Optional argument causes Misplaced \noalign

Question

I have the following piece of code from this answer for generating a dashed table rule:

\documentclass{article}
\usepackage{booktabs}
\usepackage{arydshln}
\begin{document}
\makeatletter
\def\adl@drawiv#1#2#3{%
    \hskip.5\tabcolsep
    \xleaders#3{#2.5@tempdimb #1{1}#2.5@tempdimb}%
        #2\z@ plus1fil minus1fil\relax
        \hskip.5\tabcolsep}
\newcommand{\cdashrule}[2][]{%
    \noalign{\vskip\aboverulesep
        \global\let@dashdrawstore\adl@draw
        \global\let\adl@draw\adl@drawiv}
    \cdashline{#2}
    \noalign{\global\let\adl@draw@dashdrawstore
        \vskip\belowrulesep}}
\makeatother
\begin{table}
\begin{tabular}{lll}
A & B & C \
\cdashrule{1-2}
D & E & F
\end{tabular}
\end{table}
\end{document}

I'd like to adapt \cdashrule to take an optional argument (empty by default) that it passes to \cdashline so that I can also control the formatting of the dashed line (line/gap width). However, even just defining an optional argument (and not using it anywhere) already causes the above example to throw errors:

\newcommand{\cdashrule}[2][]{%
    \noalign{\vskip\aboverulesep
        \global\let\@dashdrawstore\adl@draw
        \global\let\adl@draw\adl@drawiv}
    \cdashline{#2}
    \noalign{\global\let\adl@draw\@dashdrawstore
        \vskip\belowrulesep}}

I've also set up an Overleaf project that demonstrates this issue. How do I properly define the \cdashrule command so that it passes any optional formatting argumenets to the underlying \cdashline command?

as \cdashline original syntax is for optional argument after mandatory one, it would probably be better to do the same here. — user691586, Mar 28 '23 at 12:31

user202729 · Answer 1 · 2023-03-28T13:13:18.497

It has something to do with expandability.

Refer to the following question for some explanation on similar issue:

Misplaced \omit. \multispan with \newcommand defined with optional argument

or one of 1 2 3 4 5 6.

One (relatively easy) workaround is to use the following code, although it has certain limitations as documented in xparse's documentation:

\documentclass{article}
\usepackage{booktabs}
\usepackage{arydshln}
\begin{document}
\makeatletter
\def\adl@drawiv#1#2#3{%
    \hskip.5\tabcolsep
    \xleaders#3{#2.5@tempdimb #1{1}#2.5@tempdimb}%
        #2\z@ plus1fil minus1fil\relax
        \hskip.5\tabcolsep}
\NewExpandableDocumentCommand{\cdashrule}{O{} m}{%
    \noalign{\vskip\aboverulesep
        \global\let@dashdrawstore\adl@draw
        \global\let\adl@draw\adl@drawiv}
    \cdashline{#2}
    \noalign{\global\let\adl@draw@dashdrawstore
        \vskip\belowrulesep}}
\makeatother
\begin{table}
\begin{tabular}{lll}
A & B & C \
\cdashrule{1-2}
D & E & F
\end{tabular}
\end{table}
\end{document}

user691586 · Accepted Answer · 2023-03-28T12:44:38.570

(the other answer is more complete and was posted a bit earlier)

You can use the enhanced declarative interface of recent LaTeX.

\documentclass{article}
\usepackage{booktabs}
\usepackage{arydshln}
\begin{document}
\makeatletter
\def\adl@drawiv#1#2#3{%
    \hskip.5\tabcolsep
    \xleaders#3{#2.5@tempdimb #1{1}#2.5@tempdimb}%
        #2\z@ plus1fil minus1fil\relax
        \hskip.5\tabcolsep}
\NewExpandableDocumentCommand\cdashrule{om}{%
    \noalign{\vskip\aboverulesep
        \global\let@dashdrawstore\adl@draw
        \global\let\adl@draw\adl@drawiv}
    \IfNoValueTF{#1}{\cdashline{#2}}{\cdashline{#2}[#1]}%
    \noalign{\global\let\adl@draw@dashdrawstore
        \vskip\belowrulesep}}
\makeatother
\begin{table}
\begin{tabular}{lll}
A & B & C \
\cdashrule{1-2}
D & E & F \
\cdashrule[.4pt/1pt]{1-2}
G & H & I
\end{tabular}
\end{table}
\end{document}

But I feel it would be better to implement the same syntax as original \cdashline and the following code does it:

\documentclass{article}
\usepackage{booktabs}
\usepackage{arydshln}
\begin{document}
\makeatletter
\def\adl@drawiv#1#2#3{%
    \hskip.5\tabcolsep
    \xleaders#3{#2.5@tempdimb #1{1}#2.5@tempdimb}%
        #2\z@ plus1fil minus1fil\relax
        \hskip.5\tabcolsep}
\newcommand\cdashrule[1]{%
    \noalign{\vskip\aboverulesep
        \global\let@dashdrawstore\adl@draw
        \global\let\adl@draw\adl@drawiv\ifnum0=}\fi \@ifnextchar[%] {\cdashrule@i[#1]}% {\cdashrule@i[#1][\dashlinedash/\dashlinegap]}% }% \def\cdashrule@i[#1][#2]{% \ifnum0={\fi}%
    \cdashline{#1}[#2]%
    \noalign{\global\let\adl@draw@dashdrawstore
        \vskip\belowrulesep}}
\makeatother
\begin{table}
\begin{tabular}{lll}
A & B & C \
\cdashrule{1-2}
D & E & F \
\cdashrule{1-2}[.4pt/1pt]
G & H & I
\end{tabular}
\end{table}
\end{document}

Same output as before.

Optional argument causes Misplaced \noalign

2 Answers2

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