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<h3><a name="jel-paramref">10.2 Referencing Parameter Values</a></h3>
<p>Some tables have constant values associated with them; these may
represent such things as the epoch at which observations were taken,
the name of the catalogue, an angular resolution associated with all
observations, or any number of other things. Such constants are known as
<em>table parameters</em> (not to be confused with parameters passed
to STILTS commands) and can be thought of as extra columns which have the
same value for every row. The values of such parameters can be referenced
in STILTS algebraic expressions as follows:
<dl>
<dt><strong>param$<em>name</em></strong></dt>
<dd>If the parameter name has a suitable form (starting with a letter
and continuing with letters or numbers) it can be referenced by
prefixing that name with the string <code>param$</code>.
</dd>
<dt><strong>ucd$<em>ucd-spec</em></strong></dt>
<dd>If the parameter has a
<a href="http://www.ivoa.net/Documents/latest/UCD.html">Unified Content Descriptor</a>
it can be referenced by prefixing the UCD specifier with the
string <code>ucd$</code>. Any punctuation marks in the
UCD should be replaced by underscores, and a trailing underscore
is interpreted as a wildcard. See <a href="jel-colref.html">Section 10.1</a> for
more discussion.
</dd>
<dt><strong>utype$<em>utype-spec</em></strong></dt>
<dd>If the parameter has a Utype,
it can be referenced by prefixing the Utype specifier with the
string <code>utype$</code>. Any punctuation marks in the
Utype should be replaced by underscores. See <a href="jel-colref.html">Section 10.1</a> for
more discussion.
</dd>
</dl>
Note that if a parameter has a name in an unsuitable form (e.g. containing
spaces) and has no UCD then it cannot be referenced in an expression.
</p>
<p>There are also a couple of special values:
<dl>
<dt><strong>$ncol</strong></dt>
<dd>The number of columns in the table.
</dd>
<dt><strong>$nrow</strong></dt>
<dd>The number of rows in the table. Note in some cases this is
not known (e.g. if the table is being streamed), in which case
the value of this variable is null.
Note also that this value is a <code>long</code> (8-byte integer);
when using it in certain expressions you may find it necessary to convert
it to an <code>int</code> (4-byte integer) using the
<code>toInteger()</code> function.
</dd>
</dl>
</p>
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<b>Previous: </b><a href="jel-colref.html">Referencing Column Values</a><br>
<hr><i>STILTS - Starlink Tables Infrastructure Library Tool Set<br>Starlink User Note256<br>STILTS web page:
<a href="http://www.starlink.ac.uk/stilts/">http://www.starlink.ac.uk/stilts/</a><br>Author email:
<a href="mailto:m.b.taylor@bristol.ac.uk">m.b.taylor@bristol.ac.uk</a><br>Mailing list:
<a href="mailto:topcat-user@jiscmail.ac.uk">topcat-user@jiscmail.ac.uk</a><br></i></body>
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