使用R仅查找一个点（不是整个矩阵）的k-最近邻居

Question

Is there a package or a simple way to serach k-nearest neighbor (specially with kd tree) for one point using R? All the packages who provide this function (example RANN or FNN...) compute the knn for all the points in a matrix, I need to do it for only one point.

For example I have a matrix with 10 points "A" to "E" and I want to find for "A" the 2 nearest neighbors between the 4 other points ("B" to "E") without doing the same calculation for all the rows in the dataset (without computing knn for "B", "C", "D", "E")

I hope my question is clear, my english is not good.

Thank you for help,

Answer 1

If I understand correctly, you can do this with the FNN package:

> library(FNN)
> X <- matrix(runif(100), 5, 5)
> X
          [,1]      [,2]      [,3]      [,4]      [,5]
[1,] 0.7475301 0.6725876 0.2511358 0.5048512 0.1196027
[2,] 0.5777907 0.6337206 0.8334608 0.5067914 0.6410024
[3,] 0.5488786 0.9613076 0.2217271 0.6906149 0.7396482
[4,] 0.8230380 0.8596784 0.6348114 0.6211107 0.3089131
[5,] 0.6531433 0.8682462 0.2555402 0.2443061 0.5292509
> knnx.dist(X[-1,], X[1, , drop=FALSE], k=2)
          [,1]     [,2]
[1,] 0.4870996 0.531889
> knnx.index(X[-1,], X[1, , drop=FALSE], k=2)
     [,1] [,2]
[1,]    3    4

Note that the result of knnx.index relates to the matrix passed to the function so that 3, and 4 actually means rows 4 and 5 the original data set.