[Tex/LaTex] Forcing tikz/pgf to expand macros within commands

expansionmacrostikz-pgf

I would like to use simple macros within tikz commands, but it seems tikz doesn't expand them in the ways I would expect.

For instance, the code

\pgfpointcurveattime{.25}{\pgfpoint{0cm}{0cm}}{\pgfpoint{0cm}{1cm}}{\pgfpoint{1cm}{1cm}}{\pgfpoint{1cm}{0cm}}

nicely returns the point one quarter of the way along the bezier curve specified by those four points. However the code

\newcommand{\bcurve}{{\pgfpoint{0cm}{0cm}}{\pgfpoint{0cm}{1cm}}{\pgfpoint{1cm}{1cm}}{\pgfpoint{1cm}{0cm}}}  
\pgfpointcurveattime{.25}\bcurve

gives an error, even though it expands to the same string. Putting braces around \bcurve doesn't help.

What is the cause of this trouble, and more generally how can you use a macro in such a situation in tikz?

Best Answer

My knowledge of TeX expansion is rather limited, but I think the problem is not with PGF but with TeX itself. It tries to expand \pgfpoincurveattime first and looks for its arguments before expanding \bcurve. Hence \bcurve is taken as one of the arguments of \pgfpointcurveattime and TeX looks for the other arguments in the text that follows.

So what you want to do, is to expand \bcurve first (once). To change the order of expansion, one uses \expandafter: \expandafter\a\b expands \b and adds the result after \a and then continues on to process \a. The problem here is that you want to skip over five tokens ({.25}), which would require a lot of \expandafters:

\expandafter\pgfpointcurveattime\expandafter{\expandafter.\expandafter2\expandafter5\expandafter}\bcurve

After the execution of the \expandafters, this will leave TeX with

\pgfpointcurveattime{.25}{\pgfpoint{0cm}{0cm}}{\pgfpoint{0cm}{1cm}}{\pgfpoint{1cm}{1cm}}{\pgfpoint{1cm}{0cm}}

One possible solution to save on typing all those \expandafters and getting more readable code is to save .25 into a macro that can be skipped over in one step:

\newcommand*\pos{.25}
\expandafter\pgfpointcurveattime\expandafter\pos\bcurve

After the execution of the \expandafters, this will leave TeX with

\pgfpointcurveattime\pos{\pgfpoint{0cm}{0cm}}{\pgfpoint{0cm}{1cm}}{\pgfpoint{1cm}{1cm}}{\pgfpoint{1cm}{0cm}}

Related Solutions

[Tex/LaTex] using macros in tikz/pgf

\draw plot expects the x and y coordinates to be separated by a comma, but the way your function is set up now, it only sees one chunk of stuff (not correct LaTeX terminology).

One way to get around this is to put the \draw plot into the macro as well. You can make the macro accept an optional argument that is passed to the draw command, so you'll be able to alter the appearance of individual plots:

\documentclass{article}
\usepackage{tikz}

\begin{document}
    \begin{tikzpicture}[domain=-2:2]
        \newcommand\hyper[4][]{\draw [#1] plot ({#2*exp(#4)+#3*exp(-#4)}, {#2*exp(#4)-#3*exp(-#4)})}
         \hyper[red]{1}{1}{\x};
         \hyper[blue]{0.5}{0.5}{\x};
    \end{tikzpicture}
\end{document}

Instead of using the \draw plot functionality, you may want to take a look at PGFPlots, which makes it easier to create plots and add axes and legends etc.

\documentclass{article}
\usepackage{pgfplots}
\begin{document}
\newcommand\hyper[4][]{\addplot [#1] ({#2*exp(#4)+#3*exp(-#4)}, {#2*exp(#4)-#3*exp(-#4)});}
    \begin{tikzpicture}[domain=-2:2]
         \begin{axis}[xmin=0, xmax=4, smooth, axis lines=middle]
         \foreach \n in {0.25,0.5,...,2}{
            \hyper[domain=-1.5/\n:1.5/\n] {\n}{\n}{x}
         }
         \hyper[domain=-3:3, thick, red]{0.75}{0.75}{x}
         \end{axis}
    \end{tikzpicture}
\end{document}

[Tex/LaTex] Expansion of macros within chemmacros’ \ch{} macro

\ch is not provided by chemmacros but by the chemformula package.

As you've noticed it doesn't expand its argument. As a consequence when it sees \compound (unaware of its definition) it does nothing with it and just inputs it. Only afterwards \compound expands to its definition but then it is too late.

That means you either need to do that manually by adding the appropriate number of \expandafters or you can define a \chX (say) which expands its argument once before parsing it:

\documentclass{article}
\usepackage{chemformula}

\ExplSyntaxOn
\NewDocumentCommand \chX { O{}m }
  { \chemformula_ch:no {#1} {#2} }
\cs_generate_variant:Nn \chemformula_ch:nn {no}
\ExplSyntaxOff

\newcommand{\compound}{H2O}

\begin{document}

\ch{H2O} \par
\expandafter\ch\expandafter{\compound} \par
\chX{\compound}

\end{document}

If you choose the \chX way together with chemmacros you either should also load chemformula explicitly (\usepackage{chemmacros}\usepackage{chemformula}) or tell chemformula to do it for you (\chemsetup{formula = chemformula}) before defining the variant.

Best Answer

Related Solutions

[Tex/LaTex] using macros in tikz/pgf

[Tex/LaTex] Expansion of macros within chemmacros’ \ch{} macro

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