python - NumPy:将索引选择功能应用于多个轴

Question

我正在编写代码来优化取决于可变数量参数的数量。为了优化，我想一次跨多个轴应用索引选择函数，例如 numpy.argmax 和 numpy.argmin。下面是我现在正在使用的代码。是否有更内置或更有效的方法来跨任意数量的轴执行此任务，这些轴可能是顺序的，也可能不是顺序的？

def nd_arg_axes(func, array, start):
    """Applies an index selecting function over trailing axes from start."""

    n_trail = len(array.shape[start:])  # Number of trailing axes to apply to.

    indices = np.zeros((n_trail,)+array.shape[:start], dtype=np.intp)
    for i in np.ndindex(array.shape[:start]):
        indices[(Ellipsis,)+i] = np.unravel_index(func(array[i]),
                                               array.shape[start:])
    return tuple(indices)

# Test showing nd_arg_axes does indeed return the correct indices.
array = np.arange(27).reshape(3,3,3)
max_js = nd_arg_axes(np.argmax, array, 1)

(array[tuple(np.indices(array))+max_js] ==
np.squeeze(np.apply_over_axes(np.amax, array, axes=[1,2])))

Answer 1

如果您选择了尾随轴，您可以将尾随轴的形状 reshape 为 -1，并将 func 应用于 axis=-1:

def f(func, array, start):
    shape = array.shape
    tmp = array.reshape(shape[:start] + (-1,))
    indices = func(tmp, axis=-1)
    return np.unravel_index(indices, shape[start:])