Misplaced \noalign when trying to define a command for cmidrules

Question

When trying to maintain large tables with \cmidrules, I find them tedious to use, so I thought I'd define my own shortcut. Instead of writing \cmidrule{1-3} \cmidrule{4-5} I would simply write \cmidrulez{3,2} (because the first rule is 3 columns wide and the second one 2.

In order to support an arbitrary amount of comma-seperated arguments, I tried to adapt this answer:

\newcounter{mainargs}
\pgfkeys{mainargs/.is family, mainargs,
step counter/.code=\stepcounter{mainargs},
add argument/.style={step counter, arg\themainargs/.initial={#1}},
}
\newcounter{optargs}
\newif\ifoptargs
\pgfkeys{optargs/.is family, optargs,
    opt args present/.is if=optargs,
    step counter/.code=\stepcounter{optargs},
    add argument/.style={opt args present=true, step counter, arg\theoptargs/.initial={#1}},
}
\newcommand{\cmidrulez}[2][]{%
    \setcounter{mainargs}{0}%
    \pgfkeys{mainargs, add argument/.list={#2}}%
    \setcounter{optargs}{0}%
    \pgfkeys{optargs, add argument/.list={#1}}%
    %
    \newcounter{cmrstart}%
    \newcounter{cmrend}%
    \setcounter{cmrstart}{1}%
    \ifoptargs%
        \foreach \n in {1,...,\theoptargs}{%
            \setcounter{cmrstart}{\pgfkeysvalueof{/optargs/arg\n}}%
        }%
    \fi%
    \foreach \n in {1,...,\themainargs}{%
    \setcounter{cmrend}{\value{cmrstart}}%
   \addtocounter{cmrend}{\pgfkeysvalueof{/mainargs/arg\n}}%
    \addtocounter{cmrend}{-1}%
    \cmidrule{\arabic{cmrstart}-\arabic{cmrend}}%
    \setcounter{cmrstart}{\value{cmrend}}%
    \stepcounter{cmrstart}%
    }%
    %
}

Essentially what I'm doing is using two counters to calculate the corresponding numbers I need to give to the \cmidrule command. When I print the numbers with \arabic{cmrstart} and \arabic{cmrend} instead of calling \cmidrule the correct values appear, so this part is working.

However, when I try to use this in a table (where a \cmidrule would work) it complains about a "misplaced \noalign":

\cmidrule ->\noalign 
                     {\ifnum 0=`}\fi \@ifnextchar [{\@cmidrule }{\@cmidrule ...
l.66         \cmidrulez{2,1}
                             \\
I expect to see \noalign only after the \cr of
an alignment. Proceed, and I'll ignore this case.
! Missing } inserted.
<inserted text> 
                }
l.66         \cmidrulez{2,1}
                             \

I first thought this might have something to do with how I hand over the values, so I tried \value{cmrstart} and thecmrstart, the other two ways of which I know to get a value from a counter, but that didn't work. In fact, if I just replace the arguments of \cmidrule with hardcoded integers (inside my command definition), the problem persists, so it can't (only) be that.

When I define a much simpler command that just does something like \cmidrule{1-3} it works without problems, but as soon as there are arguments it fails:

% this works:
\newcommand{\cmidrules}{%
    \cmidrule{1-3}%
}
% this doesn't:
\newcommand{\cmidrules}[1][]{%
    \cmidrule{1-3}%
}

What am I doing wrong?

Answer 1

You cannot do assignments before issuing \cmidrule, unless they're inside \noalign.

Here's my version, where I also add * to leave a column empty. I also added (lr) for \cmidrule, otherwise \cmidrule{1-3}\cmidrule{4-5} would be exactly the same as \cmidrule{1-5}.

\documentclass{article}
\usepackage{booktabs}
\usepackage{xparse}
\ExplSyntaxOn
\NewExpandableDocumentCommand{\cmidrulez}{m}
 {
  \noalign { __leviathan_cmidrulez:n { #1 } }
  \tl_use:N \g__leviathan_cmidrulez_tl
 }
\tl_new:N \g__leviathan_cmidrulez_tl
\int_new:N \l__leviathan_cmidrulez_int
\cs_new_protected:Nn __leviathan_cmidrulez:n
 {
  \tl_gclear:N \g__leviathan_cmidrulez_tl
  \int_zero:N \l__leviathan_cmidrulez_int
  \clist_map_inline:nn { #1 }
   {
    \str_if_eq:nnTF { ##1 } { * }
     {% no rule here
      \int_incr:N \l__leviathan_cmidrulez_int
     }
     {
      \tl_gput_right:Nx \g__leviathan_cmidrulez_tl
       {
        \exp_not:N \cmidrule(lr)
         {% start
          \int_eval:n { \l__leviathan_cmidrulez_int + 1 }
          -
          \int_eval:n { \l__leviathan_cmidrulez_int + ##1 }
         }
       }
      \int_add:Nn \l__leviathan_cmidrulez_int { ##1 }
     }
   }
 }
\ExplSyntaxOff
\begin{document}
\begin{tabular}{{9}{c}}
\toprule
1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 & 9 \
\cmidrulez{3,2}
1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 & 9 \
\cmidrulez{,3,*,2}
1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 & 9 \
\bottomrule
\end{tabular}
\end{document}

The comma separated list is scanned and if the item is * the internal counter is just stepped. Otherwise the proper argument for \cmidrule is prepared by using the current value of the counter (plus 1) and the required number of steps. Then the counter is updated. The command is appended to a token list variable that's issued after \noalign has ended.

I'm not sure how much this can be useful, though. How many \cmidrule commands do you have in your document?

Answer 2

A solution with an 'expandable' programmation.

\documentclass{article}
\usepackage{array}
\usepackage{booktabs}
\usepackage{xparse}
\ExplSyntaxOn
\NewExpandableDocumentCommand { \clinez } { m } { __leviathan:nn { 1 } { #1 , } }
\cs_new:Nn __leviathan:nn 
  { \tl_if_empty:nF { #2 } { __leviathan:nnn #2 \q_stop { #1 } } }
\cs_new:Npn __leviathan:nnn #1 , #2 \q_stop #3
  {
    \str_if_eq:nnTF { #1 } { * }
      { __leviathan:nn { \int_eval:n { #3 + 1 } } { #2 } }
      {
        __leviathan:nn { \int_eval:n { #1 + #3 } } { #2 }
        \cmidrule (lr) { #3 - \int_eval:n { #3 + #1 - 1 } } 
      }
  }
\ExplSyntaxOff
\begin{document}
\begin{tabular}{{10}{c}}
a & b & c & d & e & f & g & h & i & j \
\clinez{3,2,2}
a & b & c & d & e & f & g & h & i & j \
\clinez{3,,2,2}
a & b & c & d & e & f & g & h & i & j \
\end{tabular}
\end{document}

Answer 3

Macros in that position need to be expandable to avoid starting the next table cell.

It may be easier to see the expansion working directly rather than using package code in this case.

\documentclass{article}
\usepackage{array,booktabs}
\def\cmidrulez#1{\cmz{}1#1,{-1},}
\def\gobblethree#1#2#3{}
\def\cmz#1#2#3,{%
\ifnum#3=-1 
#1\expandafter\gobblethree\fi
\cmz{#1\cmidrule(lr){#2-\the\numexpr#2+#3-1\relax}}{\the\numexpr#2+#3\relax}}
\begin{document}
\begin{tabular}{ccccc}
 1&2&3&4&5\
\cmidrulez{3,2}
 1&2&3&4&5\
 1&2&3&4&5\
\end{tabular}
\end{document}