UK TeX FAQ -- question label conditional

### Conditional compilation and “comments”

While LaTeX (or any other TeX-derived package) isn’t really like a compiler, people regularly want to do compiler-like things using it. Common requirements are conditional ‘compilation’ and ‘block comments’, and several LaTeX-specific means to this end are available.

The simple `\``newcommand{`\``gobble`}[1]``{``}` and `\``iffalse``...` `\``fi` aren’t really satisfactory (as a general solution) for comments, since the matter being skipped is nevertheless scanned by TeX, not always as you would expect. The scanning imposes restrictions on what you’re allowed to skip; this may not be a problem in today’s job, but could return to bite you tomorrow. For an example of surprises that may come to bite you, consider the following example (derived from real user experience):

>     \iffalse % ignoring this bit
>     consider what happens if we
>     use \verb+\iftrue+ -- a surprise
>     \fi

The `\``iftrue` is spotted by TeX as it scans, ignoring the `\``verb` command; so the `\``iffalse` isn’t terminated by the following `\``fi`. Also, `\``gobble` is pretty inefficient at consuming anything non-trivial, since all the matter to be skipped is copied to the argument stack before being ignored.

If your requirement is for a document from which whole chapters (or the like) are missing, consider the LaTeX `\``include`/`\``includeonly` system. If you ‘`\``include`’ your files (rather than `\``input` them — see What’s going on in my `\``include` commands?), LaTeX writes macro traces of what’s going on at the end of each chapter to the `.aux` file; by using `\``includeonly`, you can give LaTeX an exhaustive list of the files that are needed. Files that don’t get `\``include`d are skipped entirely, but the document processing continues as if they were there, and page, footnote, and other numbers are not disturbed. Note that you can choose which sections you want included interactively, using the askinclude package.

The inverse can be done using the excludeonly package: this allows you to exclude a (list of) `\``include`d files from your document, by means of an `\``excludeonly` command.

If you want to select particular pages of your document, use Heiko Oberdiek’s pagesel or the selectp packages. You can do something similar with an existing PDF document (which you may have compiled using pdflatex in the first place), using the pdfpages package. The job is then done with a document looking like:

>     \documentclass{article}
>     \usepackage[final]{pdfpages}
>     \begin{document}
>     \includepdf[pages=30-40]{yoursource.pdf}
>     \end{document}

(To include all of the document, you write

>     \includepdf[pages=-]{yoursource.pdf}

omitting the start and end pages in the optional argument.)

If you want flexible facilities for including or excluding small portions of a file, consider the comment, version or optional packages.

The comment package allows you to declare areas of a document to be included or excluded; you make these declarations in the preamble of your file. The command `\``includecomment{`version-name`}` declares an environment `version-name` whose content will be included in your document, while `\``excludecomment{`version-name`}` defines an environment whose content will be excluded from the document. The package uses a method for exclusion that is pretty robust, and can cope with ill-formed bunches of text (e.g., with unbalanced braces or `\``if` commands).

These FAQs employ the comment package to alter layout between the printed (two-column) version and the PDF version for browsing; there are `narrowversion` and `wideversion` for the two versions of the file.

version offers similar facilities to comment.sty (i.e., `\``includeversion` and `\``excludeversion` commands); it’s far “lighter weight”, but is less robust (and in particular, cannot deal with very large areas of text being included/excluded).

A significant development of version, confusingly called versions (i.e., merely a plural of the old package name). Versions adds a command `\``markversion{`version-name`}` which defines an environment that prints the included text, with a clear printed mark around it.

optional defines a command `\``opt`; its first argument is an ‘inclusion flag’, and its second is text to be included or excluded. Text to be included or excluded must be well-formed (nothing mismatched), and should not be too big — if a large body of text is needed, `\``input` should be used in the argument. The documentation (in the package file itself) tells you how to declare which sections are to be included: this can be done in the document preamble, but the documentation also suggests ways in which it can be done on the command line that invokes LaTeX, or interactively.

And, not least of this style of conditional compilation, verbatim (which should be available in any distribution) defines a `comment` environment, which enables the dedicated user of the source text editor to suppress bits of a LaTeX source file. The memoir class offers the same environment.

An interesting variation is the xcomment package. This defines an environment whose body is all excluded, apart from environments named in its argument. So, for example:

>     \begin{xcomment}{figure,table}
>       This text is not included
>       \begin{figure}
>         This figure is included
>       \end{figure}
>       This is not included, either
>       \begin{table}
>         This table also included
>       \end{table}
>       ...
>     \end{xcomment}

A further valuable twist is offered by the extract package. This allows you to produce a “partial copy” of an existing document: the package was developed to permit production of a “book of examples” from a set of lecture notes. The package documentation shows the following usage:

>     \usepackage[
>       active,
>       generate=foobar,
>       extract-env={figure,table},
>       extract-cmd={chapter,section}
>     ]{extract}

which will cause the package to produce a file foobar.tex containing all the `figure` and `table` environments, and the `\``chapter` and `\``section` commands, from the document being processed. The new file foobar.tex is generated in the course of an otherwise ordinary run on the ‘master’ document. The package provides a good number of other facilities, including (numeric or labelled) ranges of environments to extract, and an `extract` environment which you can use to create complete ready-to-run LaTeX documents with stuff you’ve extracted.

`askinclude.sty`
  Distributed with Heiko Oberdiek’s packages — macros/latex/contrib/oberdiek (or browse the directory)
`comment.sty`
  macros/latex/contrib/comment (or browse the directory)
`excludeonly.sty`
  macros/latex/contrib/misc/excludeonly.sty
`extract.sty`
  macros/latex/contrib/extract (or browse the directory)
`memoir.cls`
  macros/latex/contrib/memoir (or browse the directory)
`optional.sty`
  macros/latex/contrib/misc/optional.sty
`pagesel.sty`
  Distributed with Heiko Oberdiek’s packages — macros/latex/contrib/oberdiek (or browse the directory)
`pdfpages.sty`
  macros/latex/contrib/pdfpages (or browse the directory)
`selectp.sty`
  macros/latex/contrib/misc/selectp.sty
`verbatim.sty`
  Distributed as part of macros/latex/required/tools (or browse the directory)
`version.sty`
  macros/latex/contrib/misc/version.sty
`versions.sty`
  macros/latex/contrib/versions/versions.sty
`xcomment.sty`
  Distributed as part of macros/latex/contrib/seminar (or browse the directory)

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