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<h2>11. Object Groups
<a name="11. Object Groups"></a>
</h2>


<p style="margin-top: 1em"><font color="#000000">There are
two different ways to group objects in <b>pic</b>; <i>brace
grouping</i> and <i>block composites</i>.</font></p>

<h3>11.1. Brace Grouping
<a name="11.1. Brace Grouping"></a>
</h3>


<p style="margin-top: 1em"><font color="#000000">The
simpler method is simply to group a set of objects within
curly bracket or brace characters. On exit from this
grouping, the current position and direction are restored to
their value when the opening brace was
encountered.</font></p>

<h3>11.2. Block Composites
<a name="11.2. Block Composites"></a>
</h3>


<p style="margin-top: 1em"><font color="#000000">A block
composite object is created a series of commands enclosed by
square brackets. The composite can be treated for most
purposes like a single closed object, with the size and
shape of its bounding box. Here is an example. The program
fragment</font></p>

<pre style="margin-left:10%; margin-top: 1em">A: [
    circle;
    line up 1 at last circle .n;
    line down 1 at last circle .s;
    line right 1 at last circle .e;
    line left 1 at last circle .w;
    box dashed with .nw at last circle .se + (0.2, -0.2);
    Caption: center of last box;
]</pre>


<p style="margin-top: 1em"><font color="#000000">yields the
block in figure 11-1, which we show both with and without
its attachment points. The block&rsquo;s location becomes
the value of <b>A</b>.</font></p>


<p align="center" style="margin-top: 1em"><font color="#000000"><img src="img/pic39.png" alt="Image img/pic39.png"></font></p>


<p align="center" style="margin-top: 1em"><font color="#000000">Figure
11-1: A sample composite object</font></p>

<p style="margin-top: 1em"><font color="#000000">To refer
to one of the composite&rsquo;s attachment points, you can
say (for example) <b>A .s</b>. For purposes of object
naming, composites are a class. You could write <b>last []
.s</b> as an equivalent reference, usable anywhere a
location is needed. This construction is very important for
putting together large, multi-part diagrams.</font></p>

<p style="margin-top: 1em"><font color="#000000">Blocks are
also a variable-scoping mechanism, like a <i>groff</i>(1)
environment. All variable assignments done inside a block
are undone at the end of it. To get at values within a
block, write a name of the block followed by a dot, followed
by the label you want. For example, we could refer the the
center of the box in the above composite as <b>last []
.Caption</b> or <b>A.Caption</b>.</font></p>

<p style="margin-top: 1em"><font color="#000000">This kind
of reference to a label can be used in any way any other
location can be. For example, if we added <b>&quot;Hi!&quot;
at A.Caption</b> the result would look like this:</font></p>


<p align="center" style="margin-top: 1em"><font color="#000000"><img src="img/pic40.png" alt="Image img/pic40.png"></font></p>


<p align="center" style="margin-top: 1em"><font color="#000000">Figure
11-2: Adding a caption using interior labeling</font></p>

<p style="margin-top: 1em"><font color="#000000">You can
also use interior labels in either part of a <b>with</b>
modifier. This means that the example composite could be
placed relative to its caption box by a command containing
<b>with A.Caption at</b>.</font></p>

<p style="margin-top: 1em"><font color="#000000">Note that
both width and height of the block composite object are
always positive:</font></p>


<p align="center" style="margin-top: 1em"><font color="#000000"><img src="img/pic41.png" alt="Image img/pic41.png"></font></p>


<p align="center" style="margin-top: 1em"><font color="#000000">Figure
11-3: Composite block objects always have positive width and
height</font></p>

<p style="margin-top: 1em"><font color="#000000">Blocks may
be nested. This means you can use block attachment points to
build up complex diagrams hierarchically, from the inside
out. Note that <b>last</b> and the other sequential naming
mechanisms don&rsquo;t look inside blocks, so if you have a
program that looks like</font></p>

<pre style="margin-left:10%; margin-top: 1em">.PS
P: [box &quot;foo&quot;; ellipse &quot;bar&quot;];
Q: [
     [box &quot;baz&quot;; ellipse &quot;quxx&quot;]
     &quot;random text&quot;;
   ]
arrow from 2nd last [];
.PE</pre>


<p style="margin-top: 1em"><font color="#000000">the arrow
in the last line is attached to object <b>P</b>, not object
<b>Q</b>.</font></p>

<p style="margin-top: 1em"><font color="#000000">In DWB
<b>pic</b>, only references one level deep into enclosed
blocks were permitted. GNU <b>gpic</b> removes this
restriction.</font></p>

<p style="margin-top: 1em"><font color="#000000">The
combination of block variable scoping, assignability of
labels and the macro facility that we&rsquo;ll describe
later on can be used to simulate functions with local
variables (just wrap the macro body in block
braces).</font></p>
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