## Wednesday, March 31, 2010

### Global font size changes

The question of how to change the font size for a row in a table came up recently. It's tricky since each cell in a tabular is in its own group. My thought was a global font size change for the row that needed it and then a global change back. Since commands like \tiny actually perform many different assignments, \global\tiny does not do what we need. TeX provides an \afterassignment primitive that takes the next token and places it after the next assignment. For example,
\afterassignment\TeX
\count255=3
\the\count255
acts like
\count255=3
\TeX
\the\count255
The basic idea is to use \afterassignment with a helper macro which uses \afterassignment and \global.
\def\helper{\afterassignment\helper\global}
This doesn't quite work since there's no good way to stop it. Modifying it slightly to make it conditional allows us to turn it off:
\newif\ifhelper
\def\helper{\ifhelper\afterassignment\helper\global\fi}
Two more snags remain. First, the font size changing macros actually make use of \afterassignment to allow default units (a neat trick in its own right) and \global\afterassignment is an error so we need to reimplement it. Similarly, vrule is used and \global \vrule is an error so we have to work around that. The final snag is that the first time TeX executes one of the font size commands like \tiny it performs extra work and that extra work doesn't play so nicely with our hack. The workaround for this is to simply execute the font size changing macro first. The complete code looks like this.
\newif\ifhelper
\makeatletter
\def\unithelper#1\@nnil{\global\helpertrue\helper}
\def\helper{\ifhelper\afterassignment\helper\global\fi}
\newcommand*\globalfontsize[1]{%
\begingroup
#1%
\def\@defaultunits{\helperfalse\afterassignment\unithelper\global}%
\let\realvrule\vrule
\def\vrule{\helperfalse\global\helpertrue\afterassignment\helper\realvrule}%
\global\helpertrue
\helper#1%
\helperfalse
\endgroup}
\makeatother
\newcommand*\globaltiny{\globalfontsize\tiny}
\newcommand*\globalnormalsize{\globalfontsize\normalsize}
Now we can use \globaltiny and \globalnormalsize in the table to change the font for the whole row.