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<h1><a class="anchor" id="filter">Selecting Table Data </a></h1><p>This function demonstrates the operation of filtering a table by selecting rows that satisfy a condition and writing them to a new file, or overwriting a table with the filtered data. A third mode, where a filtered dataset is appended to the file containing the source data, will be available shortly, but is currently not supported by cfitsio.</p>
<p>The expression syntax for the conditions that may be applied to table data are described in the <a href="http://heasarc.gsfc.nasa.gov/docs/software/fitsio/fitsio.html">cfitsio manual.</a> In the example below, we illustrate filtering with a boolean expression involving one of the columns.</p>
<p>The two flags at the end of the call to FITS::filter are an `overwrite' flag - which only has meaning if the inFile and outFile are the same, and a 'read' flag. overwrite defaults to true. The second flag is a 'read' flag which defaults to false. When set true the user has immediate access to the filtered data.</p>
<p>(Also see the section "Reading with Extended File Name Syntax")</p>
<div class="fragment"><pre class="fragment"><span class="keywordtype">int</span> selectRows()
{
<span class="keyword">const</span> <span class="keywordtype">string</span> inFile(<span class="stringliteral">"atestfil.fit"</span>);
<span class="keyword">const</span> <span class="keywordtype">string</span> outFile(<span class="stringliteral">"btestfil.fit"</span>);
<span class="keyword">const</span> <span class="keywordtype">string</span> newFile(<span class="stringliteral">"ctestfil.fit"</span>);
<span class="keyword">remove</span>(newFile.c_str());
<span class="comment">// test 1: write to a new file</span>
std::auto_ptr<FITS> pInfile(<span class="keyword">new</span> FITS(inFile,Write,<span class="keywordtype">string</span>(<span class="stringliteral">"PLANETS_ASCII"</span>)));
FITS* infile(pInfile.get());
std::auto_ptr<FITS> pNewfile(<span class="keyword">new</span> FITS(newFile,Write));
ExtHDU& source = infile->extension(<span class="stringliteral">"PLANETS_ASCII"</span>);
<span class="keyword">const</span> <span class="keywordtype">string</span> expression(<span class="stringliteral">"DENSITY > 3.0"</span>);
Table& sink1 = pNewfile->filter(expression,source,<span class="keyword">false</span>,<span class="keyword">true</span>);
std::cout << sink1 << std::endl;
<span class="comment">// test 2: write a new HDU to the current file, overwrite false, read true.</span>
<span class="comment">// AS OF 7/2/01 does not work because of a bug in cfitsio, but does not</span>
<span class="comment">// crash, simply writes a new header to the file without also writing the</span>
<span class="comment">// selected data.</span>
Table& sink2 = infile->filter(expression,source,<span class="keyword">false</span>,<span class="keyword">true</span>);
std::cout << sink2 << std::endl;
<span class="comment">// reset the source file back to the extension in question.</span>
source = infile->extension(<span class="stringliteral">"PLANETS_ASCII"</span>);
<span class="comment">// test 3: overwrite the current HDU with filtered data.</span>
Table& sink3 = infile->filter(expression,source,<span class="keyword">true</span>,<span class="keyword">true</span>);
std::cout << sink3 << std::endl;
<span class="keywordflow">return</span> 0;
}
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