CODEWITHSUNDEEP
c++c++23mdspan
Porsche9II
Porsche9II

Reputation: 661

How to create mdspan when indexing does not begin at 0?

I have binary-files of the form

Inhabitants_Year_1913_2023_Month_1_12_ZipCode_20000_20999.bin

with 100 * 12 * 1000 = 1.2 million lines, each line containing an int representing the number of inhabitants for a given year / month / ZipCode combination. I have a function that parses the file and returns a std::vector<int> with 1.2mn elements. I can create a lightweight view

auto my_md_span = std::mdspan(vec.data(), 100, 12, 1000);

and can pass it around to other functions and then access the right element via

my_md_span[year - 1913, month - 1, ZipCode - 20000]

Question: Is it possible to create a std::mdspan such that I can access the rigth element directly via my_md_span[year, month, ZipCode]?

Upvotes: 2

Views: 81

Answers (1)

Steve
Steve

Reputation: 217

First I would recommend using strong types for your axes:

enum class Year  : uint16_t; // or a std::chrono::year
enum class Month : uint8_t;  /*1-based*/ // or a std::chrono::month
enum class Zip   : uint16_t;

Then wrap (or inherit privately) the mdspan, adding/exposing just the needed method(s):

class DataSet
{
    mdspan< .... > data;
    
public:
    int operator[]( Year y, Month m, Zip z ) const {
        return data[ uint16_t(y)-1913, uint8_t(m) - 1, uint16_t(z) - 20000 ];
    }
    // and of course add a constructor...
};

Upvotes: 2

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