cross-referencingmacros
I have tried to follow another question but I cannot get it to work. My question is: How do print a text based on if a reference exists or not?
I have tried the following:
\usepackage{etoolbox}
\usepackage{xparse}
...
\NewDocumentCommand\calledName{m}{%
\ifcsundef{r@KN:#1}{\ref{FN:#1}}{\ref{KN:#1}}%
}
\makeatletter and \makeatother are needed in order to allow using command names with @ in them. There is nothing really special about it.
The first idea is to use \NewDocumentCommand{\DTLfetchifnull}{mmmmmmm} if it is available, otherwise the standard \newcommand{\DTLfetchifnull}[7] is used (for compatibility with older documents).
With newer releases we essentially get
\NewDocumentCommand\DTLfetchifnull{mmmmmmm}
{%
\begingroup%
\protected@edef\@dtl@dogetrowforvalue{%
\noexpand%
\endgroup
\noexpand\dtlgetrowindex{\noexpand\dtl@rowidx}{#1}{\dtlcolumnindex{#1}{#2}}{#3}%
}\@dtl@dogetrowforvalue%
\ifx \dtl@rowidx\dtlnovalue%
\expandafter\@secondoftwo%
\else%
\expandafter\@firstoftwo%
\fi%
{%
\edtlgetrowforvalue{#1}{\dtlcolumnindex{#1}{#2}}{#3}%
\dtlgetentryfromcurrentrow{\dtlcurrentvalue}{\dtlcolumnindex{#1}{#4}}%
\DTLifnull{\dtlcurrentvalue}{#6}{#5}%
}{%
#7%
}%
}%
that shows not really good programming style. For instance, there are several useless %, but they're not harmful, just annoying. Also the first \noexpand is useless, because `\endgroup is not expandable. I'd write it as
\NewDocumentCommand\DTLfetchifnull{mmmmmmm}
{%
\begingroup
\protected@edef\@dtl@dogetrowforvalue{%
\endgroup
\noexpand\dtlgetrowindex{\noexpand\dtl@rowidx}{#1}{\dtlcolumnindex{#1}{#2}}{#3}%
}\@dtl@dogetrowforvalue
\ifx \dtl@rowidx\dtlnovalue
\expandafter\@secondoftwo
\else
\expandafter\@firstoftwo
\fi
{%
\edtlgetrowforvalue{#1}{\dtlcolumnindex{#1}{#2}}{#3}%
\dtlgetentryfromcurrentrow{\dtlcurrentvalue}{\dtlcolumnindex{#1}{#4}}%
\DTLifnull{\dtlcurrentvalue}{#6}{#5}%
}{%
#7%
}%
}
If an older LaTeX release, the first line would be \newcommand{\DTLfetchifnull}[7].
The idea is to define \@dtl@dogetrowforvalue to yield
\endgroup
\dtlgetrowindex{\dtl@rowidx}
{expansion of #1}
{expansion of \dtlcolumnindex{#1}{#2}}
{expansion of #3}
and then the scratch macro \@dtl@dogetrowforvalue will be executed, which will assign a meaning to \dtl@rowidx, which can be \dtlnovalue (depending on the arguments #1, #2 and #3. The initial \endgroup will balance the already digested \begingroup and the definition of \@dtl@dogetrowforvalue will be forgotten as soon as \endgroup is executed (but the expansion of \@dtl@dogetrowforvalue has already been placed on the input stream, so there is no problem with undefining it).
Next the token is compared with \dtlnovalue. The construction
\if<something>
\expandafter\@secondoftwo
\else
\expandafter\@firstoftwo
\fi
{tokens A}
{tokens B}
is very common in LaTeX programming. If the test returns true, tokens B will be used, otherwise tokens A.
So, if \dtl@rowidx turns out to be \dtlnovalue, the argument #7 will be used. Otherwise it will be some legal row index and
\edtlgetrowforvalue{#1}{\dtlcolumnindex{#1}{#2}}{#3}%
\dtlgetentryfromcurrentrow{\dtlcurrentvalue}{\dtlcolumnindex{#1}{#4}}%
\DTLifnull{\dtlcurrentvalue}{#6}{#5}%
will be used. Finding out what this does needs studying the manual of datatool.
amsmath allows the end user to insert their own tags for numbered equations and therefore collects the body of math environments before processing them. This allows for proper parsing of the content, possibly delaying the execution of \label only after the equation counter has been stepped. It does, however, retain access to the original LaTeX \label via \ltx@label, which is appropriate for your use:
\documentclass[fleqn]{article}
\usepackage{amsmath}
\makeatletter
\newcommand{\ltxlabel}{\ltx@label}% User-level access to original LaTeX \label
\makeatother
\begin{document}
\newcounter{mycounter}
\refstepcounter{mycounter}\label{foo}\ref{foo}% 1
\refstepcounter{mycounter}\label{bar}\ref{bar}% 2
\begin{equation} x\refstepcounter{mycounter}\ltxlabel{baz}\ref{baz} \end{equation}
\ref{baz}% 3
\refstepcounter{mycounter}\label{qux}\ref{qux}% 4
\end{document}
\ltxlabel (or \ltx@label) will be executed immediately, using whatever counter was previously stepped (via \refstepcounter) as a label reference. In contrast, \label will only be used to reference the equation counter under amsmath.
Best Answer
Here's a way with
\@ifundefinedand usinglatex.ltxcore features.If the label
KN:#1is undefined, a reference toFN:#1is used (or at least tried to do so.Another way would use
\getrefnumberetc. byrefcountpackage.