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What exactly do csname and endcsname do? What are their job?

I have glanced at the Texbook and some other books, but none of them was clear enough to me.

Can anyone please give a simple example to clarify this issue?

Normally, control sequence names are made only of letters or of one non-letter character.

A letter is, more precisely, a character having category code 11 at the moment the control sequence name is read. So, any character can become part of a control sequence name, provided we change its catcode before the definition and each usage.

With csname...endcsname we are freed from this limitation and every character can go inside them to form a control sequence name (of course, % is excluded because it disappears together what remains on the line before TeX is doing its work on characters).

However, this is not the main purpose of csname...endcsname. This construction is used to build commands from "variable parts". Think, for instance to LaTeX's newcounter: after newcounter{foo}, TeX knows thefoo that is built precisely in this way. Roughly, what LaTeX does is

newcommand{newcounter}[1]{%

   expandafternewcountcsname c@#1endcsname

   expandafterdefcsname the#1endcsname{arabic{#1}}%

 }

so that newcounter{foo} does the right job. It's more complicated than this, of course, but the main things are here; newcount is the low-level command to allocate a counter. The expandafter is just to build the control sequence before newcount and def see the token.

Inside csname...endcsname, category codes don't matter (with one main exception: active characters will be expanded if not preceded by string, see final note). LaTeX exploits this in order to build control sequence names that users won't be able to access (easily). For example, the control sequence to choose the default ten point font is OT1/cmr/m/n/10, which can be easily split internally (by the "reverse" operation that is string) and is not available to the casual user.

Another important use is in environments: when you say newenvironment{foo}, LaTeX really defines foo and endfoo. Upon finding begin{foo}, LaTeX does some bookkeeping and then executes csname fooendcsname (that's why one can say also newenvironment{foo*}); similarly, at end{foo} LaTeX executes csname endfooendcsname and after this it does some bookkeeping again.

Other uses: label{foo} will define control sequences based on foo via csname...endcsname that can be used by ref.

When one says csname fooendcsname, LaTeX will look whether foo is defined; if not, it will execute relax and from then on (respecting grouping), foo will be interpreted as relax. An interesting usage for this feature is that one can say

chapter*{Introduction}

csname phantomsectionendcsname

addcontentsline{toc}{chapter}{Introduction}

and keep hyperref happy if it's loaded, while doing nothing if the package is not loaded.

It's possible to give many other interesting uses of this trick. But one should always keep in mind that TeX does complete expansion of what it finds in that context and that only characters must remain. So

csname abcrelax defendcsname

is forbidden. But, after defxyz{abc},

csname xyz defendcsname

will be legal and equivalent to saying csname abcdefendcsname or abcdef.

Final note

It's better to add something about category codes. An active character in csname...endcsname will be expanded, so to get a literal ~ one has to write string~. Comment (category 14), ignored (category 9) and invalid (category 15) characters will remain such. So

csname %endcsname

will give an error (Missing endcsname); in csname ^^@endcsname there will be no character and csname ^^?endcsname will raise an error.

For reference, from the TeX Book (with slight formatting changes), Chapter 7: How TeX Reads What You Type (p 40):

...you can go from a list of character tokens to a control sequence by
saying csname<tokens>endcsname. The tokens that appear in this
construction between csname and endcsname may include other
control sequences, as long as those control sequences ultimately
expand into characters instead of TeX primitives; the final characters
can be of any category, not necessarily letters. For example, csname
TeXendcsname is essentially the same as TeX; but
csnameTeXendcsname is illegal, because TeX expands into tokens
containing the kern primitive. Furthermore,
csnamestringTeXendcsname will produce the unusual control
sequence \TeX, i.e., the token <TeX>, which you can't ordinarily
write.

I have used this indirectly by using the label-ref system and defining labels based on counters:

newcounter{mycount}

%...

newcommand{mycmd}{%

  stepcounter{mycount}%

  label{abcthemycount}%

  %...

}

This creates a "successive label abc1, abc2, ... for every call to mycmd, in order to avoid creating multiply defined labels with the same name. Indirectly, label{abcthemycount} calls @namedef{r@abcthemycount}, which calls

expandafterdefcsname r@abcthemycountendcsname

thereby expanding r@abcthemycount to r@abc1 and defining r@abc1 for the first label, r@abc2 for the second label, etc. Yes, labels in LaTeX are actually control sequences prepended with r@ and is constructed using csname ... endcsname which then allows numerals.

csname/endcsname allows you to build commands whose names contains 1. non-letters (e.g. dots or colons or numbers) and - more importantly - 2. commands which are expanded when you define or use the command. Both is useful if you want to construct a command name from various pieces of informations.

As an example: The xskak-Package loops through the notation of a chess game and stores a lot of informations of every move in commands. Its code contains a lot of definition of this type:

expandafterxdef

     csname Xskak.xskak@val@gameid.thec@move.WhiteToMove{w}{b}.pieceendcsname{%

      ....

      }

where xskak@val@gameid is the id of the current game, thec@move gives the current move number, WhiteToMove{w}{b} gives w or b depending on which player currently moves. So in the 10th move of black in the game with id "mygame" this xdef defines a "command" Xskak.mygame.10.b.piece which contains the name of the piece which has been moved by black in the tenth move.

Suppose you want to define a command foo2. You cannot do this because 2 is not a letter. However, this construction works: csname foo2endcsname. Sometimes this is useful, e.g. when you need a series of commands, foo1, foo2, etc (another way is to use roman numerals). Another example, suppose you want to define a series of commands like endsection, endsubsection, etc. Then you can use a loop with expandafterdefcsname end#1csname...

Short answer: csname and endcsname are a "macro environment" whose contents, if they evaluate (after expanding macros) to "ordinary text", are converted into the name of a macro (or control sequence, hence "csname").

Actually, it is a perhaps strange joke that by the rules of LaTeX macros, you can actually write begin{csname}...end{csname} and it will act as you expect, to a certain extent. For example:

defmacro{text}

defo{o}

begin{csname}%

 macro

end{csname}

% Same as csname macro endcsname

(when run with latex rather than tex) will produce the word "text" in the output. The % sign is there so that unnecessary spaces don't creep into the name of the macro we are constructing.

As a very beginner for this antique problem, I just provide a simple example that may tell future novice something about csname. Though I do not fully understand I have tried it and successfully solved one problem of mine.

My problem was to test different effect of different font command, like:

textit{textit} textbf{textbf} {bfseries bfseries} {small small}

and I tried to create a new command newcommand{FontTest}[1]{#1 #1}, I had quite a struggle wondering how to turn the first #1 to a command. And after many trials I used csname and it worked, so it was:

newcommand{FontTest}[1]{{ csname #1endcsname{#1}}}

and I can freely test different type of font families and shapes like: FontTest{textit} FontTest{sffamily} etc.

And its effect seems like turning a string into command I guess.