Fhyujk

I often use short macros in mathematical formulas for frequently used symbols, e.g. d x for a differential dx or, say, v if I need a vector v very often in my text to make things more readable.
However, I get a clash with predefined macros because d stands for a dot under the next character and v for the hacek accent, which I don't want to override (maybe I need them in the bibliography...).

So, I came up with the following to override those macros only in math mode:

documentclass{article}



usepackage{everyhook}

newcommand{mathdef}[2]{%

  PushPostHook{math}{expandafterdefcsname #1endcsname{#2}}%

  PushPostHook{display}{expandafterdefcsname #1endcsname{#2}}}

mathdef{d}{mathrm{d}}%



begin{document}

  d x  is an x with a dot below and $int f(x) d x$ is an integral over $x$.

end{document}

However, I would like the command to be defined like normal macros too, i.e. mathdef{d}{mathrm{d}}, and also be able to take arguments, but all my experiments with expandafter, unexpanded, etc. didn't work out and led only to the usual strange error messages. Any hint how I can do that?

Furthermore, do you think there is a big performance penalty using everymath like that in a large document?

I would not use everymath.

documentclass{article}

usepackage{letltxmacro}



makeatletter

newcommand{mathdef}[2]{%

  @ifundefined{#1}{@mathdef@new{#1}{#2}}{@mathdef@remember{#1}{#2}}

}



newcommand{@mathdef@remember}[2]{%

  expandafterLetLtxMacro

    csname textmode@#1expandafterendcsname

    csname #1endcsname

  expandafterdefcsname mathmode@#1endcsname{#2}%

  expandafterDeclareRobustCommandcsname#1endcsname{%

    ifmmodeexpandafter@firstoftwoelseexpandafter@secondoftwofi

     {csname mathmode@#1endcsname}{csname textmode@#1endcsname}%

  }%

}

newcommand{@mathdef@new}[2]{%

  expandafterDeclareRobustCommandcsname#1endcsname{#2}%

}

makeatother



mathdef{d}{mathop{}!mathrm{d}}



begin{document}



d{x} is an x with a dot below and $int f(x) d x$ is an integral over $x$.



end{document}

I don't think this is a good way to go, though. It's confusing and prone to errors.

See When to use LetLtxMacro? for information about LetLtxMacro.

It's a bit easier with etoolbox:

documentclass{article}

usepackage{etoolbox}



makeatletter

newcommand{mathdef}[2]{%

  @ifundefined{#1}{@mathdef@new{#1}{#2}}{@mathdef@remember{#1}{#2}}

}



newcommand{@mathdef@remember}[2]{%

  expandafterrobustifycsname#1endcsname

  csletcs{textmode@#1}{#1}%

  csdef{mathmode@#1}{#2}%

  protectedcsdef{#1}{%

    ifmmodeexpandafter@firstoftwoelseexpandafter@secondoftwofi

     {csuse{mathmode@#1}}{csuse{textmode@#1}}%

  }%

}

newcommand{@mathdef@new}[2]{%

  protectedcsdef{#1}{#2}%

}

makeatother



mathdef{d}{mathop{}!mathrm{d}}



begin{document}



d{x} is an x with a dot below and $int f(x) d x$ is an integral over $x$.



end{document}

My idea is very similar to egreg's, but I'd like to add an optional argument, so the math command could process arguments itself. The code:

documentclass{article}



usepackage{xparse}

DeclareDocumentCommand{mathdef}{mO{0}m}{%

  expandafterletcsname oldstring#1endcsname=#1

  expandafternewcommandcsname newstring#1endcsname[#2]{#3}

  renewcommand#1{%

    ifmmode

      expandafterletexpandafternextcsname newstring#1endcsname

    else

      expandafterletexpandafternextcsname oldstring#1endcsname

    fi

    next

  }%

}



mathdef{v}[1]{tilde{#1}}

mathdef{d}{mathrm{d}}



begin{document}

Hav{c}ek and tilde $v{a}+v{b}=1$.



d x  is an x with a dot below and $int f(x) d x$ is an integral over $x$.

end{document}

The result: