如何获取从固定点到特定距离的坐标邻居

Question

我的任务是实现 GetNeighbors 方法，该方法从一组点返回仅与具有整数坐标 x 和 y 的点的 h 邻居的点。 （阅读代码中的摘要以获得更多理解）

其实我和Find k nearest neighbor in C#这个人有同样的任务，但是那里的答案似乎不起作用。而且我想只用我猜应该期望的循环来做这件事:)>。

到目前为止我的代码：

        /// <summary>
        /// Gets from a set of points only points that are h-neighbors for a point with integer coordinates x and y.
        /// </summary>
        /// <param name="point">Given point with integer coordinates x and y.</param>
        /// <param name="h">Distance around a given point.</param>
        /// <param name="points">A given set of points.</param>
        /// <returns>Only points that are h-neighbors for a point with integer coordinates x and y.</returns>
        /// <exception cref="ArgumentNullException">Throw when array points is null.</exception>
        /// <exception cref="ArgumentException">Throw when h-distance is less or equals zero.</exception>
        public static Point[] GetNeighbors(Point point, int h, params Point[] points)
        {
            if (points is null)
            {
                throw new ArgumentNullException(nameof(points));
            }

            if (h <= 0)
            {
                throw new ArgumentException(null);
            }

            List<Point> neighbors = new List<Point>();

            int left = point.X - h;
            int right = point.X + h;
            int bottom = point.Y - h;
            int top = point.Y + h;


            for (int y = top; y <= bottom; y++)
            {
                for (int x = left; x <= right; x++)
                {
                    // Yeah...
                }
            }

            return neighbors.ToArray();
        }

所以到目前为止我所做的是在附近找到顶部、底部、左侧和右侧边界。我认为我只需要做一个 if 语句，它可以比较我拥有的点和来自点数组的点。嗯，怎么做，我对使用structs不是很熟悉，每次都是失败的。

这就是 struct Point 的构建方式：

/// <summary>
    /// Represents a point on the coordinate plane.
    /// </summary>
    public readonly struct Point : System.IEquatable<Point>
    {
        public Point(int x, int y)
        {
            this.X = x;
            this.Y = y;
        }

        public int X { get; }

        public int Y { get; }
        
        public static bool operator ==(Point left, Point right)
        {
            return left.Equals(right);
        }
        
        public static bool operator !=(Point left, Point right)
        {
            return !(left == right);
        }
        
        public override int GetHashCode()
        {
            return this.X.GetHashCode() ^ this.Y.GetHashCode();
        }
        
        public override bool Equals(object obj)
        {
            if (obj is null)
            {
                return false;
            }

            if (!(obj is Point))
            {
                return false;
            }

            if (obj.GetType() != this.GetType())
            {
                return false;
            }

            Point point = (Point)obj;
            
            return this.Equals(point);
        }

        public bool Equals(Point other)
        {
            return this.X == other.X && this.Y == other.Y;
        }
    }

Answer 1

您不能在坐标上循环，而是在点上循环。这是使用 LINQ 的解决方案：

public static Point[] GetNeighbors(Point point, int h, params Point[] points)
{
    int minX = point.X - h;
    int maxX = point.X + h;
    int minY = point.Y - h;
    int maxY = point.Y + h;
    return points
        .Where(p => p.X >= minX && p.X <= maxX && p.Y >= minY && p.Y <= maxY)
        .ToArray();
}

使用 C# 9.0，您可以简化与模式匹配的比较

        .Where(p => p.X is >=minX and <=maxX && p.Y is >=minY and <=maxY)

---

这是一个使用显式循环的解决方案。由于我们不知道结果数组的大小，我将结果存储在一个列表中：

    ...
    var list = new List<Point>();
    foreach (Point p in points) {
        if (p.X is >=minX and <=maxX && p.Y is >=minY and <=maxY) {
            list.Add(p);
        }
    }
    return list.ToArray();

或者您可以返回一个列表。

---

另一种解决方案是使用iterator method：

public static IEnumerable<Point> GetNeighbors(Point point, int h, params Point[] points)
{
    int minX = point.X - h;
    int maxX = point.X + h;
    int minY = point.Y - h;
    int maxY = point.Y + h;
    foreach (Point p in points) {
        if (p.X is >=minX and <=maxX && p.Y is >=minY and <=maxY) {
            yield return p;
        }
    }
}

请注意，这种方法不会将点存储在集合中，而是在循环结果时执行方法内部的循环（显式在 foreach 循环中或通过应用 .ToArray() 或 .Count() 等隐式执行.).

Point[] result = GetNeighbors(center, 2, p1, p2, p3, p4).ToArray();