Show one line of code but result from another line without adding any comment

[cderv]

Tested from quarto-dev/quarto-cli#863 (comment)

mtcars
rmarkdown::paged_table(mtcars)

Can I show 'mtcars' in the code chunk after the render but not 'rmarkdown::paged_table(mtcars)' and in addition to that have only the result of the paged_table() shown?

At first I tried

```{r, echo = -2, eval= 2}
mtcars
rmarkdown::paged_table(mtcars)
```

but it will add a ## prefix, without option to opt-out

knitr/R/block.R

Lines 235 to 240 in ca398c2

	if (is.numeric(ev <- options$eval)) {
	# group source code into syntactically complete expressions
	if (isFALSE(options$tidy)) code = sapply(xfun::split_source(code), one_string)
	iss = seq_along(code)
	code = comment_out(code, '##', setdiff(iss, iss[ev]), newline = FALSE)
	}

## mtcars

I would have thought this could be equivalent of this

```{r, eval= FALSE}
mtcars
```

```{r, echo = FALSE}
rmarkdown::paged_table(mtcars)
```

I already encountered this in demo slide when trying to show an example I think. Would it be interesting to add chunk option to allow this ?

Maybe the longer form in two chunks is enough... 🤔

[yihui]

I remember @jaredlander also requested this feature eight or nine years ago at a Strata conf (time flies), i.e., evaluate and echo different lines of code in the same chunk. I didn't do it because I thought it would be a little tricky to implement, and also felt like "cheating" :) Since this was brought up again, I think we can reconsider implementing it. Basically, after evaluate()ing the code, we remove the ## from the source code expressions with indices setdiff(iss, iss[ev]); or we use a special comment like #~ / #* etc to mask the code instead of ##, then simply remove these comment masks (this will be a little easier implementation-wise).

@cderv Do you want to try it, or you would rather me do it? I won't have time until a few weeks later.

[cderv]

I'll give it a try following your advice.

[atusy]

I would rather suggest use the render chunk option.
It has finer control and is less tricky.

Things like {r, echo=-2, eval= 2} is not a good idea because author has to correct the values when line number changes.

```{r, render = function(x, ...) knitr::knit_print(rmarkdown::paged_table(x), ...)}
mtcars
```

[yihui]

For this particular case, yes, the problem can be solved by a custom render function. For general cases, we still need a more general solution.