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Code Editor : pic-9.html

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<h2>9. Naming Objects
<a name="9. Naming Objects"></a>
</h2>

The most
natural way to name locations in pic is relative to
objects. In order to do this, you have to be able to name
objects. The pic language has rich facilities for
this that try to emulate the syntax of English.

<h3>9.1. Naming Objects By Order Of Drawing
<a name="9.1. Naming Objects By Order Of Drawing"></a>
</h3>

The
simplest (and generally the most useful) way to name an
object is with a last clause. It needs to be followed
by an object type name; box, circle,
ellipse, line, arrow, spline,
&quot;&quot;, or [] (the last type refers to a
composite object which we&rsquo;ll discuss later).
So, for example, the last circle clause in the
program attached to Figure 9.1.3 refers to the last circle
drawn.

More
generally, objects of a given type are implicitly numbered
(starting from&nbsp;1). You can refer to (say) the third
ellipse in the current picture with 3rd ellipse, or
to the first box as 1st box, or to the fifth text
string (which isn&rsquo;t an attribute to another object) as
5th &quot;&quot;.

Objects
are also numbered backwards by type from the last one. You
can say 2nd last box to get the second-to-last box,
or 3rd last ellipse to get the third-to-last
ellipse.

In places
where nth is allowed,
&lsquo;expr&rsquo;th is also allowed.
Note that &rsquo;th is a single token: no space is
allowed between the &rsquo; and the th. For
example,

for
i = 1 to 4 do { 
line from &lsquo;i&rsquo;th box.nw to &lsquo;i+1&rsquo;th
box.se 
}

<h3>9.2. Naming Objects With Labels
<a name="9.2. Naming Objects With Labels"></a>
</h3>

You can
also specify an object by referring to a label. A label is a
word (which must begin with a capital letter) followed by a
colon; you declare it by placing it immediately before the
object drawing command. For example, the program

.PS
 
A: box &quot;first&quot; &quot;object&quot; 
move; 
B: ellipse &quot;second&quot; &quot;object&quot; 
move; 
arrow right at A .r; 
.PE

declares
labels A and B for its first and second
objects. Here&rsquo;s what that looks like:

Figure
9-1: Example of label use

The
at statement in the fourth line uses the label
A (the behavior of at is explained in the next
section). We&rsquo;ll see later on that labels are most
useful for referring to block composite objects.

Labels are
not constants but variables (you can view colon as a sort of
assignment). You can say something like A: A + (1,0);
and the effect is to reassign the label A to
designate a position one inch to the right of its old
value.
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Code Editor : pic-9.html