Htydjtk

I would like to define a macro that takes two arguments, one of which is optional. If it is not given, then it should take the other one as default value.

Unfortunately,

newcommand{mycommand}[2][#1]{ ... }

does not work, neither does

newcommand{mycommand}[2][#2]{ ... }

Does anybody know how to do this?

You can use xparse to easily condition on whether or not an optional argument was present or not, and supply the appropriate combination to another (auxiliary) function. Here's an example:

documentclass{article}



usepackage{xparse}



newcommand{printthis}[2]{%

  Optional: #1; Mandatory: #2%

}



NewDocumentCommand{mycommand}{o m}{%

  IfValueTF{#1}

    {printthis{#1}{#2}}% mycommand[..]{...}

    {printthis{#2}{#2}}% mycommand{...}

}



begin{document}



mycommand{first}



mycommand[first]{second}



end{document}

A slightly different version of this stems from the use of caption, where you can supply an optional argument for the LoT/LoF, but if you don't, the mandatory arguments is sent instead (similarly for sectional units with optional arguments destined for the ToC). This uses the kernel's @dblarg:

documentclass{article}



newcommand{printthis}[2]{%

  Optional: #1; Mandatory: #2%

}



makeatletter

newcommand{mycommand}{%

  @dblargprintthis

}

makeatother



begin{document}



mycommand{first}



mycommand[first]{second}



end{document}

Good news: you can do it very easily with xparse:

documentclass{article}

usepackage{xparse}



NewDocumentCommand{foo}{O{#2}m}{%

  Optional=``#1'', mandatory=``#2''par

}

NewDocumentCommand{oof}{mO{#1}}{%

  Mandatory=``#1'', optional=``#2''par

}



begin{document}



foo{x}



foo[y]{x}



oof{x}



oof{x}[y]



end{document}

The argument specifier O{...} takes as argument what to substitute as default when the argument itself doesn't appear at call time. This can well be a parameter token referring to another argument.

This is an attempt to add protection as with other macros that process optional arguments:

%%errorcontextlines=1000

documentclass[a4paper]{article}



makeatletter

newcommandmakefirstmandatorytheoptional[1]{%

  expandafterinnermakefirstmandatorytheoptional

  expandafter{csnamestring#1endcsname}{#1}%

}%

newcommandinnermakefirstmandatorytheoptional[2]{%

  def#2{%

    ifxprotect@typeset@protect

      expandafter@firstoftwo

    else

      expandafter@secondoftwo

    fi

    {kernel@ifnextchar[{#1}{@dblarg{#1}}}%

    {protect#2}%

  }%

}%

newcommandmycommand[2][dummyoptional]{%

  This is taken for the optional argument: #1.\

  This is taken for the mandatory argument: #2.

}%

makefirstmandatorytheoptional{mycommand}%

makeatother



parindent=0ex

parskip=medskipamount



begin{document}



No optional argument given---verb|mycommand{A}|:



mycommand{A}



Optional argument "B" given---verb|mycommand[B]{A}|:



mycommand[B]{A}



end{document}