Cfrgtkky

I construct a macro named mycal(with an optional argument) to calculate value of coordinates which will be used in tikzdraw.
I find that with this optioal arg, the code can not compile. When the optional arg become mandatory, the code works all right. But I do need this arg optional.
Why and how to solve this problem?

MWE:

documentclass{article}

usepackage{tikz,picture}



begin{document}

% The following part can not compile

% newcommand{mycal}[3][0pt]{thedimexpr #2 + #3 + #1}

% newcommandmydraw[3]{tikzdraw(0,0)--(mycal[#1]{#2}{#3},0);}

% mydraw{1pt}{2cm}{3cm}



% The following part ok!

newcommand{mycal}[3]{thedimexpr #2 + #3 + #1}

newcommandmydraw[3]{tikzdraw(0,0)--(mycal{#1}{#2}{#3},0);}

mydraw{1pt}{2cm}{3cm}

end{document}

Your macro does not work because macros with optional arguments are not expandable and therefore cannot be used in the middle of a path construction. Tex has to perform some assignment to determine whether the optional argument is present or not. The solution may be to use pgfextra which allows you to interrupt temporarily the construction of the path and execute some code. You have to modify your code to save the result of the computation to some register and then use it.

documentclass{article}



usepackage{tikz}



newdimenmydimen

newcommandmycal[3][0cm]{mydimen=dimexpr #2 + #3 + #1relax}

newcommandmydraw[3][0cm]{tikzdraw(0cm,0cm) pgfextra{mycal[#1]{#2}{#3}} --(mydimen,0cm);}



begin{document}

mydraw[1cm]{2cm}{3cm}



mydraw{2cm}{3cm}

end{document}

By the way, in your example, mycal does not really need to take an optional argument. mydraw does.

newcommandmycal[3]{dimexpr #2 + #3 + #1relax}

newcommandmydraw[3][0cm]{begin{tikzpicture}draw(0cm,0cm)-- (mycal{#1}{#2}{#3},0cm);

EDIT:

If you really insist on using mycal with an optional argument without using pgfextra, you can use the following trick. It defines an expandable mycalc with an optional argument.

documentclass{article}



usepackage{tikz}



makeatletter

defmycal#1{%

  ifx[#1expandafter@firstoftwoelseexpandafter@secondoftwofi

  {@mycal[}{@mycal[0cm]{#1}}}

def@mycal[#1]#2#3{dimexpr#1+#2+#3relax}

makeatother



begin{document}

tikzdraw(0cm,0cm)--(mycal{2cm}{3cm},0cm);



tikzdraw(0cm,0cm)--(mycal[1cm]{2cm}{3cm},0cm);

end{document}

NEW EDIT:

To write mycal{2cm,3cm} or mycal[1cm]{2cm,3cm} instead of mycal{2cm}{3cm} or mycal[1cm]{2cm}{3cm} you can define mycal as follows

documentclass{article}



usepackage{tikz}



makeatletter

defmycal#1{%

  ifx[#1expandafter@firstoftwoelseexpandafter@secondoftwofi

  {@mycal[}{@mycal[0cm]{#1}}}

def@mycal[#1]#2{@@mycal{#1}#2@nil}

def@@mycal#1#2,#3@nil{dimexpr#1+#2+#3relax}

makeatother



begin{document}

tikzdraw(0cm,0cm)--(mycal{2cm,3cm},0cm);



tikzdraw(0cm,0cm)--(mycal[1cm]{2cm,3cm},0cm);

end{document}