Slice Identification in an Array
DESCRIPTION:
Returns an array filled with integers, each of which identifies where in the current slice it is. This is a generalization of the row and col functions: row(x) can be expressed as slice.index(x,1) and col(x) can be expressed as slice.index(x,2).
USAGE: 
slice.index(x, MARGIN)
REQUIRED ARGUMENTS:
x:
An array. If x has no dimensions it will be considered to be a one dimensional array.
MARGIN:
An integer giving the dimension number to slice by.
VALUE:
The result is an array with the same dimensions and dimnames as x. The subarray result[...,i,...], where i is the MARGINth dimension will be filled with the value i.
NOTE:
In the future this function may be extended to allow a vector of integers for the MARGIN argument, as in the apply function.
SEE ALSO:
apply , col , rep , row , split .
EXAMPLES: 
x <- array(data = 101:124, dim = c(4,3,2))
slice.index(x, 1)


# Produces the following output:
  , , 1
       [,1] [,2] [,3]
  [1,]    1    1    1
  [2,]    2    2    2
  [3,]    3    3    3
  [4,]    4    4    4


  , , 2
       [,1] [,2] [,3]
  [1,]    1    1    1
  [2,]    2    2    2
  [3,]    3    3    3
  [4,]    4    4    4


slice.index(x,3)


# Produces the following output:
  , , 1
       [,1] [,2] [,3]
  [1,]    1    1    1
  [2,]    1    1    1
  [3,]    1    1    1
  [4,]    1    1    1


  , , 2
       [,1] [,2] [,3]
  [1,]    2    2    2
  [2,]    2    2    2
  [3,]    2    2    2
  [4,]    2    2    2


split(x, group = slice.index(x,2)) # compare to x[,1,], x[,2,], x[,3,]


# Produces the following output:
  $"1":
  [1] 101 102 103 104 113 114 115 116


  $"2":
  [1] 105 106 107 108 117 118 119 120


  $"3":
  [1] 109 110 111 112 121 122 123 124