How do I set a variable inside of \rlap

Question

Two questions please about this code example:

1) Why does \rlap prevent me from setting a variable?
2) How do I set a variable from within an \rlap that is visible outside of \rlap?

I'm using MikTex 2.8. Thanks for any clue!

\documentclass{article}
\newif\ifmycheck
\begin{document}
\rlap{\mychecktrue}
% \mychecktrue
\ifmycheck
Check True!
\else
Check False!  % always takes this path!
\fi
\end{document}

Answer 1

1. Changes within a group stay inside that group, and \rlap puts its argument inside an \hbox, therefore within its own group. So if you did:

   \rlap{\mychecktrue % group is still open!
   % \mychecktrue
   \ifmycheck
   Check True!
   \else
   Check False!  
   \fi}
   

   You will get

   Check True!

   So you see that you did set the variable inside \rlap.

2. You want the changes to persist outside the group. So use the prefix \global:

   \rlap{\global\mychecktrue}
   % \mychecktrue
   \ifmycheck
   Check True!
   \else
   Check False!  
   \fi
   

   That will result again in "Check True!"

Answer 2

To complement Matthew's answer, a solution other than setting the variable globally is to use aftergroup to delay the assignment until after the group (produced by \hbox) is finished. Your example becomes

\documentclass{article}
\newif\ifmycheck
\begin{document}
\rlap{\aftergroup\mychecktrue}
\ifmycheck
Check True!
\else
Check False!  
\fi
\end{document}

and now typesets Check True!.