[R] Help in Recursive Function for steps reconstruction in Clusters Analysis - HCA: Single Link

[Cleber Borges]

Hello everybody,

I'm trying to build a function to illustrate, in 2D, the sequence of the 
"Single Link" algorithm (the purpose is merely didactic).

The idea is to have the scatter of points on a graph.
Iteratively, build the segments (with the "segments()" function, for 
each step).

I simulated a data set "d", and created an object "r" using the command:

r <- hclust( dist( d ), met='single' )

My problem:

I created a recursive function, "f()", to find the shortest distances. 
It is defined below.
But the return has only one column, while I thought there would be two 
columns.


What I get:
###############
# Testing f()
f( c(2, 3) )   #### is the fourth step of hclust

      [,1]
xmd1   -6
xmd2  -10
xmd1   -3
xmd1   -4
xmd2   -5

What I expected:
##################

# xmd1   xmd2
# -6     -10
# -3      -5
# -4      -5


If anyone can help me with this recursive function, I would appreciate 
it in advance.
I've never used recursion before, so I don't have a good grasp of how it 
works.

Thank you very much in advance,

Cleber.



All details are below:
########################


d <- scan()
  0.986  0.900
-1.331  1.503
-0.220 -0.752
  0.102 -0.071
-0.171  0.018
-0.782  0.490
  1.154 -0.074
-0.768 -1.529
  1.761 -1.396
-0.730  0.910

d <- matrix( d, 10, byrow=T )

r <- hclust( dist( d ), met='single' )
r$merge
#      [,1] [,2]
# [1,]   -4   -5
# [2,]   -6  -10
# [3,]   -3    1
# [4,]    2    3
# [5,]   -2    4
# [6,]   -8    5
# [7,]   -1   -7
# [8,]    6    7
# [9,]   -9    8


f <- function(xmd) {

     rs <- NULL
     xmd1 <- 999999999
     xmd2 <- 999999999

     if (xmd[1] > 0) {
         xmd1 <- f(r$merge[ xmd[1], ])
     } else {
         xmd1 <- xmd[1]
     }

     if (xmd[2] > 0) {
         xmd2 <- f(r$merge[ xmd[2], ])
     } else {
         xmd2 <- xmd[2]
     }

     return( rbind( rs, rbind( xmd1, xmd2 ) ) )
}



# Testing f()
f( c(2, 3) )

# result of f()
#      [,1]
# xmd1   -6
# xmd2  -10
# xmd1   -3
# xmd1   -4
# xmd2   -5

# My expectative:
# xmd1   xmd2
# -6     -10
# -3      -5
# -4      -5


# Testing f()
  f( c( 6, 7) )

# result of f()
#      [,1]
# xmd1   -8
# xmd1   -2
# xmd1   -6
# xmd2  -10
# xmd1   -3
# xmd1   -4
# xmd2   -5
# xmd1   -1
# xmd2   -7


# My expectative:
# xmd1   xmd2
# -8     -10
# -2     -10
# -6     -10
# -3      -5
# -4      -5
# -1      -7