Using Macro Defined Lists in TikZ/PGFplots

Question

I want to be able to use a macro to specify a list (it is a constant list, i.e. will not change throughout the document) and use this macro rather than hard code the list contents in the TikZ/PGFplots for readability. This problem was mentioned in this earlier question about foreach loops, but was not the main thrust of that question.

Here is a related question on how to maintain consistency with TikZ and pgfplots, and its continuation on the use of a macro as coordinate in pgfplots, but these related to specifying coordinates, and not lists.

In the MWE below, Example 1's solution is based on this question on using a macro defined list in a foreach which suggested to not enclose the macro defined list in a curly brace. This works fine for TikZ, so no problem yet.

However, in PGFplots things are different. The solution in Example 2 to using a foreach is from the above referenced question, and works great as long as I do not use a macro for the list, but I would prefer to use a macro here.

In Example 3, I can easily use a macro defined list for the xtick, but am not able use it for xticklabels.

So, is there a consistent way that I can always define a macro for a list and then use it without having to worry about the context of its use? If not, is there a way to change the definition of the list and/or its use for Examples 2 and 3 that work in those situations.

\documentclass{article}
\usepackage{pgfplots}

\newcommand*{\XTickLocationList}{-2, -1, 1, 2}
\newcommand*{\XTickLabelsList}{a, b, c, d}
\newcommand*{\XTickLocationsAndLabels}{-2/a, -1/b, 1/c, 2/d}

\begin{document}
\begin{tikzpicture}   % Example 1
\draw [->][gray, thin](-3,0) -- (3,0) node[blue, right] {$x$}; 
\draw [->][gray, thin](0,-3) -- (0,3) node[blue, above] {$y$};

%\foreach \x/\l in {-2/a, -1/b, 1/c, 2/d} { % Works
%\foreach \x/\l in {\XTickLocationsAndLabels} {  % Does not compile
\foreach \x/\l in \XTickLocationsAndLabels {  % Works
    \draw [thick, red]
        (\x,-2pt) -- (\x,2pt)
        node [red, above] {\l};
}
\end{tikzpicture}

\begin{tikzpicture}   % Example 2
\begin{axis}[
    minor tick num=0, 
    axis y line=center, 
    axis x line=middle, 
    xmin=-3, xmax=3, 
    ymin=-3, ymax=3,
    xlabel=$x$, ylabel=$y$,
    xtick={0},  xticklabels={}
    ]

\foreach \x/\l in {-2/a, -1/b, 1/c, 2/d}{ % Works
%\foreach \x/\l {\XTickLocationsAndLabels}  % Does not compile
%\foreach \x/\l \XTickLocationsAndLabels  % Does not compile
    \edef\temp{\noexpand\addplot
        [mark=none,color=red, thin, samples=2]%
        coordinates{ (\x,-0.05) (\x,0.05) }
            node [red, below] {\l};
    }
    \temp
} 

\end{axis}
\end{tikzpicture}

\begin{tikzpicture}    % Example 3
\begin{axis}[
    minor tick num=0, 
    axis y line=center, 
    axis x line=middle, 
    xmin=-3, xmax=3, 
    ymin=-3, ymax=3,
    xlabel=$x$, ylabel=$y$,
    %xtick={-2, -1, 1, 2}, % Works
    %xtick=\XTickLocationList, % Works
    xtick={\XTickLocationList}, % Works
    xticklabels={a, b, c, d}  % Works
    %xticklabels={\XTickLabelsList}  % Compiles, but wrong results
    %xticklabels=\XTickLabelsList  % Compiles, but wrong results
    ]

\end{axis}
\end{tikzpicture}
\end{document}

Answer 1

Given the restrictions discussed in my comments above, I suggest using a more technical approach - which is absolutely reliably and fully portable (but less commonly known in LaTeX).

I mean, the problem is that pgfplots does not understand xticklabels={\XTickLabelsList}. More precisely: it takes the argument as (single) list entry rather than expanding it. To be honest, I am not even sure if someone might want to use such a feature (a one-element-list witch a macro as content).

So, what we need is to provide the content of \XtickLabelsList programmatically.

Well, here is the promised solution:

\edef\temp{%
  \noexpand\pgfplotsset{%
      x tick labels list/.style={xticklabels={\XTickLabelsList}}
  }%
}
\temp

It defines \temp to contain the "fully expanded" content of the argument in braces ("e"xpanded "def"inition). If a \noexpand occurs in the argument of \edef, the "fully expanded" content of the item following \noexpand is the item itsself, i.e. \noexpand\pgfplotsset stays \pgfplotsset. But \XTickLabelsList will be replaced by its content. Note that any macro which might occur in \XTicjLabelsList might also be expanded (there are other techniques to avoid that. In case you need them, pose the question again or research in my TeX-programming-notes.pdf which should be shipped with pgfplots).

Anyway, the \temp macro will finally contain \pgfplotsset{.....{a,b,c,d}}} (type \show\temp to verify it). Executing \temp will activate the changes and you can readily use the style afterwards with x tick labels list.