统计学习方法P54中位数脚注

个人认为这本书中的脚注③是欠妥的，5和7与6的距离相同，何来故之说？

根据脚注②，可以发现该例子取自维基百科：

维基百科中，它的中位数选取的是7

但这是因为其Python代码中 median = len(point_list) // 2 ，使得其在偶数长度的数据集的中位数中选择了两者靠后的一个（因为数组下标从0开始）。

下附完整代码：

from collections import namedtuple
from operator import itemgetter
from pprint import pformat

class Node(namedtuple("Node", "location left_child right_child")):
    def __repr__(self):
        return pformat(tuple(self))

def kdtree(point_list, depth: int = 0):
    if not point_list:
        return None

    k = len(point_list[0])  # assumes all points have the same dimension
    # Select axis based on depth so that axis cycles through all valid values
    axis = depth % k

    # Sort point list by axis and choose median as pivot element
    point_list.sort(key=itemgetter(axis))
    median = len(point_list) // 2

    # Create node and construct subtrees
    return Node(
        location=point_list[median],
        left_child=kdtree(point_list[:median], depth + 1),
        right_child=kdtree(point_list[median + 1 :], depth + 1),
    )

def main():
    """Example usage"""
    point_list = [(7, 2), (5, 4), (9, 6), (4, 7), (8, 1), (2, 3)]
    tree = kdtree(point_list)
    print(tree)

if __name__ == "__main__":
    main()
   
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