Support for tabularx and xltabular

[amarakon]

So far kable only supports tabular and longtable.
But tabularx and xltabular are popular among LaTeX users.
I added support for them: all you have to do is use tabularx = TRUE or xltabular = TRUE.
I also added an option called header, which allows you to specify things to put before the body of the table.

[CLAassistant]

All committers have signed the CLA.

[yihui]

Instead of three boolean arguments, how about using one character argument like tabular.env? We can keep the longtable argument for backward compatibility.

[amarakon]

Instead of three boolean arguments, how about using one character argument like tabular.env? We can keep the longtable argument for backward compatibility.

tabularx and xltabular require different LaTeX code than tabular and longtable. This is why I have to create distinct options for them. If you take a tabular or longtable table and replace the environment with tabularx or xltabular, it will not work.
However, there are some people that might want to use neither tabular, longtable, tabularx, nor xltabular, which is why I openend issue #2134.
See this comment.

[amarakon]

Instead of three boolean arguments, how about using one character argument like tabular.env? We can keep the longtable argument for backward compatibility.

tabularx and xltabular require different LaTeX code than tabular and longtable. This is why I have to create distinct options for them. If you take a tabular or longtable table and replace the environment with tabularx or xltabular, it will not work. However, there are some people that might want to use neither tabular, longtable, tabularx, nor xltabular, which is why I openend issue #2134. See this comment.

Edit: Or you could add tabular.env but make it act differently for different arguments. It should work with longtable, tabularx, and xltabular.

[yihui]

Or you could add tabular.env but make it act differently for different arguments.

Yes, that's what I meant. The current tabularx argument would be equivalent to using tabular.env == 'tabularx'. In my code suggestions, I was using tabular instead of tabular.env. Are you okay with the suggested changes? (I didn't test them but just typed them directly on Github)

Thanks!

[amarakon]

Or you could add tabular.env but make it act differently for different arguments.

Yes, that's what I meant. The current tabularx argument would be equivalent to using tabular.env == 'tabularx'. In my code suggestions, I was using tabular instead of tabular.env. Are you okay with the suggested changes? (I didn't test them but just typed them directly on Github)

Thanks!

I tested your changes and tried to make the following table:

Lorem = c("Ipsum")
Dolor = c("Sit")

knitr::kable(data.frame(Lorem, Dolor), escape = FALSE, booktabs = TRUE, linesep = "\\addlinespace", tabular = "xltabular", caption = "Caption")

But it threw this error:

Error in paste(c(if (!tabular %in% c("longtable", "xltabular")) c(env1,  :
  object 'xltabular' not found
Calls: <Anonymous> ... eval -> <Anonymous> -> do.call -> kable_latex -> paste

[amarakon]

I committed the suggestions.

[yihui]

I made a couple of changes to your code and I hope you are okay with them:

1. I removed the table.length argument. If you need to specify the length for tabularx or xltabular, you may use the existing valign argument (despite the seemingly inappropriate name; technically the two arguments are doing essentially the same thing).
2. I removed the header argument to keep the number of arguments small. If you need it, you can pass it to midrule.

I'll merge for now. Please feel free to let me know if these changes don't make sense and we can amend this PR later. Thanks!

[amarakon]

Thanks, your changes are perfect.

1. The valign argument is satisfactory so there is no need for table.length.
2. Same goes for the midrule argument.