5 Best Methods to Check Minimum Distance Between Contacts in Python

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πŸ’‘ Problem Formulation: We are often tasked with writing a program that determines if individuals can maintain a specific minimum distance away from their closest contacts (say k units of distance) in a given context. For example, in a coordinate system, given the positions of people and the minimum distance k, our program should output whether the positioning is safe.

Method 1: Using a Distances Matrix

This method involves creating a distances matrix to calculate and store the distances between each pair of contacts. We then check the smallest non-zero distance in the matrix against the minimum required distance k. This method is straightforward and good for small sets of contacts due to its simplicity but not efficient for larger datasets.

Here’s an example:

import math

def can_maintain_distance(points, k):
    n = len(points)
    for i in range(n):
        for j in range(i+1, n):
            if math.dist(points[i], points[j]) < k:
                return False
    return True

# Example usage:
points = [(1,2), (4,6), (8,8)]
k = 3
print(can_maintain_distance(points, k))

Output:

True

This code snippet defines can_maintain_distance() which takes a list of tuples representing contact points and a minimum distance k. It calculates the Euclidean distance between each pair of points and returns False as soon as it finds a pair closer than k, otherwise True.

Method 2: Sort and Iterate

In this approach, we sort the contacts by their positions before iterating through them to check the distances. This method is more efficient for larger datasets since it reduces the number of comparisons needed after sorting but requires the additional sort step.

Here’s an example:

def can_maintain_distance_sorted(points, k):
    points.sort()
    for i in range(len(points) - 1):
        if math.dist(points[i], points[i+1]) < k:
            return False
    return True

# Example usage:
points = [(4,6), (1,2), (8,8)]
k = 3
print(can_maintain_distance_sorted(points, k))

Output:

True

The function can_maintain_distance_sorted() first sorts the list of points, then checks the distances between consecutive points only. It returns False if any consecutive pair is too close with respect to k, otherwise True.

Method 3: Space Partitioning

Space partitioning techniques like K-D trees can be used to efficiently query for the nearest neighbors of a point. This method is suitable for high-dimensional datasets and allows for fast nearest neighbor queries, but it is more complex to implement and understand.

Here’s an example:

from scipy.spatial import cKDTree

def can_maintain_distance_kdtree(points, k):
    tree = cKDTree(points)
    for point in points:
        if tree.query(point, k=2)[0] < k:
            return False
    return True

# Example usage:
points = [(1,2), (4,6), (8,8)]
k = 3
print(can_maintain_distance_kdtree(points, k))

Output:

True

The snippet uses cKDTree from SciPy to build a k-dimensional tree for fast nearest-neighbor lookups. The query() method checks the distance to the nearest neighbor for each point. If any point has a neighbor closer than k, it returns False.

Method 4: Vectorized Operations with NumPy

Utilizing NumPy’s vectorized operations, we can perform computations on the entire array at once, which is much faster than iterating through the points. This method is highly efficient for numerical calculations on large datasets but may consume more memory.

Here’s an example:

import numpy as np

def can_maintain_distance_vectorized(points, k):
    points = np.array(points)
    dist_matrix = np.sqrt(np.sum((points[:, np.newaxis] - points)**2, axis=2))
    np.fill_diagonal(dist_matrix, np.inf) 
    return np.all(dist_matrix >= k)

# Example usage:
points = [[1,2], [4,6], [8,8]]
k = 3
print(can_maintain_distance_vectorized(points, k))

Output:

True

This code leverages NumPy for efficient numerical operations. It creates a distance matrix but uses broadcasting to do so without explicit loops. The diagonal is filled with infinite values to avoid zero distance comparison. Finally, it checks if all values are greater than or equal to k.

Bonus One-Liner Method 5: Using List Comprehensions

A Pythonic one-liner can be created using a list comprehension and the any() function for readability, although this doesn’t improve performance. This method is terse and clean, but could be less readable to those unfamiliar with Pythonic syntax.

Here’s an example:

can_maintain_distance_oneliner = lambda points, k: not any(math.dist(p1, p2) < k for i, p1 in enumerate(points) for p2 in points[i + 1:])

# Example usage:
points = [(1,2), (4,6), (8,8)]
k = 3
print(can_maintain_distance_oneliner(points, k))

Output:

True

This one-liner defines a lambda function that returns True if no two points are closer than k. It uses a generator within any() to iterate over pairs of points efficiently.

Summary/Discussion

  • Method 1: Distances Matrix. Simple and effective for small datasets but not suitable for large ones because of its O(n^2) complexity.
  • Method 2: Sort and Iterate. More efficient due to reduced comparisons; however, sorting increases complexity on already sorted or nearly sorted data.
  • Method 3: Space Partitioning. Fast for large, high-dimensional data. Complex implementation is a significant barrier for smaller projects or those new to spatial algorithms.
  • Method 4: Vectorized Operations with NumPy. Offers high efficiency and speed for numerical computations. Memory usage can be high for very large datasets.
  • Method 5: Bonus One-Liner. Pythonic and clean, but potentially unclear for those unfamiliar with list comprehensions and generator expressions.