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I tried to create my custom class for lab reports, which needs to have a tabular on the titlepage which contains the dates on that I gave the report to my assistant for correction. In order to make things for the user as easy as possible I wanted to appear the dates and text automatically when they are defined by a command like \firsthandin{2017-01-06}. As I need to define \firsthandin, my thought was that it is useless to check whether this macro is defined, it is of course; but rather whether the argument is empty or differs from the default value. So I tried using \ifmtarg from ifmtarg.sty. Here is what I have so far:

protokoll.cls

\ProvidesClass{protokoll}[2017/01/05 v1.0]
\DeclareOption*{\PassOptionsToClass{\CurrentOption}{scrartcl}}
\ProcessOptions
\LoadClass[a4paper,fontsize=11pt,oneside]{scrartcl}
%=====================================================
\RequirePackage[utf8]{inputenc}
\RequirePackage{ifmtarg}
%=====================================================
\newcommand{\@firsthandin}{}
\newcommand{\firsthandin}[1]{\renewcommand{\@firsthandin}{#1}}

\newcommand{\@secondhandin}{Unknown}
\newcommand{\secondhandin}[1]{\renewcommand{\@secondhandin}{#1}}

\renewcommand{\maketitle}{
\begin{titlepage}
\begin{table}
\centering
\begin{tabular}{ll}
\ifmtarg{\@firsthandin}{}{First date} & \ifmtarg{\@firsthandin{}{\@firsthandin} \\
Second date & \@secondhandin \\
\end{tabular}
\end{table}
\end{titlepage}
}

The second line is just to check whether there is another mistake regarding the variables. I also tried changing the first line of the tabular to \ifmtarg{\@firsthandin}{}{First date & \@firsthandin \\} but this produces the same errors.

mwe_protokoll.tex

\documentclass{protokoll}
\firsthandin{05.01.2017}
\begin{document}
\maketitle
\end{document}

I get errors about Misplaced \cr which is according to @DavidCarlisle (What does Misplaced \cr in latex error mean) used in the definition of \\, so I guess \ifmtarg messes up with the linebreaks of the tabular. Any ideas how to solve this? (It is not important whether an empty space or "really nothing" is given out)

Community
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Enno
  • 606
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    The command is \@ifmtarg, but there is another issue with \noalign, I suppose –  Jan 06 '17 at 14:05
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    why not use etoolbox its macro examination macros are much more power full. Plus it is meant to he used in user space (aka no @ in macro names) – daleif Jan 06 '17 at 14:08
  • @daleif: The \@ifmtarg is not in user space here, it's class code (but I agree with you about the possibly easier etoolbox approach) –  Jan 06 '17 at 14:18
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    @ChristianHupfer: Yes the missing @ was actually the problem...quite easy and a quite stupid mistake from me. But thanks for your eagle eyes. I will accept your answer as you solved my problem but I think I will use the etoolbox method because it is even easier. – Enno Jan 06 '17 at 14:23
  • @ChristianHupfer you're right, but it seems I do not need the \edef part in the version I posted. – daleif Jan 06 '17 at 14:39
  • @daleif: True. I did not check the definition of \ifdefvoid but it seems to expand internally. (And I did not mean to steal the tick!) –  Jan 06 '17 at 15:29
  • @ChristianHupfer I don't collect points – daleif Jan 06 '17 at 15:30

3 Answers3

5

Here is an example using etoolbox, I've also renamed the holder macros so they can be used in user space for this example.

Note that etoolbox contains a \ifblank macro, but that cannot be used to test the emptiness of a given macro.

\documentclass[a4paper]{article}

\usepackage{etoolbox}
\newcommand{\firsthandinXX}{}
\newcommand{\firsthandin}[1]{\renewcommand{\firsthandinXX}{#1}}

\newcommand{\secondhandinXX}{Unknown}
\newcommand{\secondhandin}[1]{\renewcommand{\secondhandinXX}{#1}}

\renewcommand{\maketitle}{
\begin{titlepage}
\begin{table}
\centering
\begin{tabular}{ll}
\ifdefvoid{\firsthandinXX}{}{First date} & \ifdefvoid{\firsthandinXX}{}{\firsthandinXX} \\
Second date & \secondhandinXX \\
\end{tabular}
\end{table}
\end{titlepage}
}


\firsthandin{05.01.2017}

\begin{document}

\maketitle

\end{document}
daleif
  • 54,450
5

Conditionals in tabular cells are tricky. They should be expanded first using something like \edef\x{\if...}\x

I did not change the driver file protokoll.tex, so please load from the O.P. 

\ProvidesClass{protokoll}[2017/01/05 v1.0]
\DeclareOption*{\PassOptionsToClass{\CurrentOption}{scrartcl}}
\ProcessOptions
\LoadClass[a4paper,fontsize=11pt,oneside]{scrartcl}
%=====================================================
\RequirePackage[utf8]{inputenc}
\RequirePackage{ifmtarg}
%=====================================================
\newcommand{\@firsthandin}{}
\newcommand{\firsthandin}[1]{\renewcommand{\@firsthandin}{#1}}

\newcommand{\@secondhandin}{Unknown}
\newcommand{\secondhandin}[1]{\renewcommand{\@secondhandin}{#1}}

\renewcommand{\maketitle}{
\begin{titlepage}
\begin{table}
\centering
\begin{tabular}{ll}
  \edef\x{\@ifmtarg{\@firsthandin}{}{First Date}}\x & \edef\x{\@ifmtarg{\@firsthandin}{}{\@firsthandin}}\x \\
  Second date & \@secondhandin \\
\end{tabular}
\end{table}
\end{titlepage}
}

enter image description here

2

You have a few errors in your code: check the differences.

protokoll.cls

\ProvidesClass{protokoll}[2017/01/05 v1.0]
\DeclareOption*{\PassOptionsToClass{\CurrentOption}{scrartcl}}
\ProcessOptions
\LoadClass[a4paper,fontsize=11pt,oneside]{scrartcl}
%=====================================================
\RequirePackage[utf8]{inputenc}
\RequirePackage{ifmtarg}
%=====================================================
\newcommand*{\@firsthandin}{}
\newcommand{\firsthandin}[1]{\renewcommand{\@firsthandin}{#1}}

\newcommand{\@secondhandin}{Unknown}
\newcommand{\secondhandin}[1]{\renewcommand{\@secondhandin}{#1}}

\renewcommand{\maketitle}{%
\begin{titlepage}
  \centering
  \begin{tabular}{ll}
  \expandafter\@ifnotmtarg\expandafter{\@firsthandin}{First date & \@firsthandin \\}%
  Second date & \@secondhandin \\
  \end{tabular}
  \end{titlepage}%
}

No need for a table environment, first of all; then the correct command is \@ifmtarg, but \@ifnotmtarg is easier here; you were missing a brace, too.

You need to expand \@firsthandin in order to see whether it is empty or not.

mwe.tex

\documentclass[a4paper,fontsize=11pt,oneside]{protokoll}

\firsthandin{05.01.2017}

\begin{document}

\maketitle

\end{document}

Output

enter image description here

egreg
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