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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’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’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>"Hi!"
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’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 "foo"; ellipse "bar"];
Q: [
[box "baz"; ellipse "quxx"]
"random text";
]
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’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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