Check if macro is already defined

Question

How can I check if a macro is already defined? I want to make a command \dd. However, if the physics package is loaded I need to do \renewcommand and if it is not loaded I need to do \newcommand. So I want some sort of way to check if the macro \dd is defined or not.

Here is some pseudocode which showes what I want:

if <\dd> is defined:
    \renewcommand ....
else:
     \newcommand ...

Here is a MWE:

\documentclass{article}

\usepackage{physics} % may or may not be loaded

% Definition of \dd
% \newcommand*{\dd}{\mathop{}\!{\operator@font d}}
% or
% \renewcommand*{\dd}{\mathop{}\!{\operator@font d}}

\begin{document}

Hello $\dd f$

\end{document}

Answer 1

To answer the question in the first sentence, "How can I check if a macro is already defined?":

\ifdefined\dd
    %% it is already defined
\else
    %% it is not yet defined
\fi

I actually use this a lot, e.g., as a boolean switch (which I can switch on with \def\myswitch{} and switch off with \let\myswitch\undefined).

Answer 2

The simplest way is to do

\def\dd{<whatever you like>}

Answer 3

The simplest way is to do

\providecommand{\dd}{}% defines \dd if not already defined
\renewcommand{\dd}{<whatever you like>}

If it's for your personal use, fine.

Don't do it in a package code. A user loading your package might want to use \dd from physics (I wouldn't, but people seem to like packages that promise much more than they can actually offer).

_{I don't like physics: it's badly written and has really weird syntax.}

Answer 4

I found that the command \ifcsdef from etoolbox works like this. It can check whether a macro is defined or not, and executes some code. Here is how it works:

\ifcsdef{<macro>}
    {<true code>}
    {<false code>}

If the macro is defined, <true code> is executed, and if not, <false code> is executed. Here is the MWE:

\documentclass{article}

\RequirePackage{etoolbox} 
\usepackage{physics} % may or may not be loaded

\makeatletter
\ifcsdef{dd}
    {\renewcommand*{\dd}{\mathop{}\!{\operator@font d}}}
    {\newcommand*{\dd}{\mathop{}\!{\operator@font d}}}
\makeatother

\begin{document}

Hello $\dd f$

\end{document}

Answer 5

In this particular case, I found this solution:

\makeatletter
\DeclareRobustCommand{\dd}{\relax\ifmmode\ddaux\relax\fi}
\newcommand*{\ddaux}{\mathop{}\!{\operator@font d}}
\makeatother

Which 1) overwrites \dd from the physics package if it is imported, and 2) ensures that the command only works in mathmode.