Function argpartition

numpy/_core/fromnumeric.py:849–932

Perform an indirect partition along the given axis using the algorithm specified by the `kind` keyword. It returns an array of indices of the same shape as `a` that index data along the given axis in partitioned order. Parameters ---------- a : array_like Array

(a, kth, axis=-1, kind='introselect', order=None)

@array_function_dispatch(_argpartition_dispatcher)
def argpartition(a, kth, axis=-1, kind='introselect', order=None):
    """
    Perform an indirect partition along the given axis using the
    algorithm specified by the `kind` keyword. It returns an array of
    indices of the same shape as `a` that index data along the given
    axis in partitioned order.

    Parameters
    ----------
    a : array_like
        Array to sort.
    kth : int or sequence of ints
        Element index to partition by. The k-th element will be in its
        final sorted position and all smaller elements will be moved
        before it and all larger elements behind it. The order of all
        elements in the partitions is undefined. If provided with a
        sequence of k-th it will partition all of them into their sorted
        position at once.

    axis : int or None, optional
        Axis along which to sort. The default is -1 (the last axis). If
        None, the flattened array is used.
    kind : {'introselect'}, optional
        Selection algorithm. Default is 'introselect'
    order : str or list of str, optional
        When `a` is an array with fields defined, this argument
        specifies which fields to compare first, second, etc. A single
        field can be specified as a string, and not all fields need be
        specified, but unspecified fields will still be used, in the
        order in which they come up in the dtype, to break ties.

    Returns
    -------
    index_array : ndarray, int
        Array of indices that partition `a` along the specified axis.
        If `a` is one-dimensional, ``a[index_array]`` yields a partitioned `a`.
        More generally, ``np.take_along_axis(a, index_array, axis=axis)``
        always yields the partitioned `a`, irrespective of dimensionality.

    See Also
    --------
    partition : Describes partition algorithms used.
    ndarray.partition : Inplace partition.
    argsort : Full indirect sort.
    take_along_axis : Apply ``index_array`` from argpartition
                      to an array as if by calling partition.

    Notes
    -----
    The returned indices are not guaranteed to be sorted according to
    the values. Furthermore, the default selection algorithm ``introselect``
    is unstable, and hence the returned indices are not guaranteed
    to be the earliest/latest occurrence of the element.

    `argpartition` works for real/complex inputs with nan values,
    see `partition` for notes on the enhanced sort order and
    different selection algorithms.