How to use expressions in subcaptions

[dmkaplan2000]

Description

I want to use expressions in subcaptions, but I can't seem to figure out how to do it. For example, when I render the following document:

---
format: html
---

```{r}
x = "A test label"
```

```{r}
#| label: fig-test
#| fig-cap: !expr x
#| fig-subcap:
#|   - !expr x
#|   - !expr x
plot(1:10)
plot(1:20)
```

The overall figure caption appears as it should ("A test label"), but the captions for the subfigures are not correct, appearing as "expression(x)". Is there a way to fix this? I have tried various forms of quoting around x, but nothing seems to work.

[mcanouil]

Which version are you using?

Note that !expr is not recommended (even if is is supported) in Quarto.
With 1.4 you can do the following:

---
format: html
---

```{r}
x <- "A test label"
```

:::: {#fig-test}

::: {#fig-test-1}
```{r}
plot(1:10)
```

`{r} x`
:::

::: {#fig-test-2}
```{r}
plot(1:10)
```

`{r} x`
:::

`{r} x`
::::

[mcanouil]

@cderv is !expr even supported in fig-subcap?

[cscheid]

(In passing, you'll note that the source code for these subfigures is currently being included in the figure itself. That's a quarto bug that we'll resolve before 1.4 is out.)

[harrelfe]

In other contexts the notation for R expressions in text is backtick r expression backtick without putting r in braces. Why not use that notation for consistency?

[mcanouil]

It's the knitr syntax which is unrelated to consistency as far as Quarto is concerned.
Quarto brings inline expression beyond knitr.
See https://quarto.org/docs/prerelease/1.4/inline.html.

Also knitr in my opinion is not consistent ```r is not evaluated but ```{r} is so why `r also is?

Quarto makes this consistent for R but also for other languages.
Computed code cells and inline expressions use brackets.

[dmkaplan2000]

Huh, OK, I have used that approach for external images, but never for figures that are output from R. Are fig-cap and fig-subcap being deprecated then? We could essentially eliminate them and generate all figures and tables the way you are proposing.

To answer @cderv, !expr does do things for fig-subcap, but not at all what you expect. It seems to wrap the argument in expression().

I am using Quarto 1.4.429.

[cscheid]

Huh, OK, I have used that approach for external images, but never for figures that are output from R. Are fig-cap and fig-subcap being deprecated then?

They're not deprecated, but !expr is going to become less supported as time goes by, simply because it's a knitr-specific hack that makes a lot of quarto features impossible to implement correctly.

We're encouraging people to use the div syntax because that makes it particularly easy to write captions that include "fancy" markdown, including crossreferences, shortcodes, and computational content. Those are hard to do correctly in !expr in all settings.

[dmkaplan2000]

Yes, #1959 was my question way back on the issue of fancy caption. This is a great fix.

[dmkaplan2000]

Also, is !expr not recommended for all chunk arguments or just things like figure captions? For example, I often include things like:

#| eval: !expr file.exists("my-results.RDS")

If there a different approach to achieving this result?

[cscheid]

How is that sort of thing done in python if !expr doesn't work?

Conditionals, like you can also do in {r}:

```{r}
#| eval: !expr foo
code
```

is fairly close to

```{r}
if (foo) {
  code
}
```

[cderv]

eval: false is different than if(FALSE) in the sense that knitr will really skip evaluation of code. And when conditionally using a chunk, using if syntax when echo: true will show you source code like you don't really want. That is why it is an option and not just à if in the code chunk.

Anyhow, this is a knitr feature that still works well. One just need to know how to use it correctly in chunk and know the limitation.