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Fix vstack for mixed fixed-shape inputs #2897

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Fix vstack for mixed fixed-shape inputs #2897

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4d58987
Add vstack functionality for fixed shape tensors in xbuilder
Alex-PLACET Apr 28, 2026
b833209
Add documentation
Alex-PLACET Apr 28, 2026
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Alex-PLACET Apr 28, 2026
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35 changes: 35 additions & 0 deletions include/xtensor/generators/xbuilder.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -910,6 +910,26 @@ namespace xt

namespace detail
{
template <class S>
struct vstack_fixed_shape;

template <std::size_t N>
struct vstack_fixed_shape<fixed_shape<N>>
{
using type = fixed_shape<1, N>;
};

template <std::size_t I, std::size_t... J>
struct vstack_fixed_shape<fixed_shape<I, J...>>
{
using type = fixed_shape<I, J...>;
};

template <class... CT>
using vstack_fixed_shape_t = concat_fixed_shape_t<
0,
typename vstack_fixed_shape<typename std::decay_t<CT>::shape_type>::type...>;
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@JohanMabille JohanMabille May 11, 2026

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This is not following the standard naming pattern, where usually xxx_t = typename xxx::type. Renaming vstack_fixed_shape into vstack_fixed_shape_impl, and having something like:

template <class... CT>
struct vstack_fixed_shape
{
     using type = concat_fixed_shape_t<0, typename vstack_fixed_shape_impl<typename std::decay_t<CT>::shape_type>::type...>;
};

template <class... CT>
using vstack_fixed_shape_t = typename vstack_fixed_shape_t<CT...>::type

would be more consistent with the standard


template <class S, class... CT>
inline auto vstack_shape(std::tuple<CT...>& t, const S& shape)
{
Expand Down Expand Up @@ -948,6 +968,21 @@ namespace xt
return detail::make_xgenerator(detail::vstack_impl<CT...>(std::move(t), size_t(0)), new_shape);
}

/**
* @brief Stack fixed-shape xexpressions in sequence vertically (row wise).
* This overload preserves the result shape at compile time by treating
* 1-D fixed shapes as ``(1, N)`` row vectors before concatenation.
*
* @param t \ref xtuple of fixed-shape xexpressions to stack
* @return xgenerator evaluating to stacked elements with a fixed compile-time shape
*/
template <fixed_shape_container_concept... CT>
inline auto vstack(std::tuple<CT...>&& t)
{
using shape_type = detail::vstack_fixed_shape_t<CT...>;
return detail::make_xgenerator(detail::vstack_impl<CT...>(std::move(t), size_t(0)), shape_type{});
}

namespace detail
{

Expand Down
15 changes: 15 additions & 0 deletions test/test_xbuilder.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -424,6 +424,21 @@ namespace xt
ASSERT_TRUE(arange(8) == w1);
ASSERT_TRUE(w1 == w2);
}

TEST(xbuilder, vstack_fixed)
{
xtensor_fixed<float, fixed_shape<1, 2>> a = {{1.f, 2.f}};
xtensor_fixed<float, fixed_shape<2, 2>> b = {{3.f, 4.f}, {5.f, 6.f}};

auto c = vstack(xtuple(a, b));

using expected_shape_t = fixed_shape<3, 2>;
ASSERT_EQ(expected_shape_t{}, c.shape());
EXPECT_EQ(1.f, c(0, 0));
EXPECT_EQ(2.f, c(0, 1));
EXPECT_EQ(3.f, c(1, 0));
EXPECT_EQ(6.f, c(2, 1));
}
#endif

TEST(xbuilder, access)
Expand Down
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