Where to define new TikZ commands

Question

I have to start by saying that I am a newbie to LaTeX, let alone TikZ.

In order to solve another problem, I found a solution in another question in a comment by Tom Bombadil. He defines the new command:

    \def\centerarc[#1](#2)(#3:#4:#5)% 
[draw options] (center) (initial angle:final angle:radius) { \draw[#1] ($(#2)+({#5*cos(#3)},{#5*sin(#3)})$) arc (#3:#4:#5); }

This basically draws an arc with a given circle centre, initial and final angle. Now I am not sure about where I should define the command. I tried at the beginning of the Tikz picture, but it does not work.

\begin{tikzpicture}[thick,>=latex]
\def\centerarc[#1](#2)(#3:#4:#5)% 
[draw options] (center) (initial angle:final angle:radius) { \draw[#1] ($(#2)+({#5*cos(#3)},{#5*sin(#3)})$) arc (#3:#4:#5); };
\centerarc[red, thick](0,0)(5:85:1);
\end{tikzpicture}

What am I doing wrong?

Answer 1

Code in comments is badly formatted: the correct input should be

\def\centerarc[#1](#2)(#3:#4:#5)% [draw options] (center) (initial angle:final angle:radius)
  { \draw[#1] ($(#2)+({#5*cos(#3)},{#5*sin(#3)})$) arc (#3:#4:#5); }

because the bits after the % are just comments showing the syntax.

You should have this in the preamble (before \begin{document}) and probably you should also add a \newcommand just to be sure \def doesn't overwrite an existing command.

\newcommand\centerarc{} % just for safety
\def\centerarc[#1](#2)(#3:#4:#5){%
  % Synopsis
  % \centerarc[draw options](center)(initial angle:final angle:radius)
  \draw[#1] ($(#2)+({#5*cos(#3)},{#5*sin(#3)})$) arc (#3:#4:#5);
}

Answer 2

You could use extra #n to add flexibility. [#1]#2(#3)#4(#5:#6:#7) instead of [#1](#2)(#3:#4:#5).

\def\centerarc[#1]#2(#3)#4(#5:#6:#7)% [draw options] (center) (initial angle:final angle:radius)
  {\draw[#1]($(#3)+({#7*cos(#5)},{#7*sin(#5)})$)arc(#5:#6:#7);}

and then you can use

\centerarc[thick] (1,2) (10:20:1); % which has spaces for readability

You could probably use the power of xparse too, but I'm not sure about spaces since at this moment it doesn't accept spaces before certain arguments.

Answer 3

You should define your new command outside, i.e. before the tikzpicture environment where you want to use it.