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<title>ploticus: select attributes</title>
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<td><br><h2>select attributes</h2></td>
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<small>
<a href="../doc/welcome.html"><img src="../doc/ploticus.gif" border=0></a><br>
Version 2.41 Mar2009
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<title>Manual page for select_attributes(PL)</title>
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<body>
<h2>select attributes</h2>
These are used to select certain records from the
<a href="dataformat.html#currentds">
current data set.<tt> </tt>
</a>
Many of the ploticus procs support a <tt>select</tt> attribute.<tt> </tt>
<br><br><br>
<h2>Examples</h2>
<p>
<tt>select: @@2 = A</tt>
(select all records where 2nd field is A)
<p>
<tt>select: @@2 < 100</tt>
(select all records where 2nd field less than 100)
<p>
<tt>select: @@group in C,D</tt>
(select all records where the <tt>group</tt> field is <tt>C</tt> or <tt>D</tt>.<tt> </tt>
this assumes that data fields have been given names)
<p>
<tt>select: @@group in C,D && @@sex = f</tt>
(select all records where the <tt>group</tt> field is <tt>C</tt> or <tt>D</tt> and
the <tt>sex</tt> field is <tt>f</tt>.<tt> </tt>
<br><br><br>
<h2>Notes</h2>
As seen in the above examples, two at-signs (<tt>@@</tt>) must be used when the <tt>select</tt>
statement resides within a ploticus script and will be subjected to the script interpreter.<tt> </tt>
Character constants should not be enclosed in quotes.<tt> </tt>
Parentheses may not be used to establish precedence.<tt> </tt>
<br><br><br>
<h2>Operands</h2>
Operands may be
<ul>
<li>
field numbers, e.g. <tt>@@1</tt> is the first data field
<li>
field names, e.g. <tt>@@group</tt> (field names must be in effect)
<li>
Literal values (numerics, unquoted strings,
<a href="commalist.html">
comma-delimited lists
</a>
)
<li>
Ploticus @variables or variables that you set
(use one at-sign)
<li>
<a href="functions.html">
functions
</a>
<br><br><br>
</ul>
<h2>Comparison operators</h2>
<dl>
<dt><dd><p>
<pre>
<b>= </b> Equal to. This is case sensitive for strings.
<b>!=</b> Not equal to.
<b>> </b> Greater than.
<b>>=</b> Greater than or equal to.
<b>< </b> Less than.
<b><=</b> Less than or equal to.
</pre>
</dl>
<p>
Data types: if both sides of a comparison are numeric, a numeric comparison will be done.<tt> </tt>
Otherwise, a string comparison will be done.<tt> </tt>
<p>
<b>Wild card matching</b>: Wild card matching may be done
using <b>like</b>. Wild card characters are <b>*</b> which matches
any number of any character, and <b>?</b> which matches a single
instance of any character. This matching is case-insensitive.<tt> </tt>
<dl>
<dt><dd><p>
<pre>
<b>like</b> Wild card match. Example: hello like h* (true)
<b>!like</b> Not a wild card match.
</pre>
</dl>
<p>
<b>Commalist operators</b>: these take a
<a href="commalist.html">
commalist
</a>
on the right side.<tt> </tt>
<dl>
<dt><dd><p>
<pre>
<b>in</b> Member of list. Example: z in x,y,z (true)
<b>!in</b> Not a member of list. (Alt: <b>ni</b>)
<b>inlike</b> Same as <b>in</b> but wild card matching is used.
(Wild cards may be used in list members.)
<b>!inlike</b> Same as <b>!in</b> but wild card matching is used.
</pre>
<br><br><br>
</dl>
<h2>Logical connectors</h2>
Individual conditional expressions may be connected together using
logical AND (<b>&&</b>) or OR (<b>||</b>).<tt> </tt>
An entire expression may be negated by putting <b>not:</b> at the beginning
of the expression.<tt> </tt>
<p>
Because parentheses may not be used to establish precedence
mixing AND and OR in the same expression requires care.<tt> </tt>
When the expression is parsed, it is first
split into OR terms, then each of the OR terms is split into AND terms.<tt> </tt>
For example: <tt>A = B and C = D or E = E of F = G</tt> would evaluate to true,
because it is interpreted as if it were written
<tt>(a = B and C = D) or (e = e) or (f = g)</tt>.<tt> </tt>
It may be best to avoid mixing AND and OR in the same expression
and use multiple expressions instead.<tt> </tt>
<br><br><br>
<h2>Limitations and bugs</h2>
<p>
Because parentheses may not be used to establish precedence,
the mixing of AND and OR in the same expression is problematic (see above).<tt> </tt>
<br>
<br>
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