如何在列表中找到离我的点最近的两个点的坐标

Question

I have a list of values and a given point and I want to find the coordinates of the two closest points (one inferior and one superior).

For example:

list = [0, 0.5, 1, 1.5, 2, 2.5, 3]
point = 0.7

I want my return value to be [1, 2] since 0.7 is between 0.5 and 1.

Any idea of how to do this?

Answer 1

Assuming the given list is always sorted as it is in your example and that it has for sure one value below and one value above your given point, then you can loop over the list until you find a value larger than the given point and then return a list containing that index and the index below it.

def two_closest(_list, point):
    for index, value in enumerate(_list):
        if value > point:
            return (index-1, index)
    raise Exception("Malformed parameters")

For larger lists, you could speed this up with binary search or other heuristics. For example, you said your list is created by np.linespace, you could find the exact place your coordinate would lie between if you knew how the list was created.

Answer 2

you can compute distances and then exploit numpy.argsort :

import numpy as np

def closest_n_indexes(distances, n=2):
    return np.argsort(distances)[:n]

point_list = [0, 0.5, 1, 1.5, 2, 2.5, 3]
point = 0.7

distances = [abs(p-point) for p in point_list]

closest_n_indexes(distances)
Out[8]: array([1, 2])

Answer 3

If list is in sorted order you can use python's built-in binary search library. It's very fast:

import bisect

def get_bracket(lst, target):
    lindex = bisect.bisect_right(lst, target) - 1
    rindex = lindex + 1
    return [lindex, rindex]

print(get_bracket([0, 0.5, 1, 1.5, 2, 2.5, 3], 0.7))

which prints:

[1, 2]