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<html>
   
   <head>
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      <link rel="stylesheet" type="text/css" href="sun-style.css">
      <title>Algebraic Expression Syntax</title>
   </head>
   
   <body>
      <hr>
      <a href="jel-colref.html">Next</a> <a href="plotSuffixes.html">Previous</a> <a href="index.html">Up</a> <a href="index.html">Contents</a> <br> <b>Next: </b><a href="jel-colref.html">Referencing Column Values</a><br>
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      <hr>
      <hr>
      <h2><a name="jel">10 Algebraic Expression Syntax</a></h2>
      <p>Many of the STILTS commands 
         allow you to use algebraic expressions based on table columns when doing
         things like making row selections, defining new columns, selecting values
         to plot or match, and so on.
         In these cases you are defining an expression which
         has a value in each row as a function of the values in the existing
         columns in that row.
         This is a powerful feature which permits you to manipulate and select
         table data in very flexible ways.
         The syntax for entering these expressions is explained in this section.
         
      </p>
      <p>What you write are actually expressions in
         the Java language, which are compiled into Java bytecode before
         evaluation.  However, this does not mean that you need to be a
         Java programmer to write them.  The syntax is pretty similar to C,
         but even if you've never programmed in C most simple things,
         and many complicated ones, are quite intutitive.
         
      </p>
      <p>The following explanation gives
         some guidance and <a href="jelExamples.html">examples</a>
         for writing these expressions.
         Unfortunately a complete tutorial on writing Java is beyond
         the scope of this document, but it should provide enough information
         for even a novice to write useful expressions.
         
      </p>
      <p>The expressions that you can write are basically any function
         of all the column values which apply
         to a given row; the function result can then be used where STILTS
         needs a per-row value, for instance to define a new column.
         If the built-in operators and functions are not sufficient,
         or it's unwieldy to express your function in one line of code,
         it is possible to add new functions by writing your own classes -
         see <a href="jelExtend.html">Section 10.7.3</a>.
         
      </p>
      <p>Note that since these algebraic expressions often contain spaces,
         you may need to enclose them in single or double quotes so that
         they don't get confused with other parts of the command string.
         
      </p>
      <p><strong>Note:</strong> if Java is running in an environment with
         certain security restrictions (a security manager which
         does not permit creation of custom class loaders) then algebraic
         expressions won't work at all.  It's not particularly likely
         that security restrictions will be in place if you are running
         from the command line though.
         
      </p>
      <ul>
         <li><a href="jel-colref.html">10.1 Referencing Column Values</a></li>
         <li><a href="jel-paramref.html">10.2 Referencing Parameter Values</a></li>
         <li><a href="sec10.3.html">10.3 Null Values</a></li>
         <li><a href="sec10.4.html">10.4 Operators</a></li>
         <li><a href="staticMethods.html">10.5 Functions</a></li>
         <ul>
            <li><a href="uk.ac.starlink.ttools.func.Strings.html">10.5.1 Strings</a></li>
            <li><a href="uk.ac.starlink.ttools.func.Tilings.html">10.5.2 Tilings</a></li>
            <li><a href="uk.ac.starlink.ttools.func.Fluxes.html">10.5.3 Fluxes</a></li>
            <li><a href="uk.ac.starlink.ttools.func.Distances.html">10.5.4 Distances</a></li>
            <li><a href="uk.ac.starlink.ttools.func.Times.html">10.5.5 Times</a></li>
            <li><a href="uk.ac.starlink.ttools.func.Arrays.html">10.5.6 Arrays</a></li>
            <li><a href="uk.ac.starlink.ttools.func.Arithmetic.html">10.5.7 Arithmetic</a></li>
            <li><a href="uk.ac.starlink.ttools.func.TrigDegrees.html">10.5.8 TrigDegrees</a></li>
            <li><a href="uk.ac.starlink.ttools.func.KCorrections.html">10.5.9 KCorrections</a></li>
            <li><a href="uk.ac.starlink.ttools.func.Lists.html">10.5.10 Lists</a></li>
            <li><a href="uk.ac.starlink.ttools.func.Coverage.html">10.5.11 Coverage</a></li>
            <li><a href="uk.ac.starlink.ttools.func.CoordsRadians.html">10.5.12 CoordsRadians</a></li>
            <li><a href="uk.ac.starlink.ttools.func.Conversions.html">10.5.13 Conversions</a></li>
            <li><a href="uk.ac.starlink.ttools.func.CoordsDegrees.html">10.5.14 CoordsDegrees</a></li>
            <li><a href="uk.ac.starlink.ttools.func.Maths.html">10.5.15 Maths</a></li>
            <li><a href="uk.ac.starlink.ttools.func.Formats.html">10.5.16 Formats</a></li>
         </ul>
         <li><a href="jelExamples.html">10.6 Examples</a></li>
         <li><a href="jelAdvanced.html">10.7 Advanced Topics</a></li>
         <ul>
            <li><a href="sec10.7.1.html">10.7.1 Expression evaluation</a></li>
            <li><a href="instanceMethods.html">10.7.2 Instance Methods</a></li>
            <li><a href="jelExtend.html">10.7.3 Adding User-Defined Functions</a></li>
         </ul>
      </ul>
      <hr><a href="jel-colref.html">Next</a> <a href="plotSuffixes.html">Previous</a> <a href="index.html">Up</a> <a href="index.html">Contents</a> <br> <b>Next: </b><a href="jel-colref.html">Referencing Column Values</a><br>
       <b>Up: </b><a href="index.html">Top</a><br>
       <b>Previous: </b><a href="plotSuffixes.html">Parameter Suffixes</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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