# Tensor Rank

## Rank overload

All xtensor’s classes have a member `rank` that can be used to overload based on rank using SFINAE. Consider the following example:

    template <class E, std::enable_if_t<!xt::has_rank_t<E, 2>::value, int> = 0>
    inline E foo(E&& a)
    {
        ... // act on object of flexible rank, or fixed rank != 2
    }

    template <class E, std::enable_if_t<xt::has_rank_t<E, 2>::value, int> = 0>
    inline E foo(E&& a)
    {
        ... // act on object of fixed rank == 2
    }

    int main()
    {
        xt::xarray<size_t> a = {{9, 9}, {9, 9}};
        xt::xtensor<size_t, 1> b = {9, 9};
        xt::xtensor<size_t, 2> c = {{9, 9}, {9, 9}};

        foo(a); // flexible rank -> first overload
        foo(b); // fixed rank == 2 -> first overload
        foo(c); // fixed rank == 2 -> second overload

        return 0;
    }

Note

If one wants to test for more than a single value for `rank`, one can use the default value `SIZE_MAX` used for flexible rank objects. For example, one could have the following overloads:

    // flexible rank
    template <class E, std::enable_if_t<!xt::has_fixed_rank_t<E>::value, int> = 0>
    inline E foo(E&& a);

    // fixed rank == 1
    template <class E, std::enable_if_t<xt::has_rank_t<E, 1>::value, int> = 0>
    inline E foo(E&& a);

    // fixed rank == 2
    template <class E, std::enable_if_t<xt::has_rank_t<E, 2>::value, int> = 0>
    inline E foo(E&& a);

Note that fixed ranks other than 1 and 2 will raise a compiler error.

Of course, if one wants a more limited scope, one could also do the following:

    // flexible rank
    inline void foo(xt::xarray<double>& a);

    // fixed rank == 1
    inline void foo(xt::xtensor<double,1>& a);

    // fixed rank == 2
    inline void foo(xt::xtensor<double,2>& a);

## Rank as member

If you want to use the rank as a member of your own class you can use `xt::get_rank<E>`. Consider the following example:

    template <class T>
    struct Foo
    {
        static const size_t rank = xt::get_rank<T>::value;

        static size_t value()
        {
            return rank;
        }
    };

    int main()
    {
        xt::xtensor<double, 1> A = xt::zeros<double>({2});
        xt::xtensor<double, 2> B = xt::zeros<double>({2, 2});
        xt::xarray<double> C = xt::zeros<double>({2, 2});

        assert(Foo<decltype(A)>::value() == 1);
        assert(Foo<decltype(B)>::value() == 2);
        assert(Foo<decltype(C)>::value() == SIZE_MAX);

        return 0;
    }

`xt::get_rank` ‘returns’ the rank of the xtensor object if its rank is fixed. In all other cases it ‘returns’ `SIZE_MAX`. Indeed `xt::get_rank<xt::array<double>>::value` is equal to `SIZE_MAX`, but equally so is `xt::get_rank<double>::value`.