Collecting into a command values of a counter

Question

I am trying to collect into a command called \drawtasks a series of \drawtask commands that call TikZ. Every time I call \wptask, the command should increment a counter and append to \drawtasks a call to \drawtask with the current value counter. Unfortunately, when I finally call \drawtasks, I always get the last counter value. I've tried out solutions proposed in similar questions on here (e.g., here, but it just doesn't work: it seems that my problem is different in that I want to append a call to another command which should not be expanded. Here is an example:

\documentclass{article}
\usepackage{tikz}

\newcommand{\drawtask}[4]{
    \node[anchor=west] at (-2,-0.5 * #1 - 0.25) {Task #2} ;
    \filldraw[fill=green!20,draw=green!50!black] (#3 / 3,-0.5 * #1 - 0.15) rectangle (#4 / 3,-0.5 * #1 - 0.35) ;

}
\newcounter{wptaskcounter}
\setcounter{wptaskcounter}{0}
\newcommand{\drawtasks}{}
\makeatletter
\newcommand{\appendwptask}[4]{%
    \protected@edef\@tempa{#1}%
    \expandafter\g@addto@macro\expandafter\drawtasks\expandafter{%
        \drawtask{\@tempa}{#2}{#3}{#4}%
    }%
}
\makeatother
\newcommand{\wptask}[3]{%
    \appendwptask{\thewptaskcounter}{#1}{#2}{#3}%
    \stepcounter{wptaskcounter}%
}

\wptask{1.1}{0}{3}
\wptask{1.2}{3}{6}
\wptask{1.3}{6}{9}

\begin{document}

\begin{tikzpicture}
    \drawtasks
\end{tikzpicture}

\end{document}

I'm really confused with the expansion rules of LaTeX and would really appreciate some help! Thanks in advance.

Answer 1

Based on egreg's comments on the answer you linked in the question, you might do it this way.

I always read \expandafter<token 1><token 2> as meaning something like come back and expand <token 1> after expanding <token 2> i.e. expand this next thing after the thing following it.

However, it seems instead to mean expand <token 2> next and then come back to <token 1> i.e. expand the thing after the next thing next.

In that case, what you want within the \drawtasks is for TeX to consider \drawtask only after looking at \@tempa. So you might try

\expandafter\drawtask\expandafter{\@tempa ...

which says to look at \drawtask only after expanding (looking at \{ only after expanding \@tempa).

... er, I think. However, I'm almost certainly wrong.

Nonetheless,

\newcommand{\appendwptask}[4]{%
    \protected@edef\@tempa{#1}%
    \expandafter\g@addto@macro\expandafter\drawtasks\expandafter{%
        \expandafter\drawtask\expandafter{\@tempa}{#2}{#3}{#4}%
    }%
}

seems to work, I think.

\documentclass{article}
\usepackage{tikz}

\newcommand{\drawtask}[4]{
    \node[anchor=west] at (-2,-0.5 * #1 - 0.25) {Task #2} ;
    \filldraw[fill=green!20,draw=green!50!black] (#3 / 3,-0.5 * #1 - 0.15) rectangle (#4 / 3,-0.5 * #1 - 0.35) ;

}
\newcounter{wptaskcounter}
\setcounter{wptaskcounter}{0}
\newcommand{\drawtasks}{}
\makeatletter
\newcommand{\appendwptask}[4]{%
    \protected@edef\@tempa{#1}%
    \expandafter\g@addto@macro\expandafter\drawtasks\expandafter{%
        \expandafter\drawtask\expandafter{\@tempa}{#2}{#3}{#4}%
    }%
}
\makeatother
\newcommand{\wptask}[3]{%
    \appendwptask{\thewptaskcounter}{#1}{#2}{#3}%
    \stepcounter{wptaskcounter}%
}

\wptask{1.1}{0}{3}
\wptask{1.2}{3}{6}
\wptask{1.3}{6}{9}

\begin{document}
\begin{tikzpicture}
    \drawtasks
\end{tikzpicture}
\end{document}

Answer 2

Rather than doing it this way I would make \wptask generate a comma separated list and then have \drawtasks loop over the list using tikz/pgf's \foreach command.

Doing it this way, your MWE produces:

Here's the code:

\documentclass{article}
\usepackage{tikz}

\let\wptasklist\relax% will become a comma separate list of the task parameters
\newcommand{\wptask}[3]{%  \addtask might be a better name:)
   \ifx\wptasklist\relax\def\wptasklist{#1/#2/#3}% first task
   \else\xdef\wptasklist{\wptasklist,#1/#2/#3}% add a task
   \fi%
}

\newcommand\drawtasks{% loop over \wptasklist and draw the tasks
  \begin{tikzpicture}
     \foreach \a/\b/\c [count=\n] in \wptasklist {% loop over task list
        \node[anchor=west] at (-2,-0.5 * \n - 0.25) {Task \a};
        \filldraw[fill=green!20,draw=green!50!black] 
                 (\b/3,-0.5*\n-0.15) rectangle (\c/3,-0.5*\n-0.35);
     }
  \end{tikzpicture}
}

\wptask{1.1}{0}{3}
\wptask{1.2}{3}{6}
\wptask{1.3}{6}{9}

\begin{document}

    \drawtasks

\end{document}

Hopefully the code is fairly self-explanatory. Notice that there are no expansion issues, except for the use of \xdef to add to the task list, and there is no need for a counter because you can use [count=\n] as part of the \foreach loop to determine the "task number".

There is one caveat, which doesn't seem to apply in your use-case: any of the parameters to \wptask that contain a slash / will need to be put inside braces {.../...}.

EDIT

Here is a second solution that is closer to the OP's MWE. It uses \eappto from the etoolbox package, which is a robust and general implementation of the \appendwptask macro in the question, together with \noexpand. For me at least, this gives a is more intuitive way of coping with these expansion issues. The output is the same as above.

\documentclass{article}
\usepackage{etoolbox}
\usepackage{tikz}

\newcommand{\drawtask}[4]{
    \node[anchor=west] at (-2,-0.5 * #1 - 0.25) {Task #2} ;
    \filldraw[fill=green!20,draw=green!50!black] (#3 / 3,-0.5 * #1 - 0.15) rectangle (#4 / 3,-0.5 * #1 - 0.35) ;

}
\newcounter{wptaskcounter}% set to 0 by default
\newcommand{\drawtasks}{}
\newcommand{\wptask}[3]{%
    \eappto\drawtasks{\noexpand\drawtask{\thewptaskcounter}{#1}{#2}{#3}}% append to \drawtasks
    \stepcounter{wptaskcounter}%
}

\wptask{1.1}{0}{3}
\wptask{1.2}{3}{6}
\wptask{1.3}{6}{9}

\begin{document}
  \begin{tikzpicture}
    \drawtasks
  \end{tikzpicture}
\end{document}

If you want to "accumulate many things" then using a key-val system like pgfkeys might be a better option...well, except for the initial learning curve:)

Answer 3

You can use \unexpanded:

\documentclass{article}
\usepackage{tikz}

\providecommand{\expandonce}[1]{%
  \unexpanded\expandafter{#1}%
} % etoolbox already provides this

\newcommand{\drawtask}[4]{
  \node[anchor=west] at (-2,-0.5 * #1 - 0.25) {Task #2} ;
  \filldraw[fill=green!20,draw=green!50!black]
    (#3 / 3,-0.5 * #1 - 0.15) rectangle (#4 / 3,-0.5 * #1 - 0.35) ;
}
\newcounter{wptaskcounter}
\setcounter{wptaskcounter}{0}
\newcommand{\drawtasks}{}

\newcommand{\appendwptask}[4]{%
  \edef\drawtasks{%
    \expandonce{\drawtasks}%
    \expandonce{\drawtask{#1}{#2}{#3}{#4}}%
  }%
}
\newcommand{\wptask}[3]{%
  \expandafter\appendwptask\expandafter{\the\value{wptaskcounter}}{#1}{#2}{#3}%
  \stepcounter{wptaskcounter}%
}

\wptask{1.1}{0}{3}
\wptask{1.2}{3}{6}
\wptask{1.3}{6}{9}

\begin{document}

\begin{tikzpicture}
\drawtasks
\end{tikzpicture}

\end{document}

In this way, \expandonce{\drawtasks} will only expand once \drawtasks, so to get its previous value; similarly, \expandonce{\drawtask{...}{...}{...}{...}} will perform the first level expansion of \drawtask, leaving the replacement text (with parameters replaced by the arguments). You also need to expand the value of the counter.

A much cleaner version can be obtained by using expl3 (via xparse for the interface):

\documentclass{article}
\usepackage{tikz}
\usepackage{xparse}

\newcommand{\drawtask}[4]{
  \node[anchor=west] at (-2,-0.5 * #1 - 0.25) {Task #2} ;
  \filldraw[fill=green!20,draw=green!50!black]
    (#3 / 3,-0.5 * #1 - 0.15) rectangle (#4 / 3,-0.5 * #1 - 0.35) ;
}

\ExplSyntaxOn
\tl_new:N \g_boris_drawtasks_tl
\int_new:N \g_boris_wptask_int

\NewDocumentCommand{\drawtasks}{}
 {
  \tl_use:N \g_boris_drawtasks_tl
 }

\NewDocumentCommand{\wptask}{mmm}
 {
  \boris_append_wptask:fnnn
    { \int_to_arabic:n { \g_boris_wptask_int } }
    { #1 }
    { #2 }
    { #3 }
  \int_gincr:N \g_boris_wptask_int
 }

\cs_new_protected:Nn \boris_append_wptask:nnnn
 {
  \tl_gput_right:No \g_boris_drawtasks_tl
   {
    \drawtask{#1}{#2}{#3}{#4}
   }
 }
\cs_generate_variant:Nn \boris_append_wptask:nnnn { f }
\ExplSyntaxOff

\wptask{1.1}{0}{3}
\wptask{1.2}{3}{6}
\wptask{1.3}{6}{9}

\begin{document}

\begin{tikzpicture}
\drawtasks
\end{tikzpicture}

\end{document}

We take care of getting the expanded version of the counter's value by defining a variant f (full expansion).