TeX("$\lambda$") doesn't work in R

Question

Im using the latex2exp package to make some plots in R with latex expressions in the labels. TeX("$\lambda$") on the xlabel doesn't print what it needs to. The documentation said to use TeX(r'($\lambda$)') which doesnt work, and TeX("$\\lambda$") doesnt work either. For example, plot(1:5, 1:5, xlab = Tex(r'($\lambda$)')) will not have the lambda character on the xlabel. How should I fix this?

Answer 1

With knitr and the tikz device there are no need of latex2exp and the TeX() function, only escape the backslash.

\documentclass{article}
\begin{document}
<<foo,echo=F,dev='tikz',fig.width=3, fig.height=3, fig.align='center'>>=
plot(1:5, 1:5, xlab = '$\\lambda$ with \\LaTeX')
@
\end{document}

Answer 2

The documentation of latex2exp mentions the following:

Before R 4.0, it is necessary to escape the backslash within the string literal: therefore, one writes '$\\gamma$' rather than '$\gamma$' (the latter will cause an error).

After R 4.0, it is recommended to use the new raw string literal syntax (see ?Quotes). The syntax looks like r"(...)", where ... can contain any character sequence, including \:

You can find your R version when you start R, the version number is printed in the terminal window. Note that the R command is TeX() (so upper case T, lower case e, upper case X, which is the standard capitalization of TeX, and not Tex as written in the final snippet in the question - but I guess that is a typo.

The following works in R 3.6:

library(latex2exp)
plot(1:5, 1:5, xlab = TeX("$\\lambda$"))

Result:

For R 4.0 and above you can use the new syntax. Note that you have to explicitly include the delimiter within the string. In the example above the delimiters are '( and )'. The following works when tested in R 4.0.3:

library(latex2exp)
plot(1:5, 1:5, xlab = TeX(r'($\lambda$)'))

This does not work if you have the closing sequence )' somewhere inside of your string. In that case you can extend the delimiter with dashes, for example:

plot(1:5, 1:5, xlab = TeX(r'---($\lambda\cdot(a+b)'$)---'))
# more readable:
plot(1:5, 1:5, xlab = TeX(
   r'---(
   $\lambda\cdot(a+b)'$
   )---'
))