DTML Basics (part 3) (
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Last time out, DTML Basics demonstrated conditional statements. This time around,
it's time to study loops...which, in the DTML world, aren't exactly what you're
used to. Take a look.If you've been following along, you now know how to incorporate decision-making
constructs in your DTML code. As you saw in last week's episode, DTML comes with
a fairly large family of conditional statements, which allow you to add business
intelligence to your DTML scripts.
But DTML is a full-fledged programming language, and one which allows you to
do a lot more than nest "if" statements within each other. One of its cooler capabilities
involves using powerful loop constructs to iterate over sequences of both variables
and objects. This ability to create loops is not unique to DTML - almost every
programming language on the planet allows you to do this - but DTML includes some
fairly interesting twists to the traditional approach.
Coincidentally, those twists just happen to be the subject of today's discussion.
Keep reading - you've probably never seen this before.{mospagebreak title=Playing
The Numbers} Before we get started, let's get the jargon straight - what's a sequence
anyway?
As traditionally understood, a "sequence" is a series of items, usually connected
to each other by a logical thread. For example,
0, 1, 2, 3, 4, 5...
p, q, r, s, t...
huey, dewey, louie...
are all valid sequences
In the Zope context, the definition of a sequence can be scoped down a little
further - a sequence here is usually a set of objects. For example, if you create
a folder in Zope, the objects stored within that folder can be considered a sequence.
Or, if you retrieve a set of records from a database, the resultset returned can
also be considered a sequence of data items.
A sequence, however, is just one part of the jigsaw. In order to access the elements
that make up a sequence, you usually need a programming structure that will iterate,
or loop, through the sequence, processing each element in turn. This loop can
be combined with decision-making logic (remember what you learnt last time?) to
perform specific actions or execute specific commands while processing the elements
in a sequence.
In order to demonstrate this, let's create a sequence in Zope and write a loop
to process it. Fire up Zope, log into the Zope management interface, and create
an instance of the DTML Document object. Name it "SampleSequence" and fill it
with the following code:
<h3><dtml-var title_or_id></h3><br>
<ul>
<dtml-in expr="0,
1, 2, 3, 4, 5">
<li> I am <dtml-var sequence-item></li>
</dtml-in>
</ul>
First, I've created a simple sequence by brute force - it contains numbers from
0 to 5. Then, the <dtml-in> tag is used to loop through the sequence, in a
manner similar to the "for" loops that PHP and Perl programmers are familiar with.
From the output, it's obvious that this script loops six times to display the
items in the sequence.
The individual elements of the sequence can be accessed via the special "sequence-item"
variable. This is true in all cases, except when the items in the sequence are
object that cannot be converted to strings.
Save the code and view the output of the script - you should see something like
this:
* I am 0
* I am 1
* I am 2
* I am 3
* I am 4
* I am 5
