Create a std::vector of msgs

edit

Your message contains an array, so you do not need to create and array your self. If you have instantiated it, it already contains the array (which for C++ is serialized as std::vector). Use it like:

parse_and_pub::Lookuptable your_msg_obj;  // declare message instance

// fill lookup table
for ( /* all the values you want to add */ )
{
    your_msg_obj.LuT.push_back( 12.5 /* or whatever double value */ ); 
                          //  your_msg_obj.LuT is of type std::vector<double>
                          //  just treat it like you would thread any other vector of that kind 
                          //     i. e. use .resize(), .push_back(), operator[] on it or 
                          //                   derive iterators on it as you like
}

// and publish your message with filled look up table vector
your_publisher.publish( your_msg_obj );

Update for your comment:

For creating an array of msgs you can do:

std::vector<parse_and_pub::Lookuptable> your_luts;

your_luts.resize(1);
your_luts[0].LuT.push_back( 12.5 );

However, you won't be able to publish such a vector (also not if you use share_ptrs to messages). If you want to publish an arrary of such look up tables you must have another message type containing an array of your lookup tables:

Your Lookuptable.msg:

float64[] Lut

Your 2nd LookuptablesArray.msg:

parse_and_pub/Lookuptable[] look_up_tables ## array of Lookuptable.msg

Then you could create an instance of parse_and_pub::LookuptablesArray fill that and publish that.

To create a vector of messages use

std::vector < parse_and_pub::LookuptableConstPtr> tables;

to populate the look up table use

tables.at(<index>).LuT.push_back(<value>);