Identify the largest point within the radius of another point?

Use this example data to see what I mean

tag <- as.character(c(1,2,3,4,5,6,7,8,9,10))

species <- c("A","A","A","A","B","B","B","C","C","D")

size <- c(0.10,0.20,0.25,0.30,0.30,0.15,0.15,0.20,0.15,0.15)

radius <- (size*40)

x <- c(9,4,25,14,28,19,9,22,10,2)

y <- c(36,7,15,16,22,24,39,20,34,9)

data <- data.frame(tag, species, size, radius, x, y)


# Plot the points using qplot (from package tidyverse)
qplot(x, y, data = data) +
  geom_point(aes(colour = species, size = size))

Now that you can see the plot, what I want to do is for each individual “species A” point, I’d like to identify the largest point within a radius of size*40.

For example, in the bottom left of the plot you can see that species A (tag 2) would produce a radius large enough to contain the close species D point.

However, the species A point on the far right-hand-side of the plot (tag 3) would produce a radius large enough to contain both of the close species B and species C points, in which case I’d want some sort of output that identifies the largest individual within the species A radius.

I’d like to know what I can run (if anything) on this data set to get find the largest “within radius” point for each species A point and get an output like this:

Species A point ---- Largest point within radius

Species A tag 1 ----- Species C tag 9

Species A tag 2 ----- Species D tag 10

Species A tag 3 ----- Species B tag 5

Species A tag 4 ----- Species C tag 8

I've used spatstat and CTFSpackage to make some plots in the past but I can't figure out how to "find largest neighbor within radius". Perhaps I can tackle this in ArcMAP? Also, this is just a small example dataset. Realistically I will be wanting to find the "largest neighbor within radius" for thousands of points.

Any help or feedback would be greatly appreciated.

Solution

Following finds the largest species and tag pair that is within given radius for each of the species.

all_df <- data # don't wanna have a variable called data
res_df <- data.frame()
for (j in 1 : nrow(all_df)) {

  # subset the data
  df <- subset(all_df, species != species[j])
  # index of animals within radius
  ind <- which ((df$x - x[j])^2 +  (df$y - y[j])^2 < radius[j]^2 )

  # find the max `size` in the subset df
  max_size <- max(df$size[ind])
  # all indices with max_size in df
  max_inds <- which(df$size[ind] == max_size)
  # pick the last one is there is more than on max_size  
  new_ind <- ind[max_inds[length(max_inds)]]

  # results in data.frame
  res_df <- rbind(res_df, data.frame(org_sp = all_df$species[j], 
                                     org_tag = all_df$tag[j], 
                                     res_sp = df$species[new_ind], 
                                     res_tag = df$tag[new_ind]))
}

res_df
#      org_sp org_tag res_sp res_tag
# 1       A       1      C       9
# 2       A       2      D      10
# 3       A       3      B       5
# 4       A       4      C       8
# 5       B       5      A       3
# 6       B       6      C       8
# 7       B       7      C       9
# 8       C       8      B       5
# 9       C       9      B       7
# 10      D      10      A       2