28.4.6 Using a list instead of a conditional expression

Suppose you want a different navigation bar for some of your HTML output files, depending on the name of the chapter from which the files are generated. One way would be to use a conditional expression (see §28.6.4.2 Using conditional expressions) to check the current chapter file name and choose the code for the navigation bar. For example:

[NavBar]

; Configure navigation bar for roadmap:

<$_if ($$_currbase is "user_roadmap")> <$rmap>

; Configure navigation bar for Programmer's Guide topics:

<$_elseif ($$_currbase is "bgp_user")> <$pgnav>

<$_elseif ($$_currbase is "mld_user")> <$pgnav>

 ... (long list of similar clauses)

<$_elseif ($$_currbase is "pga_user")> <$pgnav>

; Configure navigation bar for function topics, by default:

<$_else>

   <p>

   <a href="functions.htm">Function Index</a>

   </p>

<$_endif>

Instead, you could use a list indexed by the value of $$_currbase, with each list value a macro call (or HTML code):

[navmap]

0 = <p><a href="functions.htm">Function Index</a></p>

user_roadmap = <$rmap>

bgp_user = <$pgnav>

mld_user = <$pgnav>

...

pga_user = <$pgnav>

[NavBar]

<$navmap[$$_currbase]>

The 0 (zero) list item corresponds to the <$_else> clause in the original [NavBar] macro, and is used if the specified index (the value of $$_currbase) is not found. Instead of a macro call the value of this list item is straight HTML code, which works as long as the code is all on one line. You could just as well use a macro call for the zero value, like the rest of the list items.

---

> 28 Working with macros > 28.4 Using multiple-value list variables > 28.4.6 Using a list instead of a conditional expression