[R] Help with Book example of Matrix application

[David Winsemius]

On Apr 9, 2012, at 12:32 PM, James Lenihan wrote:

> I found this example in an Introductory R book in the chapter on  
> Matrices and
> Arrays
>
> The array is
>> m
>     [,1] [,2] [,3] [,4] [,5]
> [1,]    0   12   13    8   20
> [2,]   12    0   15   28   88
> [3,]   13   15    0    6    9
> [4,]    8   28    6    0   33
> [5,]   20   88    9   33    0
>
> The code is
>
> #returns the minimum value of d[i,j], i !=j and the row attaining
> #the minimum, for square symmetric d; no special policy on ties

Would you consider a more compact operation? That code seems very unR- 
ish.

  min( m[row(m) != col(m)] )
#[1] 6
  which(m ==  min( m[row(m) != col(m)] ), arr.ind=TRUE)
#
      row col
[1,]   4   3
[2,]   3   4
-------------------
  c(min= min( m[row(m) != col(m)] ),
    which(m ==  min( m[row(m) != col(m)] ), arr.ind=TRUE)[1,])
########
min row col
   6   4   3


I suppose the question could be how to debug that code, in which case  
you should be manually "stepping through" it to see what the values  
are during the implicit loop.

> mind  <- function(d) {
>     n  <- nrow(d)
>     # add a column to identify row number for apply()
>     dd  <- cbind(d,1:n)
>     wmins <- apply(dd[-n,],1,imin)
>     # wins will be 2 X n, 1st row being indices and 2nd being values
>     i <- which.min(wmins[2,])
>     j <- wmins[1,i]
>     return(c(d[i,j],i,j))
> }
>
> #this finds the location, value of the minimum in a row x
> imin  <- function(x) {
>   lx  <- length(x)
>    i  <- x[lx] # original row number
>    j <-which.min(x[(x+1):(lx-1)])
>    k <- i+j
>    return(c(k,x[k]))
> }
> The result s/b
> [1] 6 3 4
>
> I am getting:
>
> Warning messages:
> 1: In (x + 1):(lx - 1) :
>  numerical expression has 6 elements: only the first used
> 2: In (x + 1):(lx - 1) :
>  numerical expression has 6 elements: only the first used
> 3: In (x + 1):(lx - 1) :
>  numerical expression has 6 elements: only the first used
> 4: In (x + 1):(lx - 1) :
>  numerical expression has 6 elements: only the first used
>
> I have check my typing a number of times.
>
> Does anyone see an error?
>
> Thanks,
>
> Jim L.
> 	[[alternative HTML version deleted]]
>
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David Winsemius, MD
West Hartford, CT