[Boost-users] [multi_array] Slices along the N'th dimension of a multi_array in a generic function

Subject: [Boost-users] [multi_array] Slices along the N'th dimension of a multi_array in a generic function
From: Jesse Perla (jesseperla_at_[hidden])
Date: 2009-06-24 13:54:02

I have a generic function which will take in a boost::multi_array of
dimension size N.  I need to take slices along the M'th < N dimension (known
at compile time).  I can't figure out how to do this with the multi_array
library and associated ranges/array_views.  Is there another way to do this
rather than building a boost::indices object directly?  Besides the sliced
dimension, I will always want the entire ranges for the other ones.

I believe the problem is that the index_gen object needs different types as
we build the indices in parts.  The following code shows what a hardcoded
example would look like for N = 4 and M = 3

    const std::size_t n_1 = 3;
    const std::size_t n_2 = 4;
    const std::size_t n_3 = 5;
    const std::size_t n_4 = 6;

    typedef boost::multi_array<double, 4> array_type;
    array_type vals(boost::extents[n_1][n_2][n_3][n_4]);

    //...fill the array...

    typedef boost::multi_array_types::index_range range;
    typedef boost::multi_array_types::index_gen index_gen_type;

    //Trying to do this dynamically instead, but in pieces.  Note that you
need the different autos at each stage since you can't build an index
directly?
    auto slice_indices2 = boost::indices[range(0,n_1)][range(0,n_2)];
//first dimensions
    auto slice_indices3 = slice_indices2[1]; //Slice dimension.... for the
2nd index.
    auto slice_indices4 = slice_indices3[range(0,n_4)]; //last dimensions

    auto vals_slice3 = vals[ slice_indices4 ]; //Take slices with the final
object.  This is a proper multi_array view
    cout << vals_slice3[0][0][0] << endl;

Thanks, Jesse