Increment/Decrement variables in TikZ

Question

I need to increment/decrement variables in TikZ, i.e., I have

\def\a{0}

and then I need to perform

\a=\a-1.5;

The code compiles, but it does not change the value of the variable \a.

How do I do this?

Answer 1

There's not much to go on in your question. So, I'll do my best here. There are several approaches you can take. You wanted to decrement \a; I increment it in the following examples to visually emphasis how the effects are taking place and being used: just make the necessary changes for decrementing.

The first is to combine a use of \pgfmathparse with \edef

code for above image:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}

Use \verb`\pgfmathparse` together with \verb`\edef`

\begin{tikzpicture}
  \edef\mya{0}
  \pgfmathparse{\mya+1.5}
  \edef\mya{\pgfmathresult}
  \node[circle,draw] at (\mya,\mya) {\mya};
\end{tikzpicture}

\end{document}

If you want to remember the changed macro outside of the tikzpicture then you can use \xdef

code for above image:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}

To remember outside of the picture use \verb`\xdef`

\begin{tikzpicture}
  \edef\mya{0}
  \pgfmathparse{\mya+1.5}
  \xdef\mya{\pgfmathresult}
  \node[circle,draw] at (\mya,\mya) {\mya};
\end{tikzpicture}

\verb`\mya`=\mya

\end{document}

You can use a \foreach loop with the above tricks:

code for above image:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}

If used within a \verb`\foreach` loop, you have several choices:

Use \verb`\xdef`

\begin{tikzpicture}
  \edef\mya{0}
  \foreach \x in {1,2,...,8}
  {   
      \pgfmathparse{\mya+1.5}
      \xdef\mya{\pgfmathresult}
      \node[circle,draw,inner sep=\mya pt] at (\mya*0.50cm,\mya*0.25cm) {\makebox[0pt]{\mya}};
  }
\end{tikzpicture}
\end{document}

Better yet, you can use evaluate=\x as ...

code for above image:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}

Using \verb`evaluate=\x as ...`

\begin{tikzpicture}
  \edef\mya{0}
  \foreach \x [evaluate=\x as \mya using \x*1.5] in {1,2,...,8}
  {   
      \node[circle,draw,inner sep=\mya pt] at (\mya*0.5cm,\mya*0.25cm) {\makebox[0pt]{\mya}};
  }
\end{tikzpicture}

\end{document}

Finally, if you need to remember the value outside of the \foreach loop, you can do something like:

code for above image:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}

Using \verb`evaluate=\x as ...` and remembering

\begin{tikzpicture}
  \edef\mya{0}
  \foreach \x [evaluate=\x as \mya using \x*1.5] in {1,2,...,8}
  {   
      \node[circle,draw,inner sep=\mya pt] at (\mya*0.5cm,\mya*0.25cm) {\makebox[0pt]{\mya}};
      \xdef\remembermya{\mya}
  }
  \edef\mya{\remembermya}
  \node at (0,0) {\mya};
\end{tikzpicture}

\end{document}

This should probably be enough to get you started. If you could post a more thorough MWE illustrating how you're trying to use \a, then I could probably give you a more complete answer.

You could also take a rather different approach using coordinate arithmetic:

code for above image:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}

Using \verb`calc` package to perform coordinate arithmetic: \verb`($(0,0)+\x*(<coordinate>)$)` 

\begin{tikzpicture}
  \coordinate (myinc) at (1.5*0.5cm,0.25cm);
  \foreach \x  in {1,2,...,8}
  {   
      \pgfmathparse{\x*1.5}
      \edef\mya{\pgfmathresult}
      \node[circle,draw,inner sep=\mya pt] at ($(0,0)+\x*(myinc)$) {\makebox[0pt]{\mya}};
  }
\end{tikzpicture}

\end{document}