Adding current countervalue in macro to other variable?

Question

I have trouble with expanding a counter inside another macro. It seems that I cant use a simple chain of expandafter here since my counter can get very high. I found Force expansion of counter value in command argument but could not figure out how to apply it in my examplecode:

\documentclass[a4paper,10pt]{report}
\setlength{\parindent}{0pt}

\newcounter{Mynumbers}


\makeatletter
\def\printmylist{}
\makeatother

\newcommand{\addtomylist}[2]{ %
        \stepcounter{Mynumbers}%
        $^{( \theMynumbers , #1  ) }~$%
        }

\begin{document}

first\addtomylist{0}{one}\\
second\addtomylist{4}{four} \\

~
\printmylist \\

~\\~\\~\\~\\
expected outcome:\\
$first^{(1,0)}$ \\
$secound^{(2,4)}$  \\

1: one \\
2: four \\


\end{document}

Answer 1

Depending on what you are really trying to do there may be better ways of doing this. The following appears to produce your requested output, so I think that this may do what you want:

\documentclass[a4paper,10pt]{report}
\parindent=0pt

\usepackage{etoolbox}

\def\printmylist{}

\newcounter{Mynumbers}
\newcommand{\addtomylist}[2]{%
        \stepcounter{Mynumbers}%
        ${}^{(\theMynumbers, #1 )}~$%
        \xappto\printmylist{\theMynumbers: \unexpanded{#2\\}}%
        }

\begin{document}

first\addtomylist{0}{one}

second\addtomylist{4}{four}

\printmylist

\end{document}

This produces:

I have used the \xappto command from the etoolbox package. This command appends an expanded version of the second argument to the macro specified by the first argument:

\xappto<macro><stuff to append>

I have put \unexpanded because you don't want that part to expand when it is added to \printmylist.

Also, notice that I removed the space from the start of

\newcommand{\addtomylist}[2]{ % <-- unwanted space????
        \stepcounter{Mynumbers}%
        $^{( \theMynumbers , #1  ) }~$%
        }

This is what caused the gap between the one and the super-scripted (1,0) in your MWE.

Answer 2

You have to expand the counter's value before adding it to the list.

\documentclass[a4paper,10pt]{report}
\setlength{\parindent}{0pt}

\newcounter{Mynumbers}

\newcommand\printmylist{}

\makeatletter
\newcommand{\addtomylist}[2]{%
  \stepcounter{Mynumbers}%
  {\textsuperscript{(\theMynumbers,#1)}}%
  \edef\@tempa{\theMynumbers}%
  \expandafter\addto@mylist@\expandafter{\@tempa}{#2}%
}
\newcommand{\addto@mylist@}[2]{%
  \g@addto@macro\printmylist{#1: #2\par}%
}
\makeatother

\begin{document}

first\addtomylist{0}{one}

second\addtomylist{4}{four}


\printmylist

\end{document}

The mandatory expl3 version:

\documentclass[a4paper,10pt]{report}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\addtomylist}{mm}
 {
  \int_gincr:N \g_mylist_index_int
  \textsuperscript{(\int_use:N \g_mylist_index_int,#1)}
  \mylist_add:fn { \int_use:N \g_mylist_index_int }{ #2 }
 }
\NewDocumentCommand{\printmylist}{}
 {
  \seq_use:Nn \g_mylist_list_seq { \par }
 }

\int_new:N \g_mylist_index_int
\seq_new:N \g_mylist_list_seq

\cs_new_protected:Nn \mylist_add:nn
 {
  \seq_gput_right:Nn \g_mylist_list_seq { #1:~#2 }
 }
\cs_generate_variant:Nn \mylist_add:nn { f }
\ExplSyntaxOff

\setlength{\parindent}{0pt}

\begin{document}

first\addtomylist{0}{one}

second\addtomylist{4}{four}

\printmylist

\end{document}

The output is the same.

Answer 3

The output you expect differs from the output you get in two aspects:

1. The phrases "first" and "second" are not in mathmode.
2. The macro \printmylist does not deliver the output 1: one \\ 2: four \\ .

Besides this you don't need \makeatletterand \makeatother with the definition of the \printmylist-macro as that definition does not contain the symbol "@". Also the definition of your \addtomylist-command will contain space tokens that might produce horizontal space.

Here comes your code with slight modifications. I hope it now does what you wish it to do:

\documentclass[a4paper,10pt]{report}
\setlength{\parindent}{0pt}

\newcounter{Mynumbers}

\newcommand*\printmylist{}

\makeatletter
\newcommand{\addtomylist}[2]{%
  \stepcounter{Mynumbers}%
  $^{(\theMynumbers,#1) }~$%
  \expandafter\g@addto@macro\expandafter\printmylist\expandafter{\number\value{Mynumbers}: #2 \\}%
}%
\makeatother

\begin{document}

$first$\addtomylist{0}{one}\\
$second$\addtomylist{4}{four}\\

\printmylist\\

\hrulefill\\
expected output:\\

$first^{(1,0)}$\\
$second^{(2,4)}$ \\

1: one \\
2: four \\


\end{document}

You might get better results using \ensuremathinstead of $..$.

You might get better results either using \par instead of \\ or wrapping things into \hboxes while via \par ensuring that TeX is in vertical mode where each outermost \hbox is treated a s a single line:

\documentclass[a4paper,10pt]{report}
\setlength{\parindent}{1cm}

\newcounter{Mynumbers}

\newcommand*\printmylist{\par}

\makeatletter
\newcommand{\addtomylist}[2]{%
  \stepcounter{Mynumbers}%
  \ensuremath{^{(\theMynumbers,#1)}}%
  \expandafter\g@addto@macro
  \expandafter\printmylist
  \expandafter{%
  \expandafter\hbox
  \expandafter{%
  \number\value{Mynumbers}: #2}}%
}%
\makeatother

\begin{document}

\verb|\parindent| is set to 1cm so you can see the difference between \TeX{}
acting in horizontal mode (where it also does the horizontal line-indenting with
the first line of a paragraph and the line breaking within a paragraph and the
\verb|\parfillskip|-glue-thingie at the end of a paragraph) and \TeX{} acting
in vertical mode where each outermost \verb|\hbox| is treated as a single
"line" whose content in turn gets processed in restricted horizontal mode:

\noindent\hrulefill

received output:

$first\addtomylist{0}{one}$

$second\addtomylist{4}{four}$

\printmylist

\noindent\hrulefill

expected output:

$first^{(1,0)}$

$second^{(2,4)}$

\hbox{1: one}
\hbox{2: four}

\end{document}