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POGenerating default value when none is found
 

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    <p>I have an input vector that can have any size between empty and 3 elements. I want the generated string to always be 3 floats separated by spaces, where a default value is used if there aren't enough elements in the vector. So far I've managed to output only the contents of the vector:</p> <pre><code>#include &lt;iostream&gt; #include &lt;iterator&gt; #include &lt;vector&gt; #include "boost/spirit/include/karma.hpp" namespace karma = boost::spirit::karma; namespace phx = boost::phoenix; typedef std::back_insert_iterator&lt;std::string&gt; BackInsertIt; int main( int argc, char* argv[] ) { std::vector&lt;float&gt; input; input.push_back(1.0f); input.push_back(2.0f); struct TestGram : karma::grammar&lt;BackInsertIt, std::vector&lt;float&gt;(), karma::space_type&gt; { TestGram() : TestGram::base_type(output) { using namespace karma; floatRule = double_; output = repeat(3)[ floatRule ]; } karma::rule&lt;BackInsertIt, std::vector&lt;float&gt;(), karma::space_type&gt; output; karma::rule&lt;BackInsertIt, float(), karma::space_type&gt; floatRule; } testGram; std::string output; BackInsertIt sink(output); karma::generate_delimited( sink, testGram, karma::space, input ); std::cout &lt;&lt; "Generated: " &lt;&lt; output &lt;&lt; std::endl; std::cout &lt;&lt; "Press enter to exit" &lt;&lt; std::endl; std::cin.get(); return 0; } </code></pre> <p>I've tried modifying the float rule to something like this: <code>floatRule = double_ | lit(0.0f)</code>, but that only gave me compilation errors. The same for a lot of other similar stuff I tried.</p> <p>I really have no idea how to get this working. Some help would be great :)</p> <p><strong>EDIT:</strong> Just to make it clear. If I have a vector containing 2 elements: 1.0 and 2.0, I want to generate a string that looks like this: <code>"1.0 2.0 0.0"</code> (the last value should be the default value).</p>
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