Command already defined (custom command)

Question

When compiling following code, I the compiler tells me that a command is already defined (the name is \c@b...).

\documentclass{article}

\usepackage{calc}
\usepackage{ifthen}
\usepackage{tikz}

\usepackage{verbatim}

\newcommand{\piepart}[5]
{
    \draw[thick,fill=#4] (0,0) -- (#1:1) arc (#1:#2:1) -- cycle;

    \pgfmathparse{0.5*#1+0.5*#2}
    \let\midangle\pgfmathresult
    \pgfmathtruncatemacro{\trunc}{\midangle}
    \ifthenelse{\trunc < 90}{
    \draw (\midangle:1) node[above right]{#3};
    }
    {
        \ifthenelse{\trunc < 180}{
        \draw (\midangle:1) node[above left]{#3};
        }
        {
            \ifthenelse{\trunc < 270}{
            \draw (\midangle:1) node[below left]{#3};
            }
            {
                \draw (\midangle:1) node[below right]{#3};
            }
        }
    }
    \pgfmathparse{min((#2-#1-10)/110*(-0.3),0)}
    \let\temp\pgfmathresult
    \pgfmathparse{max(\temp,-0.5) + 0.8}
    \let\innerpos\pgfmathresult
    \node at (\midangle:0.8) {#5};

}

\newcommand{\piechart}[1]
{
\newcounter{a}
\newcounter{b}
\foreach \p/\mcolor/\mname in #1
    {
        \setcounter{a}{\value{b}}
        \addtocounter{b}{\p}
        \piepart{\thea/100*360}
                    {\theb/100*360}
                    {\mname}{\mcolor}{\p\%}
    }
}

\begin{document}

\begin{tikzpicture}[scale=1.5]
\tikzstyle{every node}=[font=\tiny]
\piechart{{75/gray/A, 18/green/B, 7/red/C}}                 
\end{tikzpicture}
\begin{tikzpicture}[scale=1.5]
\tikzstyle{every node}=[font=\tiny]
\piechart{{75/gray/A, 18/green/B, 7/red/C}}                 
\end{tikzpicture}

\end{document}

If somebody knows what creates the error, or how to solve it, could they please tell me.

Thanks

Answer 1

There are two \newcounter calls inside the macro \pichart. When the macro is called the second time, then the counters are already defined and you get the error messages about already defined commands \c@a and \c@b, which are the internal command representation of the counters.

Move the \newcounter calls outside and reset the counters inside the macro:

\newcounter{a}
\newcounter{b}
\newcommand*{\piechart}[1]{%
  \setcounter{a}{0}%
  \setcounter{b}{0}%
  ...
}

Full example for completeness (and with larger fonts):

\documentclass{article}

\usepackage{calc}
\usepackage{ifthen}
\usepackage{tikz}

\usepackage{verbatim}

\newcommand{\piepart}[5]
{
    \draw[thick,fill=#4] (0,0) -- (#1:1) arc (#1:#2:1) -- cycle;

    \pgfmathparse{0.5*#1+0.5*#2}
    \let\midangle\pgfmathresult
    \pgfmathtruncatemacro{\trunc}{\midangle}
    \ifthenelse{\trunc < 90}{
    \draw (\midangle:1) node[above right]{#3};
    }
    {
        \ifthenelse{\trunc < 180}{
        \draw (\midangle:1) node[above left]{#3};
        }
        {
            \ifthenelse{\trunc < 270}{
            \draw (\midangle:1) node[below left]{#3};
            }
            {
                \draw (\midangle:1) node[below right]{#3};
            }
        }
    }
    \pgfmathparse{min((#2-#1-10)/110*(-0.3),0)}
    \let\temp\pgfmathresult
    \pgfmathparse{max(\temp,-0.5) + 0.8}
    \let\innerpos\pgfmathresult
    \node at (\midangle:0.8) {#5};

}

\newcounter{a}
\newcounter{b}
\newcommand{\piechart}[1]
{
    \setcounter{a}{0}%
    \setcounter{b}{0}%
    \foreach \p/\mcolor/\mname in #1
    {
        \setcounter{a}{\value{b}}
        \addtocounter{b}{\p}
        \piepart{\thea/100*360}
                    {\theb/100*360}
                    {\mname}{\mcolor}{\p\%}
    }
}

\begin{document}

\begin{tikzpicture}[scale=1.5]
    \tikzstyle{every node}=[font=\footnotesize]
    \piechart{{75/gray/A, 18/green/B, 7/red/C}}                 
\end{tikzpicture}
\begin{tikzpicture}[scale=1.5]
    \tikzstyle{every node}=[font=\footnotesize]
    \piechart{{75/gray/A, 18/green/B, 7/red/C}}                 
\end{tikzpicture}

\end{document}