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<a name="Functions-and-Variables-for-Structures"></a>
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Previous: <a href="maxima_26.html#Introduction-to-Structures" accesskey="p" rel="previous">Introduction to Structures</a>, Up: <a href="maxima_25.html#Structures" accesskey="u" rel="up">Structures</a> [<a href="maxima_toc.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="maxima_423.html#Function-and-Variable-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Functions-and-Variables-for-Structures-1"></a>
<h4 class="subsection">5.6.2 Functions and Variables for Structures</h4>
<a name="structures"></a><a name="Item_003a-defstruct_002fdefvr_002fstructures"></a><dl>
<dt><a name="index-structures"></a>Global variable: <strong>structures</strong></dt>
<dd>
<p><code>structures</code> is the list of user-defined structures defined by <code>defstruct</code>.
</p>
<div class=categorybox>
Categories:<a href="maxima_424.html#Category_003a-Structures">Structures</a>
·<a href="maxima_424.html#Category_003a-Global-variables">Global variables</a>
·</div></dd></dl>
<a name="defstruct"></a><a name="Item_003a-defstruct_002fdeffn_002fdefstruct"></a><dl>
<dt><a name="index-defstruct"></a>Function: <strong>defstruct</strong> <em><br> <tt>defstruct</tt> (<var>S</var>(<var>a_1</var>, …, <var>a_n</var>)) <br> <tt>defstruct</tt> (<var>S</var>(<var>a_1</var> = <var>v_1</var>, …, <var>a_n</var> = <var>v_n</var>))</em></dt>
<dd>
<p>Define a structure, which is a list of named fields <var>a_1</var>, …,
<var>a_n</var> associated with a symbol <var>S</var>.
An instance of a structure is just an expression which has operator <var>S</var>
and exactly <code>n</code> arguments.
<code>new(<var>S</var>)</code> creates a new instance of structure <var>S</var>.
</p>
<p>An argument which is just a symbol <var>a</var> specifies the name of a field.
An argument which is an equation <code><var>a</var> = <var>v</var></code> specifies the field name <var>a</var>
and its default value <var>v</var>.
The default value can be any expression.
</p>
<p><code>defstruct</code> puts <var>S</var> on the list of user-defined structures, <code>structures</code>.
</p>
<p><code>kill(<var>S</var>)</code> removes <var>S</var> from the list of user-defined structures,
and removes the structure definition.
</p>
<p>Examples:
</p>
<div class="example">
<pre class="example">(%i1) defstruct (foo (a, b, c));
(%o1) [foo(a, b, c)]
(%i2) structures;
(%o2) [foo(a, b, c)]
(%i3) new (foo);
(%o3) foo(a, b, c)
(%i4) defstruct (bar (v, w, x = 123, y = %pi));
(%o4) [bar(v, w, x = 123, y = %pi)]
(%i5) structures;
(%o5) [foo(a, b, c), bar(v, w, x = 123, y = %pi)]
(%i6) new (bar);
(%o6) bar(v, w, x = 123, y = %pi)
(%i7) kill (foo);
(%o7) done
(%i8) structures;
(%o8) [bar(v, w, x = 123, y = %pi)]
</pre></div>
<div class=categorybox>
Categories:<a href="maxima_424.html#Category_003a-Structures">Structures</a>
·</div></dd></dl>
<a name="new"></a><a name="Item_003a-defstruct_002fdeffn_002fnew"></a><dl>
<dt><a name="index-new"></a>Function: <strong>new</strong> <em><br> <tt>new</tt> (<var>S</var>) <br> <tt>new</tt> (<var>S</var> (<var>v_1</var>, …, <var>v_n</var>))</em></dt>
<dd>
<p><code>new</code> creates new instances of structures.
</p>
<p><code>new(<var>S</var>)</code> creates a new instance of structure <var>S</var>
in which each field is assigned its default value, if any,
or no value at all if no default was specified in the structure definition.
</p>
<p><code>new(<var>S</var>(<var>v_1</var>, ..., <var>v_n</var>))</code> creates a new instance of <var>S</var>
in which fields are assigned the values <var>v_1</var>, …, <var>v_n</var>.
</p>
<p>Examples:
</p>
<div class="example">
<pre class="example">(%i1) defstruct (foo (w, x = %e, y = 42, z));
(%o1) [foo(w, x = %e, y = 42, z)]
(%i2) new (foo);
(%o2) foo(w, x = %e, y = 42, z)
(%i3) new (foo (1, 2, 4, 8));
(%o3) foo(w = 1, x = 2, y = 4, z = 8)
</pre></div>
<div class=categorybox>
Categories:<a href="maxima_424.html#Category_003a-Structures">Structures</a>
·</div></dd></dl>
<a name="Item_003a-defstruct_002fdeffn_002f_0040"></a><dl>
<dt><a name="index-_0040"></a>Operator: <strong>@</strong></dt>
<dd>
<p><code>@</code> is the structure field access operator.
The expression <code><var>x</var>@ <var>a</var></code> refers to the value of field <var>a</var> of the structure instance <var>x</var>.
The field name is not evaluated.
</p>
<p>If the field <var>a</var> in <var>x</var> has not been assigned a value,
<code><var>x</var>@ <var>a</var></code> evaluates to itself.
</p>
<p><code>kill(<var>x</var>@ <var>a</var>)</code> removes the value of field <var>a</var> in <var>x</var>.
</p>
<p>Examples:
</p>
<div class="example">
<pre class="example">(%i1) defstruct (foo (x, y, z));
(%o1) [foo(x, y, z)]
(%i2) u : new (foo (123, a - b, %pi));
(%o2) foo(x = 123, y = a - b, z = %pi)
(%i3) u@z;
(%o3) %pi
(%i4) u@z : %e;
(%o4) %e
(%i5) u;
(%o5) foo(x = 123, y = a - b, z = %e)
(%i6) kill (u@z);
(%o6) done
(%i7) u;
(%o7) foo(x = 123, y = a - b, z)
(%i8) u@z;
(%o8) u@z
</pre></div>
<p>The field name is not evaluated.
</p>
<div class="example">
<pre class="example">(%i1) defstruct (bar (g, h));
(%o1) [bar(g, h)]
(%i2) x : new (bar);
(%o2) bar(g, h)
(%i3) x@h : 42;
(%o3) 42
(%i4) h : 123;
(%o4) 123
(%i5) x@h;
(%o5) 42
(%i6) x@h : 19;
(%o6) 19
(%i7) x;
(%o7) bar(g, h = 19)
(%i8) h;
(%o8) 123
</pre></div>
<div class=categorybox>
Categories:<a href="maxima_424.html#Category_003a-Structures">Structures</a>
·<a href="maxima_424.html#Category_003a-Operators">Operators</a>
·</div></dd></dl>
<hr>
<div class="header">
<p>
Previous: <a href="maxima_26.html#Introduction-to-Structures" accesskey="p" rel="previous">Introduction to Structures</a>, Up: <a href="maxima_25.html#Structures" accesskey="u" rel="up">Structures</a> [<a href="maxima_toc.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="maxima_423.html#Function-and-Variable-Index" title="Index" rel="index">Index</a>]</p>
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