000]

No. I see it now, but it is kind of in an out of the way place on my screen. Sorry for the bother...

[mcanouil]

It's fine, it's kind of a new feature from GitHub.
I noticed this only few weeks ago. 😉

[mcanouil]

For all that might see this discussion: the related issue is #1959 and it is scheduled as part of the v1.2 release milestone.

[eeholmes]

I achieve this as follows. My plot captions are often hairy bits of code since I am dynamically creating the caption based on the data being plotted.

```{r}
plot_cap <-  sprintf("A caption with some math, $\\gamma = 0.5$, ($N = %s$)", nrow(iris))
```

or

```{r}
plot_cap <-  captionfunction(mydata)
```

and then

```{r}
#| fig-cap: !expr plot_cap
boxplot(Sepal.Length ~ Species, data = iris)
```

basically the same thing, but I find this cleaner.

[egouldo]

Thanks for the example code. Can the plot_cap expression be created within the same chunk where it is called, or does it have to be created in a previous chunk?

[cderv]

It can be created in the same code chunk as this is a special knitr feature where some chunk options are evaluated after chunk evaluation. fig-cap (i..e fig.cap in knitr) is one of them

Did you try it already ? It should work in Quarto for knitr engine as this is a knitr feature.

In knitr, this is controlled at package option level by eval.after knitr option (https://yihui.org/knitr/options/#package-options) and this concerns mainly figure option (fig.cap, fig.scap, fig.alt and their dash version in Quarto).

From knitr documentatin

eval.after: (c('fig.cap', 'fig.scap', 'fig.alt'); character)
A character vector of option names. These options will be evaluated after a chunk has been evaluated, and all other options will be evaluated before a chunk. For example, for fig.cap = paste('p-value is', t.test(x)$p.value), it will be evaluated after the chunk according to the value of x if eval.after = 'fig.cap'.