[Tex/LaTex] A reusable newlength (no stop/break if length already defined)

conditionalslengthsmacros

Often I have separate .tikz files, which may contain stuff like \newlength{\mytmplen}; and given I'm not very original with names, the same statement ends up in multiple such files. Which turns out to be a problem, if I want to include all those files in a single .tex document — then upon the second \newlength with the same argument, the compilation process would stop with:

! LaTeX Error: Command \mytmplen already defined.
               Or name \end... illegal, see p.192 of the manual.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              

l.30 \newlength{\mytmplen}

?

So, I thought of devising a new newlength command, which would define the new length if it doesn't exist; but it would simply typeout a message, and go on if it does exist.

Obviously, I'd have to have a method to check if a command exists; and possibly a method to obtain the command name (e.g. mytmplen) from the command token (e.g. \mytmplen). So far, I found these posts relevant:

I will be posting my solution as answer below; of course, corrections or alternative methods for solving this would be much appreciated!

Best Answer

Your answer appears to work. A simpler method might consist of first \let-ting the length variable in question to \relax and then applying \newlength to it. Put differently, if you can't remember if a certain macro name has been used before to denote a length parameter (or anything else, really!) and if you're comfortable with (re)using it anyway, you can first \let it to \relax and then issue a \newlength directive.

\documentclass{article}
\begin{document}

\newlength\mytmplen
\setlength\mytmplen{10pt}
\the\mytmplen

\let\mytmplen\relax % let \mytmplen to \relax

\newlength\mytmplen % now it's safe to "re-issue" it as a length 
\setlength\mytmplen{20pt}
\the\mytmplen

\end{document}

enter image description here

Related Solutions

[Tex/LaTex] How to see which package define a particular command/macro

You can add

\show\P

to your document as in

 \show\P
\documentclass{article}
 \show\P
\usepackage{graphics}
 \show\P
\usepackage{foo}
 \show\P
\usepackage{bar}
 \show\P
\begin{document}
 \show\P

This will make the TeX stop and show the definition of \P at each step, if it is already defined in the format the first one will show the definition, otherwise it will show undefined until the package that defines it is loaded. (The definition might be delayed with \AtBeginDocument which is why I put the final \show at that point.

[Tex/LaTex] What’s wrong with the newcommand

The first problem you have is that you cannot create a macro starting with \end... when using \newcommand. This check is defined inside the LaTeX kernel as part of an \@ifdefinable condition. To circumvent this you need to use TeX directives. That is,

\def\endexer#1{<stuff>}

rather than

\newcommand{\endexer}[1]{<stuff>}

Secondly, the \newcommand syntax has the following structure (see source2e.pdf):

\newcommand*{\foo}[<i>][<j>]{<text>}

* is optional
\foo is the command to be created
<i> is the number of arguments to gobble (up to 9); this references mandatory or a possible optional argument
<j> is the default value of the first optional argument (if it exists does not exist); can only have one
<text> is the macro definition when using \foo

For \newcommand{\foo}[1]{stuff #1} you would use it as \foo{<stuff>}, not \foo[<stuff>]. This latter usage would have required a definition of \foo of the form

\newcommand{\foo}[1][]{<stuff>}

Note the second set of [], denoting that \foo takes an optional argument (that is empty by default/if not specified).

Specific to your case, use \endexer{123}, not \endexer[123].

Here is a minimal example:

enter image description here

\documentclass{article}
\usepackage{hyperref}
\newcommand{\qqq}{\section{Question}\begin{enumerate}}
\newcommand{\exer}{\end{enumerate}\section{Exercises}\begin{enumerate}}
\def\endexer#1{\end{enumerate}\href{my.url.com/path/{#1}}{Take the quiz}}
\begin{document}

\qqq
\item What is $2+3$?
\exer
\item Write a program to compute $2+3$.
\endexer{123}

\end{document}

Best Answer

Related Solutions

[Tex/LaTex] How to see which package define a particular command/macro

[Tex/LaTex] What’s wrong with the newcommand

Related Question