fix: correct negative axis handling in roll function - #2878
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JohanMabille
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May 28, 2026
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Thanks for the fix! Commit cherry picked in #2920 rebased on master. |
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Description
This PR fixes a bug in xt::roll(e, shift, axis) where negative axis indices (e.g., -1 for the last axis) were incorrectly rejected.
The Bug
include/xtensor/misc/xmanipulation.hpp
The original code converted axis to size_t before normalization:
This caused valid negative indices like -1 to incorrectly trigger the bounds check exception.
The Fix
include/xtensor/misc/xmanipulation.hpp
auto cpy = empty_like(e); const auto& shape = cpy. shape(); - std::size_t saxis = static_cast<std::size_t>(axis); - if (axis < 0) - { - axis += std::ptrdiff_t(cpy. dimension()); - } + const auto dim = cpy.dimension(); - if (saxis >= cpy.dimension() || axis < 0) + if (axis < -static_cast<std::ptrdiff_t>(dim) || axis >= static_cast<std::ptrdiff_t>(dim)) { - XTENSOR_THROW(std::runtime_error, "axis is no within shape dimension."); + XTENSOR_THROW(std::runtime_error, "axis is not within shape dimension."); } + std::size_t saxis = normalize_axis(dim, axis); + const auto axis_dim = static_cast<std::ptrdiff_t>(shape[saxis]);Tests Added
test/test_xmanipulation.cpp
xarray<double> expected8 = {{{3, 1, 2}}, {{6, 4, 5}}, {{9, 7, 8}}}; ASSERT_EQ(expected8, xt::roll(e2, -2, /*axis*/ 2)); + // Boundary error cases + EXPECT_THROW(xt::roll(e2, 1, /*axis*/ 3), std::runtime_error); + EXPECT_THROW(xt::roll(e2, 1, /*axis*/ -4), std::runtime_error); + + // Negative axis indices + xarray<double> expected9 = {{{3, 1, 2}}, {{6, 4, 5}}, {{9, 7, 8}}}; + ASSERT_EQ(expected9, xt::roll(e2, -2, /*axis*/ -1)); + + xarray<double> expected10 = {{{1, 2, 3}}, {{4, 5, 6}}, {{7, 8, 9}}}; + ASSERT_EQ(expected10, xt::roll(e2, -2, /*axis*/ -2)); + + xarray<double> expected11 = {{{4, 5, 6}}, {{7, 8, 9}}, {{1, 2, 3}}}; + ASSERT_EQ(expected11, xt::roll(e2, 2, /*axis*/ -3)); }Note: This bug has existed since #1823 (2019).