如何仅查找某些顶点的邻域? [英] How to find the neighborhood for certain vertices only?

问题描述

问题开始了我有一个带n<100个顶点的无向图g.该图很简单.所有顶点的坐标都是整数(x_i, y_i), i=1, 2,..., n，边的集合是预定义的，它们是长度为1单位的线段. 顶点的度可以是2，3或4.

I have an undirected graph g with n<100 vertices. The graph is simple. The coordinates of all vertices are integer (x_i, y_i), i=1, 2,..., n, the set of edges is predefinded, they are line segments with the length 1 unit. The degree of vertices can be 2, 3 or 4.

library(igraph)

g <- graph_from_literal(1-2-3-4-5-6-7-8-1, 8-9-4)
B <- t(matrix(c(0,0, 0,1, 0,2, -1,2, -2,2, -2,1, -2,0, -1,0, -1,1), nrow =2));

V(g)$id <- seq_len(vcount(g))

V(g)$x <- B[,1]; V(g)$y <- B[,2]

plot(g, layout=as.matrix(B))

我需要为corner属性的顶点设置新属性.

I need to set the new attribute for vertex the corner attribute.

如果顶点的度数为2并且两个入射边不在同一条线上，则我们说顶点i是corner顶点.在顶点上方的图中，1, 3, 5, 7是拐角顶点，而其余的顶点2, 4, 6, 8, 9是非拐角.

We say the vertex i is the corner vertex if its degree is 2 and two incident edges do not lie on the same line. On the plot above vertices 1, 3, 5, 7 are corner vertices while remaining vertices 2, 4, 6, 8, 9 are non-corner.

我的尝试

我找到了度数等于2的顶点列表.

I have found the list of vertices that have degree equal to 2.

idv <- V(g)[strength(g)==2]; idv # 1 2 3 5 6 7 9

然后找到第i个顶点的邻域顶点列表.

Then the list of neighborhood vertices for i-th vertex was found.

neigh<-neighborhood(g, idv); neigh

错误在这里，因为我看到了所有顶点的邻域顶点，而不仅是度数等于2的顶点.例如，

The error is here, because I see the neighborhood vertices for all vertices, no only vertices that have degree equal to 2. For instance,

neigh[[4]]; neigh[[8]];   
#[1] 4 3 5 9
#[1] 8 1 7 9

问题.如何使用neighborhood函数查找仅度为2的顶点的邻域?

Question. How to use the neighborhood function to find the neighborhood for vertices with degree 2 only?

推荐答案

该函数正确，但是第二个参数与感兴趣的顶点无关，它是第三个参数:

The function is correct, but the second argument isn't about vertices of interest, it's the third one:

neighborhood
# function (graph, order = 1, nodes = V(graph), mode = c("all", 
#     "out", "in"), mindist = 0) 
# {
# ...

因此

length(neighborhood(g, nodes = idv))
# [1] 7

完成工作.

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