Ggtcf

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,

newcommandmycommand[2][#1] ... 

does not work, neither does

newcommandmycommand[2][#2] ... 

Does anybody know how to do this?

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

documentclassarticle
usepackagexparse

NewDocumentCommandfooO#2m%
 Optional=``#1'', mandatory=``#2''par

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


begindocument

foox

foo[y]x

oofx

oofx[y]

enddocument

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.

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:

documentclassarticle

usepackagexparse

newcommandprintthis[2]%
 Optional: #1; Mandatory: #2%


NewDocumentCommandmycommando m%
 IfValueTF#1
 printthis#1#2% mycommand[..]...
 printthis#2#2% mycommand...


begindocument

mycommandfirst

mycommand[first]second

enddocument

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:

documentclassarticle

newcommandprintthis[2][]%
 Optional: #1; Mandatory: #2%


makeatletter
newcommandmycommand%
 @dblargprintthis

makeatother

begindocument

mycommandfirst

mycommand[first]second

enddocument

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

%%errorcontextlines=1000
documentclass[a4paper]article

makeatletter
newcommandmakefirstmandatorytheoptional[1]%
 expandafterinnermakefirstmandatorytheoptional
 expandaftercsnamestring#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.
%
makefirstmandatorytheoptionalmycommand%
makeatother

parindent=0ex
parskip=medskipamount

begindocument

No optional argument given---verb|mycommandA|:

mycommandA

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

mycommand[B]A

enddocument