knitr: Use an inline expression in fig.cap chunk option

Question

My question is about the knitr option fig.cap, when using LaTeX. Is it possible to include an inline or \Sexpr in the fig.cap character string? For example, I would like to have something like (I'm using an .Rtex file): ``` \documentclass{article} \begin{document} %% begin.rcode fig.cap="x is \rinline{x}" % x <- 5 % p <- seq(0,5) % q <- x*p % plot(p,q) %% end.rcode \end{document} ``` I'd really like for that chunk to produce a plot in my .tex document, with a caption reading "x is 5". Instead, it throws an "undefined control sequence" error on pdflatex compilation. If I don't escape the rinline (i.e. use just inline{x}), then it compiles, but the caption is "x is inlinex". Is what I'm asking possible? This is my first SO question (used the answers here many times, though. Thanks!), so I'd appreciate any feedback on how to ask better questions. Thanks for the help!

Accepted Answer

`fig.cap` is evaluated as an R expression, so instead of using `\rinline` (and thus having the caption again parsed by `knitr`), you can just create the caption string in R.

```
%% begin.rcode fig.cap=paste("x is", x)
```

but because `fig.cap` is evaluated before `x` is created by default, you will need to postpone the evaluation of `fig.cap`; to do that, you can include a chunk like this in the beginning of your document:

```
%% begin.rcode setup, include=FALSE
%% opts_knit$set(eval.after = 'fig.cap')
%% end.rcode
```

It specifies `fig.cap` to be evaluated after the code chunk is evaluated, i.e. when `x` is available for you to use in the figure caption. See `eval.after` in the documentation.

The other way to do this is to create `x` in a previous chunk, and use `fig.cap=paste("x is", x)` in the next chunk.

knitr: Use an inline expression in fig.cap chunk option

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