Increment/Decrement variables in TikZ

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}