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How to "flip" mdspan using submdspan (or any other utility)? #352

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Sunday111 opened this issue Jun 30, 2024 · 0 comments
Open

How to "flip" mdspan using submdspan (or any other utility)? #352

Sunday111 opened this issue Jun 30, 2024 · 0 comments

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@Sunday111
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I've been experimenting with mdspan for a few days, and one thing remains unclear to me: Is it possible to use submdspan to create an mdspan that "views" the original data in reverse order?

For example, consider a span with 9 rows and 10 columns:

constexpr int num_rows = 9;
constexpr int num_columns = 10;
auto data = std::views::iota(0, num_columns * num_rows) | std::ranges::to<std::vector>();
const exstd::mdspan<const int, exstd::extents<int, num_rows, num_columns>, exstd::layout_right> span{data.data()};

This represents the following table:

      0,   1,   2,   3,   4,   5,   6,   7,   8,   9
     10,  11,  12,  13,  14,  15,  16,  17,  18,  19
     20,  21,  22,  23,  24,  25,  26,  27,  28,  29
     30,  31,  32,  33,  34,  35,  36,  37,  38,  39
     40,  41,  42,  43,  44,  45,  46,  47,  48,  49
     50,  51,  52,  53,  54,  55,  56,  57,  58,  59
     60,  61,  62,  63,  64,  65,  66,  67,  68,  69
     70,  71,  72,  73,  74,  75,  76,  77,  78,  79
     80,  81,  82,  83,  84,  85,  86,  87,  88,  89

Is there a way to call the submdspan function to get the same data but with the rows flipped?

     80,  81,  82,  83,  84,  85,  86,  87,  88,  89
     70,  71,  72,  73,  74,  75,  76,  77,  78,  79
     60,  61,  62,  63,  64,  65,  66,  67,  68,  69
     50,  51,  52,  53,  54,  55,  56,  57,  58,  59
     40,  41,  42,  43,  44,  45,  46,  47,  48,  49
     30,  31,  32,  33,  34,  35,  36,  37,  38,  39
     20,  21,  22,  23,  24,  25,  26,  27,  28,  29
     10,  11,  12,  13,  14,  15,  16,  17,  18,  19
      0,   1,   2,   3,   4,   5,   6,   7,   8,   9

I tried to achieve this by passing a negative stride into strided_slice, but it returned incorrect results (with negative extents):

exstd::submdspan(span, exstd::strided_slice{.offset = 5, .extent = 5, .stride = -1}, exstd::full_extent)

At this point, I think I do not understand how this is supposed to be done. As far as I understand, the original intention of this proposal was to have a tool similar to indexing in NumPy, MATLAB, or Fortran, which can handle this task easily.

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