How to gsub('%', '\%', ... in R?

I want to export a latex table with a units column that has the percent (%) symbol.

library(xtable) 
foo <- data.frame(units='%', citation = '\\citep{authorYYYYabc}')
print(xtable(foo), sanitize.text.function = function(x) {x})

note: above code has been changed since Joris' answer.

In this case, the '%' is interpreted as a comment by LaTeX.

I have tried

gsub('%', '\\%', foo)

returns

[1] "1"

how can I convert the % to \% so that LaTex comments it out?

This question is a little bit like a previous question "can R paste('\')?"; even po开发者_如何学编程lishing the same table, but I can't figure out this particular case.

Combining Joris' answer with the codes in the comment:

library(xtable)
foo <- data.frame(units='%', citation = '\\citep{authorYYYYabc}')
print(xtable(foo), sanitize.text.function = function(x)gsub('%', '\\\\%', x))

I'm not sure if I understand you completely correctly. If I do xtable(foo), then the % is correctly escaped :

...
  \hline
1 & \% \\ 
   \hline
...

If you want to make an escape slash for something else, you need a quadruple one in gsub :

> x <- gsub('%', '\\\\%', foo[,1])
> x
[1] "\\%"
> cat(x,"\n")
\% 

Mind you, you cannot gsub in a dataframe, only in a vector. This can be shown by

> as.character(foo)
[1] "1"