Hiding output of a macro

Question

Is there a way to get a macro to hide its output but still do the logic with counters etc.?

I'm stuck on the definition of macro \dontoutput below.

% \documentclass, \usepackage, ...

\newcount\innercount

\def\dontoutput#1{#1} % ???

\long\def\inner#1{\advance\innercount 1\relax}

\long\def\outer#1{
  \innercount 0\relax
  \dontoutput{#1}    % call it, increment counters in \inner, but don't output anything!
  Inner called \the\innercount times.
  \def\inner##1{##1} % we want to output it too now.
  #1                 % call again
}

\begin{document}
  \outer{
    a
    \inner{b}
    \inner{c}
    \inner{d}
    e
  }
\end{document}

% Outputs:  a e Inner called 3 times. abcde
% I'd like:     Inner called 3 times. abcde

Answer 1

\newcounter{inner}
\def\inner#1{#1}
\def\outer#1{%
  {\sbox0{\setcounter{inner}{0}\def\inner##1{\stepcounter{inner}}#1}}%
  Inner called \theinner\ times. #1}

Command on LaTeX counters act globally; so we typeset the argument of \outer in a box where we set the counter to zero and define \inner to step the counter. The default definition of \inner, which is reinstated outside the box, is to typeset its argument.

Following Martin's suggestion I have also put the box assignment in a group, for greater safety. The command \sbox builds a box saving it into a storage bin, but typesetting nothing in the document. However all commands are performed, in particular we can redefine \inner to have a different meaning, since when we enter a box construction another group is opened. The group around the \sbox operation ensures that the storage bin will be forgotten at the group's end.

Usually one writes

\newsavebox{\mybox}

in the preamble and then \sbox{\mybox}{...} in order to \usebox{\mybox} somewhere else; however, \mybox refers just to a certain internal number and the numbers 0, 2, 4, 6, and 8 are reserved for "scratch boxes" and one can freely use them for purposes such as the present one.

Answer 2

You can store the content into a box which is not used, as egreg already showed.

You could also use \nullfont to switch off any text output. However, paragraphs are still inserted AFAIK, so \let\par\relax would also be useful. Finally you want to disable that any command switches to a real font again, so \let\selectfont\relax should also be added. Either put these in a group or save and restore the original definitions (first \let\origfoo\foo then later \let\foo\origfoo). You should use \normalfont after restoring \selectfont in the latter case.

Usually the use of a box is preferable, I guess.