Passing parameters to \g@addto@macro

Question

I'm a complete newbie but have a little background in programming. I've been looking at a MWE located here provided by Yiannis Lazarides.

In the macro definition, we have

%% add an image to the DB, use the LaTeX macro
%% \g@addto@macro to store them at the DB location
\def\addtoDB#1#2{%
    \ifx\@empty#1
        \g@addto@macro{#1}{#2}
    \else
        \g@addto@macro#1{,#2}
    \fi
}

What is the difference between \g@addto@macro{#1}{#2} and \g@addto@macro#1{,#2} ?

Answer 1

The macro is aimed to incrementally build a list macro; in the document you find something like

\def\alist{fig167,fig168,fig169,fig176,%
  fig180,fig181,fig182,fig183,fig185,fig186,fig187,fig188}

Well, one can obtain the same result by

\newDB{alist}

and then

\addtoDB\alist{fig167}
\addtoDB\alist{fig168}
\addtoDB\alist{fig169}
\addtoDB\alist{fig176}

and so on.

At the first \addtoDB call, the macro \alist defined by \newDB is still empty, so the conditional is true and

\g@addto@macro\alist{fig167}

is executed. At the second macro call, \alist is not empty, so

\g@addto@macro\alist{,fig168}

is executed, resulting in \alist expanding to fig167,fig168. And so on for the successive calls.

Answer 2

In this particular case I think the two uses of \g@addto@macro are meant to be the same apart from the extra comma prepended to the second argument in the second case. In fact since the first argument of \g@addto@macro is meant to be simply the name of a macro, the brackets of the first case are not strictly necessary. However, for reasons we'll see in a second, they are a good precaution and, in my opinion, should be used in the second case as well.

To illustrate the difference they make in general (when the first argument is not necessarily a macro name) let's consider the following example:

\documentclass{article}

\def\test#1#2{this #1 is #2 a test}
\def\testA#1#2{A \test{#1}{#2} A}
\def\testB#1#2{B \test#1{#2} B}

\begin{document}
 \testA{{1}{2}}{3}

 \testB{{1}{2}}{3}
\end{document}

The \test macro takes two arguments, \testA uses \test as in your first case (with brackets around #1) while \testB uses \test as in your second case.

In the body we use \testA and \testB with the same two arguments: #1 = {1}{2} and #2 = 3.

Since \testA uses brackets around #1, it expands to A \test{{1}{2}}{3} A, as expected, binding {1}{2} to the first argument of \test. \testB, instead, expands to B \test{1}{2}{3} B which means that \test will get 1 as first argument and 2 as second argument; {3} is not consumed by \test and gets typeset after \test is expanded.

Hence the output is

A this 12 is 3 a test A

B this 1 is 2 a test3 B