You surely can define your shortcuts; indeed you should.
Let's make some examples. Suppose your document is full of Fourier transforms, for which you need a fancy F. Instead of writing every time
$\mathcal{F}(f)$
it's surely better to define a new command, say
\newcommand{\FT}{\mathcal{F}}
(choose any name you like), so that you can type
$\FT(f)$
and get the same result as before, with a big bonus! If you change your mind about the notation, you can simply modify the definition.
Another example. The "gradient" operator is not predefined; so you might want to have a command for it:
\DeclareMathOperator{\grad}{grad}
A different one; my preferred notation for vectors is, say, \mathbf{v}. However, since conventions are different, I never type vectors in that way, but prefer to have
\newcommand{\vect}[1]{\mathbf{#1}}
for the same reason as before; I might change my mind and want to modify the appearance, say for using bold italic; this would be accomplished just by saying
\usepackage{bm}
and changing the above into
\newcommand{\vect}[1]{\bm{#1}}
How do you organize this? Here's an example:
\documentclass[a4paper]{book}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc} % input encoding UTF-8
% Useful packages
\usepackage{amsmath,amssymb,amsthm}
% add all the packages you need
% Personal definitions
\newcommand{\FT}{\mathcal{F}} % Fourier transform operator
\DeclareMathOperator{\grad}{grad} % gradient
\newcommand{\vect}[1]{\mathbf{#1}} % vectors and matrices
\begin{document}
...
\end{document}
Add definitions while you find that they are useful for distinguishing logical units of your document.
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}
Best Answer
The package you need is
amsthm(besides the recommendedamsmath).Add
\usepackage{amsthm}and all will go flawlessly.