How to set an array of categories

Hello Kirupa Forum,

I got this old question ticking in my head: how to get various categories in a xml, like:

<slide category=“rooms”/>
<slide category=“rooms”/>
<slide category=“rooms”/>
<slide category=“dinner”/>
<slide category=“dinner”/>
<slide category=“dinner”/>
<slide category=“kitchen”/>
<slide category=“kitchen”/>
<slide category=“kitchen”/>
<slide category=“kitchen”/>
<slide category=“kitchen”/>

and fill and array with olny these 3 categories programaticaly, in as 3.0?