mjhjmtu

Why the following sample fails to compile in pdflatex when ifeven is used?

It fails with many errors in line 34 (where the last closing brackets are). The errors are like: Undefined control sequence. }, Extra }, or forgoten endgroup.}, Extra else.}, Extra fi }.

documentclass[tikz]standalone

newcommandstencilptreg[4]node[circle,fill,draw,inner sep=1.5pt,label=below:#4,#1] at (#2) (#3) 
newcommandstencilptstg[4]node[circle,fill,white,draw=black,inner sep=1.5pt,label=below:#4,#1] at (#2) (#3) 

begindocument
 begintikzpicture[x=2cm,y=2cm]
 pgfmathsetmacroN5;
 pgfmathsetmacroM3;

 pgfmathsetmacroNN(N*2); % Number of points in staggered grid
 pgfmathsetmacroMM(M*2); % Number of points in staggered grid 

 foreach j [evaluate=j as q using int(j/2)] in 0,1,...,MM 

 ifoddj
 draw [black!50,thin] (0,j) node[red,left] $n=fracj2$ -- (N,j); 
 else
 draw [black!50,thin] (0,j) node[red,left] $n=q$ -- (N,j);
 fi

 foreach i [evaluate=i as p using (i/2)] in 0,1,...,NN 
 
 ifoddi
 ifoddj
 stencilptstgp,jx-i-j$p$;
 fi
 else
 %ifevenj
 pgfmathsetmacropint(p); 
 stencilptregp,jx-i-j$p$;
 %fi
 fi
 
 
 endtikzpicture
enddocument

Because there is no ifeven. However, it's equivalent to

ifoddjelse
 % <ifeven> content
fi

You should look in the log file for the precise error message, rather than relying on the short tags shown by TeXStudio or other front-ends.

Running your file in batch mode, the first error message is

! Extra }, or forgotten endgroup.
<argument> ...p ,j }x-i -j $p $; fi fi }

l.36 }

Not really clear, but a few lines down you see

! Extra else.
pgffor@collectargument ...pgffor@body #1}else 
 expandafter long expand...
l.36 }

I'm ignoring this; it doesn't match any if.

! Extra fi.
pgffor@collectargument ...r pgffor@body #1fi 
 pgffor@iterate 
l.36 }

We're getting closer! Let's go down some more:

! Undefined control sequence.
pgffor@body ...i -j }$p $; fi else ifeven 
 j pgfmathsetmacro p ...
l.36 

The control sequence at the end of the top line
of your error message was never def'ed. If you have
misspelled it (e.g., `hobx'), type `I' and the correct
spelling (e.g., `Ihbox'). Otherwise just continue,
and I'll forget about whatever was undefined.

Hey! That's the problem!

TeX has no ifeven and TikZ doesn't provide it. Actually, it is not possible to define an ifeven command that works the same as ifodd just reversing the test.

You can choose between

 unlessifoddjrelax
 pgfmathsetmacropint(p);
 stencilptregp,jx-i-j$p$;
 fi

or

 ifoddjrelaxelse
 pgfmathsetmacropint(p);
 stencilptregp,jx-i-j$p$;
 fi

I recommend relax in order not to trigger premature expansions. Not a big deal here, but in other circumstances it could be crucial. However, it's not to be added for any numeric conditionals, particularly in contexts where full expansion is needed.

A safer protection might be

ifoddnumexprjrelaxelse

or

unlessifoddnumexprjrelax

but in this case it would be a waste.