How to specify degenerate dimension of boost multi

问题:

I have a 3D multi_array and I would like to make 2D slices using dimensions specified at runtime. I know the index of degenerate dimension and the index of a slice that I want to extract in that degenerate dimension. Currently the ugly workaround looks like that:

if (0 == degenerate_dimension)
{
    Slice slice = input_array[boost::indices[slice_index][range()][range()]];
}
else if (1 == degenerate_dimension)
{
    Slice slice = input_array[boost::indices[range()][slice_index][range()]];
}
else if (2 == degenerate_dimension)
{
    Slice slice = input_array[boost::indices[range()][range()][slice_index]];
}

Is there a more beautiful way to construct index_gen object? Something like that:

var slicer;
for(int i = 0; i < 3; ++i) {
    if (degenerate_dimension == i)
        slicer = boost::indices[slice_index];
    else
        slicer = boost::indices[range()];
}
Slice slice = input_array[slicer];

It seems each subsequent call to boost::indices::operator[] returns a different type depending on the dimensionality (i.e. number of previous calls), so there's no way to use a single variable, that can hold the temporary index_gen object.

回答1:

Please, try this. Сode has one disadvantage - it refers to ranges_ array variable declared at boost::detail:: multi_array namespace.

#include <boost/multi_array.hpp>                                                                                                                              

typedef boost::multi_array<double, 3> array_type;                                                                                                             
typedef boost::multi_array_types::index_gen::gen_type<2,3>::type index_gen_type;                                                                                   
typedef boost::multi_array_types::index_range range;                                                                                                          

index_gen_type                                                                                                                                                     
func(int degenerate_dimension, int slice_index)                                                                                                               
{                                                                                                                                                             
    index_gen_type slicer;                                                                                                                                         
    int i;                                                                                                                                                    
    for(int i = 0; i < 3; ++i) {                                                                                                                              
        if (degenerate_dimension == i)                                                                                                                        
            slicer.ranges_[i] = range(slice_index);                                                                                                           
        else                                                                                                                                                  
            slicer.ranges_[i] = range();                                                                                                                      
    }                                                                                                                                                         
    return slicer;                                                                                                                                            
}                                                                                                                                                             

int main(int argc, char **argv)                                                                                                                               
{                                                                                                                                                             
    array_type myarray(boost::extents[3][3][3]);                                                                                                              
    array_type::array_view<2>::type myview = myarray[ func(2, 1) ];                                                                                           
    return 0;                                                                                                                                                 
}

回答2:

What you're trying to do is move a variable from run time to compile time. This can only be done with a chain of if else statements or a switch statement.

A simplified example

// print a compile time int
template< int I >
void printer( void )
{
   std::cout << I << '\n';
}

// print a run time int
void printer( int i )
{
   // translate a runtime int to a compile time int
   switch( i )
   {
      case 1: printer<1>(); break;
      case 2: printer<2>(); break;
      case 3: printer<3>(); break;
      case 4: printer<4>(); break;
      default: throw std::logic_error( "not implemented" );
   }
}

// compile time ints
enum{ enum_i = 2 };
const int const_i = 3;
constexpr i constexper_i( void ) { return 4; }

// run time ints
extern int func_i( void ); // { return 5; }
extern int global_i; // = 6

int main()
{
   int local_i = 7;
   const int local_const_i = 8;

   printer<enum_i>();
   printer<const_i>();
   printer<constexpr_i()>();
   //printer<func_i()>();
   //printer<global_i>();
   //printer<local_i>();
   printer<local_const_i>();

   printer( enum_i );
   printer( const_i );
   printer( constexpr_i() );
   printer( func_i()      ); // throws an exception
   printer( global_i      ); // throws an exception
   printer( local_i       ); // throws an exception
   printer( local_const_i ); // throws an exception
}